diff --git a/library/alloc/benches/vec_deque_append.rs b/library/alloc/benches/vec_deque_append.rs
index 5825bdc355f2d..30b6e600e5adc 100644
--- a/library/alloc/benches/vec_deque_append.rs
+++ b/library/alloc/benches/vec_deque_append.rs
@@ -5,6 +5,11 @@ const WARMUP_N: usize = 100;
 const BENCH_N: usize = 1000;
 
 fn main() {
+    if cfg!(miri) {
+        // Don't benchmark Miri...
+        // (Due to bootstrap quirks, this gets picked up by `x.py miri library/alloc --no-doc`.)
+        return;
+    }
     let a: VecDeque<i32> = (0..VECDEQUE_LEN).collect();
     let b: VecDeque<i32> = (0..VECDEQUE_LEN).collect();
 
diff --git a/library/alloc/src/lib.miri.rs b/library/alloc/src/lib.miri.rs
new file mode 100644
index 0000000000000..89d7f49f55d2e
--- /dev/null
+++ b/library/alloc/src/lib.miri.rs
@@ -0,0 +1,4 @@
+//! Grep bootstrap for `MIRI_REPLACE_LIBRS_IF_NOT_TEST` to learn what this is about.
+#![no_std]
+extern crate alloc as realalloc;
+pub use realalloc::*;
diff --git a/library/core/src/lib.miri.rs b/library/core/src/lib.miri.rs
new file mode 100644
index 0000000000000..5c1027f20ba07
--- /dev/null
+++ b/library/core/src/lib.miri.rs
@@ -0,0 +1,4 @@
+//! Grep bootstrap for `MIRI_REPLACE_LIBRS_IF_NOT_TEST` to learn what this is about.
+#![no_std]
+extern crate core as realcore;
+pub use realcore::*;
diff --git a/library/std/src/lib.miri.rs b/library/std/src/lib.miri.rs
new file mode 100644
index 0000000000000..1f9bfb5b1b5c0
--- /dev/null
+++ b/library/std/src/lib.miri.rs
@@ -0,0 +1,4 @@
+//! Grep bootstrap for `MIRI_REPLACE_LIBRS_IF_NOT_TEST` to learn what this is about.
+#![no_std]
+extern crate std as realstd;
+pub use realstd::*;
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 8a79c28fc6c62..bacf5f0d33cec 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -595,7 +595,7 @@ impl Step for Miri {
         // This is for the tests so everything is done with the target compiler.
         let miri_sysroot = Miri::build_miri_sysroot(builder, target_compiler, target);
         builder.ensure(compile::Std::new(target_compiler, host));
-        let sysroot = builder.sysroot(target_compiler);
+        let host_sysroot = builder.sysroot(target_compiler);
 
         // # Run `cargo test`.
         // This is with the Miri crate, so it uses the host compiler.
@@ -618,7 +618,7 @@ impl Step for Miri {
 
         // miri tests need to know about the stage sysroot
         cargo.env("MIRI_SYSROOT", &miri_sysroot);
-        cargo.env("MIRI_HOST_SYSROOT", &sysroot);
+        cargo.env("MIRI_HOST_SYSROOT", &host_sysroot);
         cargo.env("MIRI", &miri);
 
         // Set the target.
@@ -681,10 +681,6 @@ impl Step for Miri {
             }
         }
 
-        // Tell `cargo miri` where to find the sysroots.
-        cargo.env("MIRI_SYSROOT", &miri_sysroot);
-        cargo.env("MIRI_HOST_SYSROOT", sysroot);
-
         // Finally, pass test-args and run everything.
         cargo.arg("--").args(builder.config.test_args());
         let mut cargo = Command::from(cargo);
@@ -2540,9 +2536,14 @@ fn prepare_cargo_test(
     //
     // Note that to run the compiler we need to run with the *host* libraries,
     // but our wrapper scripts arrange for that to be the case anyway.
-    let mut dylib_path = dylib_path();
-    dylib_path.insert(0, PathBuf::from(&*builder.sysroot_libdir(compiler, target)));
-    cargo.env(dylib_path_var(), env::join_paths(&dylib_path).unwrap());
+    //
+    // We skip everything on Miri as then this overwrites the libdir set up
+    // by `Cargo::new` and that actually makes things go wrong.
+    if builder.kind != Kind::Miri {
+        let mut dylib_path = dylib_path();
+        dylib_path.insert(0, PathBuf::from(&*builder.sysroot_libdir(compiler, target)));
+        cargo.env(dylib_path_var(), env::join_paths(&dylib_path).unwrap());
+    }
 
     if builder.remote_tested(target) {
         cargo.env(
@@ -2598,28 +2599,62 @@ impl Step for Crate {
         let target = self.target;
         let mode = self.mode;
 
+        // Prepare sysroot
         // See [field@compile::Std::force_recompile].
         builder.ensure(compile::Std::force_recompile(compiler, compiler.host));
 
-        if builder.config.build != target {
-            builder.ensure(compile::Std::force_recompile(compiler, target));
-            builder.ensure(RemoteCopyLibs { compiler, target });
-        }
-
         // If we're not doing a full bootstrap but we're testing a stage2
         // version of libstd, then what we're actually testing is the libstd
         // produced in stage1. Reflect that here by updating the compiler that
         // we're working with automatically.
         let compiler = builder.compiler_for(compiler.stage, compiler.host, target);
 
-        let mut cargo = builder::Cargo::new(
-            builder,
-            compiler,
-            mode,
-            SourceType::InTree,
-            target,
-            builder.kind.as_str(),
-        );
+        let mut cargo = if builder.kind == Kind::Miri {
+            if builder.top_stage == 0 {
+                eprintln!("ERROR: `x.py miri` requires stage 1 or higher");
+                std::process::exit(1);
+            }
+
+            // Build `cargo miri test` command
+            // (Implicitly prepares target sysroot)
+            let mut cargo = builder::Cargo::new(
+                builder,
+                compiler,
+                mode,
+                SourceType::InTree,
+                target,
+                "miri-test",
+            );
+            // This hack helps bootstrap run standard library tests in Miri. The issue is as
+            // follows: when running `cargo miri test` on libcore, cargo builds a local copy of core
+            // and makes it a dependency of the integration test crate. This copy duplicates all the
+            // lang items, so the build fails. (Regular testing avoids this because the sysroot is a
+            // literal copy of what `cargo build` produces, but since Miri builds its own sysroot
+            // this does not work for us.) So we need to make it so that the locally built libcore
+            // contains all the items from `core`, but does not re-define them -- we want to replace
+            // the entire crate but a re-export of the sysroot crate. We do this by swapping out the
+            // source file: if `MIRI_REPLACE_LIBRS_IF_NOT_TEST` is set and we are building a
+            // `lib.rs` file, and a `lib.miri.rs` file exists in the same folder, we build that
+            // instead. But crucially we only do that for the library, not the test builds.
+            cargo.env("MIRI_REPLACE_LIBRS_IF_NOT_TEST", "1");
+            cargo
+        } else {
+            // Also prepare a sysroot for the target.
+            if builder.config.build != target {
+                builder.ensure(compile::Std::force_recompile(compiler, target));
+                builder.ensure(RemoteCopyLibs { compiler, target });
+            }
+
+            // Build `cargo test` command
+            builder::Cargo::new(
+                builder,
+                compiler,
+                mode,
+                SourceType::InTree,
+                target,
+                builder.kind.as_str(),
+            )
+        };
 
         match mode {
             Mode::Std => {
diff --git a/src/bootstrap/src/core/builder.rs b/src/bootstrap/src/core/builder.rs
index c051b8183287e..e110a489cc088 100644
--- a/src/bootstrap/src/core/builder.rs
+++ b/src/bootstrap/src/core/builder.rs
@@ -631,6 +631,7 @@ pub enum Kind {
     Format,
     #[value(alias = "t")]
     Test,
+    Miri,
     Bench,
     #[value(alias = "d")]
     Doc,
@@ -673,6 +674,7 @@ impl Kind {
             Kind::Fix => "fix",
             Kind::Format => "fmt",
             Kind::Test => "test",
+            Kind::Miri => "miri",
             Kind::Bench => "bench",
             Kind::Doc => "doc",
             Kind::Clean => "clean",
@@ -822,6 +824,7 @@ impl<'a> Builder<'a> {
                 // Run run-make last, since these won't pass without make on Windows
                 test::RunMake,
             ),
+            Kind::Miri => describe!(test::Crate),
             Kind::Bench => describe!(test::Crate, test::CrateLibrustc),
             Kind::Doc => describe!(
                 doc::UnstableBook,
@@ -970,6 +973,7 @@ impl<'a> Builder<'a> {
             Subcommand::Fix => (Kind::Fix, &paths[..]),
             Subcommand::Doc { .. } => (Kind::Doc, &paths[..]),
             Subcommand::Test { .. } => (Kind::Test, &paths[..]),
+            Subcommand::Miri { .. } => (Kind::Miri, &paths[..]),
             Subcommand::Bench { .. } => (Kind::Bench, &paths[..]),
             Subcommand::Dist => (Kind::Dist, &paths[..]),
             Subcommand::Install => (Kind::Install, &paths[..]),
@@ -1235,6 +1239,7 @@ impl<'a> Builder<'a> {
         assert!(run_compiler.stage > 0, "miri can not be invoked at stage 0");
         let build_compiler = self.compiler(run_compiler.stage - 1, self.build.build);
 
+        // Prepare the tools
         let miri = self.ensure(tool::Miri {
             compiler: build_compiler,
             target: self.build.build,
@@ -1245,7 +1250,7 @@ impl<'a> Builder<'a> {
             target: self.build.build,
             extra_features: Vec::new(),
         });
-        // Invoke cargo-miri, make sure we can find miri and cargo.
+        // Invoke cargo-miri, make sure it can find miri and cargo.
         let mut cmd = Command::new(cargo_miri);
         cmd.env("MIRI", &miri);
         cmd.env("CARGO", &self.initial_cargo);
@@ -1308,7 +1313,11 @@ impl<'a> Builder<'a> {
         if cmd == "clippy" {
             cargo = self.cargo_clippy_cmd(compiler);
             cargo.arg(cmd);
-        } else if let Some(subcmd) = cmd.strip_prefix("miri-") {
+        } else if let Some(subcmd) = cmd.strip_prefix("miri") {
+            // Command must be "miri-X".
+            let subcmd = subcmd
+                .strip_prefix("-")
+                .unwrap_or_else(|| panic!("expected `miri-$subcommand`, but got {}", cmd));
             cargo = self.cargo_miri_cmd(compiler);
             cargo.arg("miri").arg(subcmd);
         } else {
@@ -1711,6 +1720,15 @@ impl<'a> Builder<'a> {
             cargo.env("RUSTC_WRAPPER_REAL", existing_wrapper);
         }
 
+        // If this is for `miri-test`, prepare the sysroots.
+        if cmd == "miri-test" {
+            self.ensure(compile::Std::new(compiler, compiler.host));
+            let host_sysroot = self.sysroot(compiler);
+            let miri_sysroot = test::Miri::build_miri_sysroot(self, compiler, target);
+            cargo.env("MIRI_SYSROOT", &miri_sysroot);
+            cargo.env("MIRI_HOST_SYSROOT", &host_sysroot);
+        }
+
         cargo.env(profile_var("STRIP"), self.config.rust_strip.to_string());
 
         if let Some(stack_protector) = &self.config.rust_stack_protector {
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index 67cde01ccdb63..96dec97525048 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -2022,7 +2022,7 @@ impl Config {
             Subcommand::Build { .. } => {
                 flags.stage.or(build_stage).unwrap_or(if download_rustc { 2 } else { 1 })
             }
-            Subcommand::Test { .. } => {
+            Subcommand::Test { .. } | Subcommand::Miri { .. } => {
                 flags.stage.or(test_stage).unwrap_or(if download_rustc { 2 } else { 1 })
             }
             Subcommand::Bench { .. } => flags.stage.or(bench_stage).unwrap_or(2),
@@ -2044,6 +2044,7 @@ impl Config {
         if flags.stage.is_none() && crate::CiEnv::current() != crate::CiEnv::None {
             match config.cmd {
                 Subcommand::Test { .. }
+                | Subcommand::Miri { .. }
                 | Subcommand::Doc { .. }
                 | Subcommand::Build { .. }
                 | Subcommand::Bench { .. }
@@ -2099,7 +2100,9 @@ impl Config {
 
     pub(crate) fn test_args(&self) -> Vec<&str> {
         let mut test_args = match self.cmd {
-            Subcommand::Test { ref test_args, .. } | Subcommand::Bench { ref test_args, .. } => {
+            Subcommand::Test { ref test_args, .. }
+            | Subcommand::Bench { ref test_args, .. }
+            | Subcommand::Miri { ref test_args, .. } => {
                 test_args.iter().flat_map(|s| s.split_whitespace()).collect()
             }
             _ => vec![],
diff --git a/src/bootstrap/src/core/config/flags.rs b/src/bootstrap/src/core/config/flags.rs
index 7262b785ee00a..7782f7e61c958 100644
--- a/src/bootstrap/src/core/config/flags.rs
+++ b/src/bootstrap/src/core/config/flags.rs
@@ -382,6 +382,25 @@ pub enum Subcommand {
         /// `/<build_base>/rustfix_missing_coverage.txt`
         rustfix_coverage: bool,
     },
+    /// Build and run some test suites *in Miri*
+    Miri {
+        #[arg(long)]
+        /// run all tests regardless of failure
+        no_fail_fast: bool,
+        #[arg(long, value_name = "ARGS", allow_hyphen_values(true))]
+        /// extra arguments to be passed for the test tool being used
+        /// (e.g. libtest, compiletest or rustdoc)
+        test_args: Vec<String>,
+        /// extra options to pass the compiler when running tests
+        #[arg(long, value_name = "ARGS", allow_hyphen_values(true))]
+        rustc_args: Vec<String>,
+        #[arg(long)]
+        /// do not run doc tests
+        no_doc: bool,
+        #[arg(long)]
+        /// only run doc tests
+        doc: bool,
+    },
     /// Build and run some benchmarks
     Bench {
         #[arg(long, allow_hyphen_values(true))]
@@ -453,6 +472,7 @@ impl Subcommand {
             Subcommand::Fix { .. } => Kind::Fix,
             Subcommand::Format { .. } => Kind::Format,
             Subcommand::Test { .. } => Kind::Test,
+            Subcommand::Miri { .. } => Kind::Miri,
             Subcommand::Clean { .. } => Kind::Clean,
             Subcommand::Dist { .. } => Kind::Dist,
             Subcommand::Install { .. } => Kind::Install,
@@ -464,7 +484,7 @@ impl Subcommand {
 
     pub fn rustc_args(&self) -> Vec<&str> {
         match *self {
-            Subcommand::Test { ref rustc_args, .. } => {
+            Subcommand::Test { ref rustc_args, .. } | Subcommand::Miri { ref rustc_args, .. } => {
                 rustc_args.iter().flat_map(|s| s.split_whitespace()).collect()
             }
             _ => vec![],
@@ -473,14 +493,16 @@ impl Subcommand {
 
     pub fn fail_fast(&self) -> bool {
         match *self {
-            Subcommand::Test { no_fail_fast, .. } => !no_fail_fast,
+            Subcommand::Test { no_fail_fast, .. } | Subcommand::Miri { no_fail_fast, .. } => {
+                !no_fail_fast
+            }
             _ => false,
         }
     }
 
     pub fn doc_tests(&self) -> DocTests {
         match *self {
-            Subcommand::Test { doc, no_doc, .. } => {
+            Subcommand::Test { doc, no_doc, .. } | Subcommand::Miri { no_doc, doc, .. } => {
                 if doc {
                     DocTests::Only
                 } else if no_doc {
diff --git a/src/bootstrap/src/utils/render_tests.rs b/src/bootstrap/src/utils/render_tests.rs
index 70f25b2cc87c1..95cd55c9a3d7c 100644
--- a/src/bootstrap/src/utils/render_tests.rs
+++ b/src/bootstrap/src/utils/render_tests.rs
@@ -231,14 +231,16 @@ impl<'a> Renderer<'a> {
         print!("\ntest result: ");
         self.builder.colored_stdout(|stdout| outcome.write_long(stdout)).unwrap();
         println!(
-            ". {} passed; {} failed; {} ignored; {} measured; {} filtered out; \
-             finished in {:.2?}\n",
+            ". {} passed; {} failed; {} ignored; {} measured; {} filtered out{time}\n",
             suite.passed,
             suite.failed,
             suite.ignored,
             suite.measured,
             suite.filtered_out,
-            Duration::from_secs_f64(suite.exec_time)
+            time = match suite.exec_time {
+                Some(t) => format!("; finished in {:.2?}", Duration::from_secs_f64(t)),
+                None => format!(""),
+            }
         );
     }
 
@@ -374,7 +376,9 @@ struct SuiteOutcome {
     ignored: usize,
     measured: usize,
     filtered_out: usize,
-    exec_time: f64,
+    /// The time it took to execute this test suite, or `None` if time measurement was not possible
+    /// (e.g. due to running inside Miri).
+    exec_time: Option<f64>,
 }
 
 #[derive(serde_derive::Deserialize)]
diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh b/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh
index 10ae7f17db7ab..38c5b173ae31b 100755
--- a/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh
@@ -62,3 +62,8 @@ case $HOST_TARGET in
     exit 1
     ;;
 esac
+# Also smoke-test `x.py miri`. This doesn't run any actual tests (that would take too long),
+# but it ensures that the crates build properly when tested with Miri.
+python3 "$X_PY" miri --stage 2 library/core --test-args notest
+python3 "$X_PY" miri --stage 2 library/alloc --test-args notest
+python3 "$X_PY" miri --stage 2 library/std --test-args notest
diff --git a/src/etc/completions/x.py.fish b/src/etc/completions/x.py.fish
index 9fc95fd09ba9f..a3539fc0be023 100644
--- a/src/etc/completions/x.py.fish
+++ b/src/etc/completions/x.py.fish
@@ -39,6 +39,7 @@ complete -c x.py -n "__fish_use_subcommand" -f -a "fix" -d 'Run cargo fix'
 complete -c x.py -n "__fish_use_subcommand" -f -a "fmt" -d 'Run rustfmt'
 complete -c x.py -n "__fish_use_subcommand" -f -a "doc" -d 'Build documentation'
 complete -c x.py -n "__fish_use_subcommand" -f -a "test" -d 'Build and run some test suites'
+complete -c x.py -n "__fish_use_subcommand" -f -a "miri" -d 'Build and run some test suites *in Miri*'
 complete -c x.py -n "__fish_use_subcommand" -f -a "bench" -d 'Build and run some benchmarks'
 complete -c x.py -n "__fish_use_subcommand" -f -a "clean" -d 'Clean out build directories'
 complete -c x.py -n "__fish_use_subcommand" -f -a "dist" -d 'Build distribution artifacts'
@@ -308,6 +309,45 @@ complete -c x.py -n "__fish_seen_subcommand_from test" -l llvm-profile-generate
 complete -c x.py -n "__fish_seen_subcommand_from test" -l enable-bolt-settings -d 'Enable BOLT link flags'
 complete -c x.py -n "__fish_seen_subcommand_from test" -l skip-stage0-validation -d 'Skip stage0 compiler validation'
 complete -c x.py -n "__fish_seen_subcommand_from test" -s h -l help -d 'Print help (see more with \'--help\')'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l test-args -d 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)' -r
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l rustc-args -d 'extra options to pass the compiler when running tests' -r
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l config -d 'TOML configuration file for build' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l build-dir -d 'Build directory, overrides `build.build-dir` in `config.toml`' -r -f -a "(__fish_complete_directories)"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l build -d 'build target of the stage0 compiler' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l host -d 'host targets to build' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l target -d 'target targets to build' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l exclude -d 'build paths to exclude' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l skip -d 'build paths to skip' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l rustc-error-format -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l on-fail -d 'command to run on failure' -r -f -a "(__fish_complete_command)"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l stage -d 'stage to build (indicates compiler to use/test, e.g., stage 0 uses the bootstrap compiler, stage 1 the stage 0 rustc artifacts, etc.)' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l keep-stage -d 'stage(s) to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l keep-stage-std -d 'stage(s) of the standard library to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l src -d 'path to the root of the rust checkout' -r -f -a "(__fish_complete_directories)"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -s j -l jobs -d 'number of jobs to run in parallel' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l warnings -d 'if value is deny, will deny warnings if value is warn, will emit warnings otherwise, use the default configured behaviour' -r -f -a "{deny	'',warn	'',default	''}"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l error-format -d 'rustc error format' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l color -d 'whether to use color in cargo and rustc output' -r -f -a "{always	'',never	'',auto	''}"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l llvm-skip-rebuild -d 'whether rebuilding llvm should be skipped, overriding `skip-rebuld` in config.toml' -r -f -a "{true	'',false	''}"
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l rust-profile-generate -d 'generate PGO profile with rustc build' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l rust-profile-use -d 'use PGO profile for rustc build' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l llvm-profile-use -d 'use PGO profile for LLVM build' -r -F
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l reproducible-artifact -d 'Additional reproducible artifacts that should be added to the reproducible artifacts archive' -r
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l set -d 'override options in config.toml' -r -f
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l no-fail-fast -d 'run all tests regardless of failure'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l no-doc -d 'do not run doc tests'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l doc -d 'only run doc tests'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -s v -l verbose -d 'use verbose output (-vv for very verbose)'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -s i -l incremental -d 'use incremental compilation'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l include-default-paths -d 'include default paths in addition to the provided ones'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l dry-run -d 'dry run; don\'t build anything'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l dump-bootstrap-shims -d 'Indicates whether to dump the work done from bootstrap shims'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l json-output -d 'use message-format=json'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l bypass-bootstrap-lock -d 'Bootstrap uses this value to decide whether it should bypass locking the build process. This is rarely needed (e.g., compiling the std library for different targets in parallel)'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l llvm-profile-generate -d 'generate PGO profile with llvm built for rustc'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l enable-bolt-settings -d 'Enable BOLT link flags'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -l skip-stage0-validation -d 'Skip stage0 compiler validation'
+complete -c x.py -n "__fish_seen_subcommand_from miri" -s h -l help -d 'Print help (see more with \'--help\')'
 complete -c x.py -n "__fish_seen_subcommand_from bench" -l test-args -r
 complete -c x.py -n "__fish_seen_subcommand_from bench" -l config -d 'TOML configuration file for build' -r -F
 complete -c x.py -n "__fish_seen_subcommand_from bench" -l build-dir -d 'Build directory, overrides `build.build-dir` in `config.toml`' -r -f -a "(__fish_complete_directories)"
diff --git a/src/etc/completions/x.py.ps1 b/src/etc/completions/x.py.ps1
index 6359b7ff086a9..7948adb2cfda1 100644
--- a/src/etc/completions/x.py.ps1
+++ b/src/etc/completions/x.py.ps1
@@ -66,6 +66,7 @@ Register-ArgumentCompleter -Native -CommandName 'x.py' -ScriptBlock {
             [CompletionResult]::new('fmt', 'fmt', [CompletionResultType]::ParameterValue, 'Run rustfmt')
             [CompletionResult]::new('doc', 'doc', [CompletionResultType]::ParameterValue, 'Build documentation')
             [CompletionResult]::new('test', 'test', [CompletionResultType]::ParameterValue, 'Build and run some test suites')
+            [CompletionResult]::new('miri', 'miri', [CompletionResultType]::ParameterValue, 'Build and run some test suites *in Miri*')
             [CompletionResult]::new('bench', 'bench', [CompletionResultType]::ParameterValue, 'Build and run some benchmarks')
             [CompletionResult]::new('clean', 'clean', [CompletionResultType]::ParameterValue, 'Clean out build directories')
             [CompletionResult]::new('dist', 'dist', [CompletionResultType]::ParameterValue, 'Build distribution artifacts')
@@ -386,6 +387,52 @@ Register-ArgumentCompleter -Native -CommandName 'x.py' -ScriptBlock {
             [CompletionResult]::new('--help', 'help', [CompletionResultType]::ParameterName, 'Print help (see more with ''--help'')')
             break
         }
+        'x.py;miri' {
+            [CompletionResult]::new('--test-args', 'test-args', [CompletionResultType]::ParameterName, 'extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)')
+            [CompletionResult]::new('--rustc-args', 'rustc-args', [CompletionResultType]::ParameterName, 'extra options to pass the compiler when running tests')
+            [CompletionResult]::new('--config', 'config', [CompletionResultType]::ParameterName, 'TOML configuration file for build')
+            [CompletionResult]::new('--build-dir', 'build-dir', [CompletionResultType]::ParameterName, 'Build directory, overrides `build.build-dir` in `config.toml`')
+            [CompletionResult]::new('--build', 'build', [CompletionResultType]::ParameterName, 'build target of the stage0 compiler')
+            [CompletionResult]::new('--host', 'host', [CompletionResultType]::ParameterName, 'host targets to build')
+            [CompletionResult]::new('--target', 'target', [CompletionResultType]::ParameterName, 'target targets to build')
+            [CompletionResult]::new('--exclude', 'exclude', [CompletionResultType]::ParameterName, 'build paths to exclude')
+            [CompletionResult]::new('--skip', 'skip', [CompletionResultType]::ParameterName, 'build paths to skip')
+            [CompletionResult]::new('--rustc-error-format', 'rustc-error-format', [CompletionResultType]::ParameterName, 'rustc-error-format')
+            [CompletionResult]::new('--on-fail', 'on-fail', [CompletionResultType]::ParameterName, 'command to run on failure')
+            [CompletionResult]::new('--stage', 'stage', [CompletionResultType]::ParameterName, 'stage to build (indicates compiler to use/test, e.g., stage 0 uses the bootstrap compiler, stage 1 the stage 0 rustc artifacts, etc.)')
+            [CompletionResult]::new('--keep-stage', 'keep-stage', [CompletionResultType]::ParameterName, 'stage(s) to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)')
+            [CompletionResult]::new('--keep-stage-std', 'keep-stage-std', [CompletionResultType]::ParameterName, 'stage(s) of the standard library to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)')
+            [CompletionResult]::new('--src', 'src', [CompletionResultType]::ParameterName, 'path to the root of the rust checkout')
+            [CompletionResult]::new('-j', 'j', [CompletionResultType]::ParameterName, 'number of jobs to run in parallel')
+            [CompletionResult]::new('--jobs', 'jobs', [CompletionResultType]::ParameterName, 'number of jobs to run in parallel')
+            [CompletionResult]::new('--warnings', 'warnings', [CompletionResultType]::ParameterName, 'if value is deny, will deny warnings if value is warn, will emit warnings otherwise, use the default configured behaviour')
+            [CompletionResult]::new('--error-format', 'error-format', [CompletionResultType]::ParameterName, 'rustc error format')
+            [CompletionResult]::new('--color', 'color', [CompletionResultType]::ParameterName, 'whether to use color in cargo and rustc output')
+            [CompletionResult]::new('--llvm-skip-rebuild', 'llvm-skip-rebuild', [CompletionResultType]::ParameterName, 'whether rebuilding llvm should be skipped, overriding `skip-rebuld` in config.toml')
+            [CompletionResult]::new('--rust-profile-generate', 'rust-profile-generate', [CompletionResultType]::ParameterName, 'generate PGO profile with rustc build')
+            [CompletionResult]::new('--rust-profile-use', 'rust-profile-use', [CompletionResultType]::ParameterName, 'use PGO profile for rustc build')
+            [CompletionResult]::new('--llvm-profile-use', 'llvm-profile-use', [CompletionResultType]::ParameterName, 'use PGO profile for LLVM build')
+            [CompletionResult]::new('--reproducible-artifact', 'reproducible-artifact', [CompletionResultType]::ParameterName, 'Additional reproducible artifacts that should be added to the reproducible artifacts archive')
+            [CompletionResult]::new('--set', 'set', [CompletionResultType]::ParameterName, 'override options in config.toml')
+            [CompletionResult]::new('--no-fail-fast', 'no-fail-fast', [CompletionResultType]::ParameterName, 'run all tests regardless of failure')
+            [CompletionResult]::new('--no-doc', 'no-doc', [CompletionResultType]::ParameterName, 'do not run doc tests')
+            [CompletionResult]::new('--doc', 'doc', [CompletionResultType]::ParameterName, 'only run doc tests')
+            [CompletionResult]::new('-v', 'v', [CompletionResultType]::ParameterName, 'use verbose output (-vv for very verbose)')
+            [CompletionResult]::new('--verbose', 'verbose', [CompletionResultType]::ParameterName, 'use verbose output (-vv for very verbose)')
+            [CompletionResult]::new('-i', 'i', [CompletionResultType]::ParameterName, 'use incremental compilation')
+            [CompletionResult]::new('--incremental', 'incremental', [CompletionResultType]::ParameterName, 'use incremental compilation')
+            [CompletionResult]::new('--include-default-paths', 'include-default-paths', [CompletionResultType]::ParameterName, 'include default paths in addition to the provided ones')
+            [CompletionResult]::new('--dry-run', 'dry-run', [CompletionResultType]::ParameterName, 'dry run; don''t build anything')
+            [CompletionResult]::new('--dump-bootstrap-shims', 'dump-bootstrap-shims', [CompletionResultType]::ParameterName, 'Indicates whether to dump the work done from bootstrap shims')
+            [CompletionResult]::new('--json-output', 'json-output', [CompletionResultType]::ParameterName, 'use message-format=json')
+            [CompletionResult]::new('--bypass-bootstrap-lock', 'bypass-bootstrap-lock', [CompletionResultType]::ParameterName, 'Bootstrap uses this value to decide whether it should bypass locking the build process. This is rarely needed (e.g., compiling the std library for different targets in parallel)')
+            [CompletionResult]::new('--llvm-profile-generate', 'llvm-profile-generate', [CompletionResultType]::ParameterName, 'generate PGO profile with llvm built for rustc')
+            [CompletionResult]::new('--enable-bolt-settings', 'enable-bolt-settings', [CompletionResultType]::ParameterName, 'Enable BOLT link flags')
+            [CompletionResult]::new('--skip-stage0-validation', 'skip-stage0-validation', [CompletionResultType]::ParameterName, 'Skip stage0 compiler validation')
+            [CompletionResult]::new('-h', 'h', [CompletionResultType]::ParameterName, 'Print help (see more with ''--help'')')
+            [CompletionResult]::new('--help', 'help', [CompletionResultType]::ParameterName, 'Print help (see more with ''--help'')')
+            break
+        }
         'x.py;bench' {
             [CompletionResult]::new('--test-args', 'test-args', [CompletionResultType]::ParameterName, 'test-args')
             [CompletionResult]::new('--config', 'config', [CompletionResultType]::ParameterName, 'TOML configuration file for build')
diff --git a/src/etc/completions/x.py.sh b/src/etc/completions/x.py.sh
index e1436dcde6704..8fb5cf4e0901a 100644
--- a/src/etc/completions/x.py.sh
+++ b/src/etc/completions/x.py.sh
@@ -42,6 +42,9 @@ _x.py() {
             bootstrap,install)
                 cmd="bootstrap__install"
                 ;;
+            bootstrap,miri)
+                cmd="bootstrap__miri"
+                ;;
             bootstrap,run)
                 cmd="bootstrap__run"
                 ;;
@@ -61,7 +64,7 @@ _x.py() {
 
     case "${cmd}" in
         x.py)
-            opts="-v -i -j -h --verbose --incremental --config --build-dir --build --host --target --exclude --skip --include-default-paths --rustc-error-format --on-fail --dry-run --dump-bootstrap-shims --stage --keep-stage --keep-stage-std --src --jobs --warnings --error-format --json-output --color --bypass-bootstrap-lock --llvm-skip-rebuild --rust-profile-generate --rust-profile-use --llvm-profile-use --llvm-profile-generate --enable-bolt-settings --skip-stage0-validation --reproducible-artifact --set --help [PATHS]... [ARGS]... build check clippy fix fmt doc test bench clean dist install run setup suggest"
+            opts="-v -i -j -h --verbose --incremental --config --build-dir --build --host --target --exclude --skip --include-default-paths --rustc-error-format --on-fail --dry-run --dump-bootstrap-shims --stage --keep-stage --keep-stage-std --src --jobs --warnings --error-format --json-output --color --bypass-bootstrap-lock --llvm-skip-rebuild --rust-profile-generate --rust-profile-use --llvm-profile-use --llvm-profile-generate --enable-bolt-settings --skip-stage0-validation --reproducible-artifact --set --help [PATHS]... [ARGS]... build check clippy fix fmt doc test miri bench clean dist install run setup suggest"
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 1 ]] ; then
                 COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
                 return 0
@@ -1290,6 +1293,124 @@ _x.py() {
             COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
             return 0
             ;;
+        x.py__miri)
+            opts="-v -i -j -h --no-fail-fast --test-args --rustc-args --no-doc --doc --verbose --incremental --config --build-dir --build --host --target --exclude --skip --include-default-paths --rustc-error-format --on-fail --dry-run --dump-bootstrap-shims --stage --keep-stage --keep-stage-std --src --jobs --warnings --error-format --json-output --color --bypass-bootstrap-lock --llvm-skip-rebuild --rust-profile-generate --rust-profile-use --llvm-profile-use --llvm-profile-generate --enable-bolt-settings --skip-stage0-validation --reproducible-artifact --set --help [PATHS]... [ARGS]..."
+            if [[ ${cur} == -* || ${COMP_CWORD} -eq 2 ]] ; then
+                COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
+                return 0
+            fi
+            case "${prev}" in
+                --test-args)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --rustc-args)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --config)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --build-dir)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --build)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --host)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --target)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --exclude)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --skip)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --rustc-error-format)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --on-fail)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --stage)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --keep-stage)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --keep-stage-std)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --src)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --jobs)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                -j)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --warnings)
+                    COMPREPLY=($(compgen -W "deny warn default" -- "${cur}"))
+                    return 0
+                    ;;
+                --error-format)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                --color)
+                    COMPREPLY=($(compgen -W "always never auto" -- "${cur}"))
+                    return 0
+                    ;;
+                --llvm-skip-rebuild)
+                    COMPREPLY=($(compgen -W "true false" -- "${cur}"))
+                    return 0
+                    ;;
+                --rust-profile-generate)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --rust-profile-use)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --llvm-profile-use)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --reproducible-artifact)
+                    COMPREPLY=($(compgen -f "${cur}"))
+                    return 0
+                    ;;
+                --set)
+                    COMPREPLY=("${cur}")
+                    return 0
+                    ;;
+                *)
+                    COMPREPLY=()
+                    ;;
+            esac
+            COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
+            return 0
+            ;;
         x.py__run)
             opts="-v -i -j -h --args --verbose --incremental --config --build-dir --build --host --target --exclude --skip --include-default-paths --rustc-error-format --on-fail --dry-run --dump-bootstrap-shims --stage --keep-stage --keep-stage-std --src --jobs --warnings --error-format --json-output --color --bypass-bootstrap-lock --llvm-skip-rebuild --rust-profile-generate --rust-profile-use --llvm-profile-use --llvm-profile-generate --enable-bolt-settings --skip-stage0-validation --reproducible-artifact --set --help [PATHS]... [ARGS]..."
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 2 ]] ; then
diff --git a/src/etc/completions/x.py.zsh b/src/etc/completions/x.py.zsh
index ea7e4ba6758f5..537d849f39ee3 100644
--- a/src/etc/completions/x.py.zsh
+++ b/src/etc/completions/x.py.zsh
@@ -389,6 +389,54 @@ _arguments "${_arguments_options[@]}" \
 '*::paths -- paths for the subcommand:_files' \
 && ret=0
 ;;
+(miri)
+_arguments "${_arguments_options[@]}" \
+'*--test-args=[extra arguments to be passed for the test tool being used (e.g. libtest, compiletest or rustdoc)]:ARGS: ' \
+'*--rustc-args=[extra options to pass the compiler when running tests]:ARGS: ' \
+'--config=[TOML configuration file for build]:FILE:_files' \
+'--build-dir=[Build directory, overrides \`build.build-dir\` in \`config.toml\`]:DIR:_files -/' \
+'--build=[build target of the stage0 compiler]:BUILD:( )' \
+'--host=[host targets to build]:HOST:( )' \
+'--target=[target targets to build]:TARGET:( )' \
+'*--exclude=[build paths to exclude]:PATH:_files' \
+'*--skip=[build paths to skip]:PATH:_files' \
+'--rustc-error-format=[]:RUSTC_ERROR_FORMAT:( )' \
+'--on-fail=[command to run on failure]:CMD:_cmdstring' \
+'--stage=[stage to build (indicates compiler to use/test, e.g., stage 0 uses the bootstrap compiler, stage 1 the stage 0 rustc artifacts, etc.)]:N:( )' \
+'*--keep-stage=[stage(s) to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)]:N:( )' \
+'*--keep-stage-std=[stage(s) of the standard library to keep without recompiling (pass multiple times to keep e.g., both stages 0 and 1)]:N:( )' \
+'--src=[path to the root of the rust checkout]:DIR:_files -/' \
+'-j+[number of jobs to run in parallel]:JOBS:( )' \
+'--jobs=[number of jobs to run in parallel]:JOBS:( )' \
+'--warnings=[if value is deny, will deny warnings if value is warn, will emit warnings otherwise, use the default configured behaviour]:deny|warn:(deny warn default)' \
+'--error-format=[rustc error format]:FORMAT:( )' \
+'--color=[whether to use color in cargo and rustc output]:STYLE:(always never auto)' \
+'--llvm-skip-rebuild=[whether rebuilding llvm should be skipped, overriding \`skip-rebuld\` in config.toml]:VALUE:(true false)' \
+'--rust-profile-generate=[generate PGO profile with rustc build]:PROFILE:_files' \
+'--rust-profile-use=[use PGO profile for rustc build]:PROFILE:_files' \
+'--llvm-profile-use=[use PGO profile for LLVM build]:PROFILE:_files' \
+'*--reproducible-artifact=[Additional reproducible artifacts that should be added to the reproducible artifacts archive]:REPRODUCIBLE_ARTIFACT: ' \
+'*--set=[override options in config.toml]:section.option=value:( )' \
+'--no-fail-fast[run all tests regardless of failure]' \
+'--no-doc[do not run doc tests]' \
+'--doc[only run doc tests]' \
+'*-v[use verbose output (-vv for very verbose)]' \
+'*--verbose[use verbose output (-vv for very verbose)]' \
+'-i[use incremental compilation]' \
+'--incremental[use incremental compilation]' \
+'--include-default-paths[include default paths in addition to the provided ones]' \
+'--dry-run[dry run; don'\''t build anything]' \
+'--dump-bootstrap-shims[Indicates whether to dump the work done from bootstrap shims]' \
+'--json-output[use message-format=json]' \
+'--bypass-bootstrap-lock[Bootstrap uses this value to decide whether it should bypass locking the build process. This is rarely needed (e.g., compiling the std library for different targets in parallel)]' \
+'--llvm-profile-generate[generate PGO profile with llvm built for rustc]' \
+'--enable-bolt-settings[Enable BOLT link flags]' \
+'--skip-stage0-validation[Skip stage0 compiler validation]' \
+'-h[Print help (see more with '\''--help'\'')]' \
+'--help[Print help (see more with '\''--help'\'')]' \
+'*::paths -- paths for the subcommand:_files' \
+&& ret=0
+;;
 (bench)
 _arguments "${_arguments_options[@]}" \
 '*--test-args=[]:TEST_ARGS: ' \
@@ -710,6 +758,7 @@ _x.py_commands() {
 'fmt:Run rustfmt' \
 'doc:Build documentation' \
 'test:Build and run some test suites' \
+'miri:Build and run some test suites *in Miri*' \
 'bench:Build and run some benchmarks' \
 'clean:Clean out build directories' \
 'dist:Build distribution artifacts' \
@@ -770,6 +819,11 @@ _x.py__install_commands() {
     local commands; commands=()
     _describe -t commands 'x.py install commands' commands "$@"
 }
+(( $+functions[_x.py__miri_commands] )) ||
+_x.py__miri_commands() {
+    local commands; commands=()
+    _describe -t commands 'x.py miri commands' commands "$@"
+}
 (( $+functions[_x.py__run_commands] )) ||
 _x.py__run_commands() {
     local commands; commands=()
diff --git a/src/tools/miri/README.md b/src/tools/miri/README.md
index 26e55b897080c..bd6efeac09ad8 100644
--- a/src/tools/miri/README.md
+++ b/src/tools/miri/README.md
@@ -507,6 +507,8 @@ binaries, and as such worth documenting:
   crate currently being compiled.
 * `MIRI_ORIG_RUSTDOC` is set and read by different phases of `cargo-miri` to remember the
   value of `RUSTDOC` from before it was overwritten.
+* `MIRI_REPLACE_LIBRS_IF_NOT_TEST` when set to any value enables a hack that helps bootstrap
+  run the standard library tests in Miri.
 * `MIRI_VERBOSE` when set to any value tells the various `cargo-miri` phases to
   perform verbose logging.
 * `MIRI_HOST_SYSROOT` is set by bootstrap to tell `cargo-miri` which sysroot to use for *host*
diff --git a/src/tools/miri/cargo-miri/src/phases.rs b/src/tools/miri/cargo-miri/src/phases.rs
index 694720ab21f2d..3f6c484a057d5 100644
--- a/src/tools/miri/cargo-miri/src/phases.rs
+++ b/src/tools/miri/cargo-miri/src/phases.rs
@@ -3,7 +3,7 @@
 use std::env;
 use std::fs::{self, File};
 use std::io::BufReader;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::process::Command;
 
 use rustc_version::VersionMeta;
@@ -412,9 +412,25 @@ pub fn phase_rustc(mut args: impl Iterator<Item = String>, phase: RustcPhase) {
     // Arguments are treated very differently depending on whether this crate is
     // for interpretation by Miri, or for use by a build script / proc macro.
     if target_crate {
-        // Forward arguments, but remove "link" from "--emit" to make this a check-only build.
+        // Forward arguments, but patched.
         let emit_flag = "--emit";
+        // This hack helps bootstrap run standard library tests in Miri. The issue is as follows:
+        // when running `cargo miri test` on libcore, cargo builds a local copy of core and makes it
+        // a dependency of the integration test crate. This copy duplicates all the lang items, so
+        // the build fails. (Regular testing avoids this because the sysroot is a literal copy of
+        // what `cargo build` produces, but since Miri builds its own sysroot this does not work for
+        // us.) So we need to make it so that the locally built libcore contains all the items from
+        // `core`, but does not re-define them -- we want to replace the entire crate but a
+        // re-export of the sysroot crate. We do this by swapping out the source file: if
+        // `MIRI_REPLACE_LIBRS_IF_NOT_TEST` is set and we are building a `lib.rs` file, and a
+        // `lib.miri.rs` file exists in the same folder, we build that instead. But crucially we
+        // only do that for the library, not the unit test crate (which would be runnable) or
+        // rustdoc (which would have a different `phase`).
+        let replace_librs = env::var_os("MIRI_REPLACE_LIBRS_IF_NOT_TEST").is_some()
+            && !runnable_crate
+            && phase == RustcPhase::Build;
         while let Some(arg) = args.next() {
+            // Patch `--emit`: remove "link" from "--emit" to make this a check-only build.
             if let Some(val) = arg.strip_prefix(emit_flag) {
                 // Patch this argument. First, extract its value.
                 let val =
@@ -429,13 +445,36 @@ pub fn phase_rustc(mut args: impl Iterator<Item = String>, phase: RustcPhase) {
                     }
                 }
                 cmd.arg(format!("{emit_flag}={}", val.join(",")));
-            } else if arg == "--extern" {
-                // Patch `--extern` filenames, since Cargo sometimes passes stub `.rlib` files:
-                // https://github.com/rust-lang/miri/issues/1705
+                continue;
+            }
+            // Patch `--extern` filenames, since Cargo sometimes passes stub `.rlib` files:
+            // https://github.com/rust-lang/miri/issues/1705
+            if arg == "--extern" {
                 forward_patched_extern_arg(&mut args, &mut cmd);
-            } else {
-                cmd.arg(arg);
+                continue;
             }
+            // If the REPLACE_LIBRS hack is enabled and we are building a `lib.rs` file, and a
+            // `lib.miri.rs` file exists, then build that instead. We only consider relative paths
+            // as cargo uses those for files in the workspace; dependencies from crates.io get
+            // absolute paths.
+            if replace_librs {
+                let path = Path::new(&arg);
+                if path.is_relative()
+                    && path.file_name().is_some_and(|f| f == "lib.rs")
+                    && path.is_file()
+                {
+                    let miri_rs = Path::new(&arg).with_extension("miri.rs");
+                    if miri_rs.is_file() {
+                        if verbose > 0 {
+                            eprintln!("Performing REPLACE_LIBRS hack: {arg:?} -> {miri_rs:?}");
+                        }
+                        cmd.arg(miri_rs);
+                        continue;
+                    }
+                }
+            }
+            // Fallback: just propagate the argument.
+            cmd.arg(arg);
         }
 
         // During setup, patch the panic runtime for `libpanic_abort` (mirroring what bootstrap usually does).
diff --git a/src/tools/miri/src/helpers.rs b/src/tools/miri/src/helpers.rs
index c12fe0e086d8c..6e320b60eecb7 100644
--- a/src/tools/miri/src/helpers.rs
+++ b/src/tools/miri/src/helpers.rs
@@ -105,24 +105,41 @@ fn try_resolve_did(tcx: TyCtxt<'_>, path: &[&str], namespace: Option<Namespace>)
         (path, None)
     };
 
-    // First find the crate.
-    let krate =
-        tcx.crates(()).iter().find(|&&krate| tcx.crate_name(krate).as_str() == crate_name)?;
-    let mut cur_item = DefId { krate: *krate, index: CRATE_DEF_INDEX };
-    // Then go over the modules.
-    for &segment in modules {
-        cur_item = find_children(tcx, cur_item, segment)
-            .find(|item| tcx.def_kind(item) == DefKind::Mod)?;
-    }
-    // Finally, look up the desired item in this module, if any.
-    match item {
-        Some((item_name, namespace)) =>
-            Some(
-                find_children(tcx, cur_item, item_name)
-                    .find(|item| tcx.def_kind(item).ns() == Some(namespace))?,
-            ),
-        None => Some(cur_item),
+    // There may be more than one crate with this name. We try them all.
+    // (This is particularly relevant when running `std` tests as then there are two `std` crates:
+    // the one in the sysroot and the one locally built by `cargo test`.)
+    // FIXME: can we prefer the one from the sysroot?
+    'crates: for krate in
+        tcx.crates(()).iter().filter(|&&krate| tcx.crate_name(krate).as_str() == crate_name)
+    {
+        let mut cur_item = DefId { krate: *krate, index: CRATE_DEF_INDEX };
+        // Go over the modules.
+        for &segment in modules {
+            let Some(next_item) = find_children(tcx, cur_item, segment)
+                .find(|item| tcx.def_kind(item) == DefKind::Mod)
+            else {
+                continue 'crates;
+            };
+            cur_item = next_item;
+        }
+        // Finally, look up the desired item in this module, if any.
+        match item {
+            Some((item_name, namespace)) => {
+                let Some(item) = find_children(tcx, cur_item, item_name)
+                    .find(|item| tcx.def_kind(item).ns() == Some(namespace))
+                else {
+                    continue 'crates;
+                };
+                return Some(item);
+            }
+            None => {
+                // Just return the module.
+                return Some(cur_item);
+            }
+        }
     }
+    // Item not found in any of the crates with the right name.
+    None
 }
 
 /// Convert a softfloat type to its corresponding hostfloat type.
@@ -968,10 +985,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
 
     fn frame_in_std(&self) -> bool {
         let this = self.eval_context_ref();
-        let Some(start_fn) = this.tcx.lang_items().start_fn() else {
-            // no_std situations
-            return false;
-        };
         let frame = this.frame();
         // Make an attempt to get at the instance of the function this is inlined from.
         let instance: Option<_> = try {
@@ -982,13 +995,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         };
         // Fall back to the instance of the function itself.
         let instance = instance.unwrap_or(frame.instance);
-        // Now check if this is in the same crate as start_fn.
-        // As a special exception we also allow unit tests from
-        // <https://github.com/rust-lang/miri-test-libstd/tree/master/std_miri_test> to call these
-        // shims.
+        // Now check the crate it is in. We could try to be clever here and e.g. check if this is
+        // the same crate as `start_fn`, but that would not work for running std tests in Miri, so
+        // we'd need some more hacks anyway. So we just check the name of the crate. If someone
+        // calls their crate `std` then we'll just let them keep the pieces.
         let frame_crate = this.tcx.def_path(instance.def_id()).krate;
-        frame_crate == this.tcx.def_path(start_fn).krate
-            || this.tcx.crate_name(frame_crate).as_str() == "std_miri_test"
+        let crate_name = this.tcx.crate_name(frame_crate);
+        let crate_name = crate_name.as_str();
+        // On miri-test-libstd, the name of the crate is different.
+        crate_name == "std" || crate_name == "std_miri_test"
     }
 
     /// Handler that should be called when unsupported functionality is encountered.