Distinguish crates with the same name in type errors

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1843,6 +1843,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             let expected_defid = expected_adt.did();
 
                             diagnostic.note(format!("{found_name} and {expected_name} have similar names, but are actually distinct types"));
+                            let have_same_crate_name = self.tcx.crate_name(found_defid.krate)
+                                == self.tcx.crate_name(expected_defid.krate);
                             for (defid, name) in
                                 [(found_defid, found_name), (expected_defid, expected_name)]
                             {
@@ -1862,7 +1864,21 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                     format!("{name} is defined in the current crate")
                                 } else {
                                     let crate_name = self.tcx.crate_name(defid.krate);
-                                    format!("{name} is defined in crate `{crate_name}`")
+                                    diagnostic.span_note(
+                                        def_span,
+                                        format!("{name} is defined in crate `{crate_name}`"),
+                                    );
+
+                                    // If these are named the same, give a hint about why the compiler thinks they're different.
+                                    if have_same_crate_name {
+                                        let crate_paths = self.tcx.crate_extern_paths(defid.krate);
+                                        diagnostic.note(format!(
+                                            "`{crate_name}` was loaded from {}",
+                                            crate_paths[0].display()
+                                        ));
+                                    }
+
+                                    continue;
                                 };
                                 diagnostic.span_note(def_span, msg);
                             }

src/bootstrap/src/core/builder.rs

@@ -1798,7 +1798,11 @@ impl<'a> Builder<'a> {
             cargo.env("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR", map_to);
         }
 
-        if self.config.rust_remap_debuginfo {
+        // Users won't have the original cargo registry from the CI builder available, even if they have `rust-src` installed.
+        // Don't show their sources in UI tests even if they're available.
+        // As a happy side-effect, this fixes a few UI tests when download-rustc is enabled.
+        // NOTE: only set this when building std so the error messages are better for rustc itself.
+        if self.config.rust_remap_debuginfo || mode == Mode::Std {
             let mut env_var = OsString::new();
             if self.config.vendor {
                 let vendor = self.build.src.join("vendor");

tests/incremental/circular-dependencies.rs

@@ -26,12 +26,14 @@ fn test() {
     //[cfail2]~| NOTE arguments to this function are incorrect
     //[cfail2]~| NOTE `Foo` and `circular_dependencies::Foo` have similar names, but are actually distinct types
     //[cfail2]~| NOTE the crate `circular_dependencies` is compiled multiple times, possibly with different configurations
+    //[cfail2]~| NOTE loaded from
     //[cfail2]~| NOTE function defined here
 
     consume_foo(aux::produce_foo());
     //[cfail2]~^ ERROR mismatched types [E0308]
     //[cfail2]~| NOTE expected `Foo`, found `circular_dependencies::Foo`
     //[cfail2]~| NOTE arguments to this function are incorrect
     //[cfail2]~| NOTE `circular_dependencies::Foo` and `Foo` have similar names, but are actually distinct types
+    //[cfail2]~| NOTE loaded from
     //[cfail2]~| NOTE the crate `circular_dependencies` is compiled multiple times, possibly with different configurations
 }

tests/run-make/type-mismatch-sysroot-crate/Makefile

@@ -0,0 +1,20 @@
+# Test that we emit useful diagnostics when the same crate is loaded from the sysroot and --extern.
+# This can't be a run-make test because it needs the aux-file to itself have a dependency passed with --extern
+# (and just bare `--extern hashbrown` errors too early, while compiling `uses_hashbrown.rs`).
+
+include ../tools.mk
+define SED_REGEX
+s#/hashbrown-[^/]*/src#/hashbrown-VERSION/src#;
+s#[^ ]*/build/[^ ]*/lib/rustlib/[^ ]*#BUILD_DIR/HOST/stageN/lib/rustlib/TARGET/lib/libhashbrown.rlib#;
+s#[^ ]*/build/[^ ]*/test/run-make/#BUILD_DIR/HOST/test/run-make/#
+s#[^ ]*tests/run-make/#TEST_DIR/#
+endef
+export SED_REGEX
+
+all:
+	$(RUSTC) hashbrown.rs --crate-type lib
+	$(RUSTC) uses-hashbrown.rs --extern hashbrown=$(TMPDIR)/libhashbrown.rlib --crate-type lib
+	$(RUSTC) mismatch.rs --extern uses_hashbrown=$(TMPDIR)/libuses_hashbrown.rlib 2>$(TMPDIR)/stderr.txt || true
+	sed -e "$$SED_REGEX" < $(TMPDIR)/stderr.txt > $(TMPDIR)/normalized.txt
+	$(RUSTC_TEST_OP) $(TMPDIR)/normalized.txt expected.txt
+	$(CGREP) "loaded from" < $(TMPDIR)/stderr.txt

tests/run-make/type-mismatch-sysroot-crate/expected.txt

@@ -0,0 +1,28 @@
+error[E0308]: mismatched types
+ --> mismatch.rs:5:25
+  |
+5 |     uses_hashbrown::foo(hashbrown::HashMap::default())
+  |     ------------------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `HashMap`, found `HashMap<_, _, _, _>`
+  |     |
+  |     arguments to this function are incorrect
+  |
+  = note: `hashbrown::HashMap<_, _, _, _>` and `hashbrown::HashMap` have similar names, but are actually distinct types
+note: `hashbrown::HashMap<_, _, _, _>` is defined in crate `hashbrown`
+ --> /rust/deps/hashbrown-VERSION/src/map.rs:190:1
+  = note: `hashbrown` was loaded from BUILD_DIR/HOST/stageN/lib/rustlib/TARGET/lib/libhashbrown.rlib
+note: `hashbrown::HashMap` is defined in crate `hashbrown`
+ --> TEST_DIR/type-mismatch-sysroot-crate/hashbrown.rs:1:1
+  |
+1 | pub struct HashMap;
+  | ^^^^^^^^^^^^^^^^^^
+  = note: `hashbrown` was loaded from BUILD_DIR/HOST/test/run-make/type-mismatch-sysroot-crate/type-mismatch-sysroot-crate/libhashbrown.rlib
+  = note: perhaps two different versions of crate `hashbrown` are being used?
+note: function defined here
+ --> TEST_DIR/type-mismatch-sysroot-crate/uses-hashbrown.rs:1:8
+  |
+1 | pub fn foo(_: hashbrown::HashMap) {}
+  |        ^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/run-make/type-mismatch-sysroot-crate/hashbrown.rs

@@ -0,0 +1 @@
+pub struct HashMap;

tests/run-make/type-mismatch-sysroot-crate/mismatch.rs

@@ -0,0 +1,6 @@
+#![feature(rustc_private)]
+extern crate hashbrown;
+
+fn main() {
+    uses_hashbrown::foo(hashbrown::HashMap::default())
+}

tests/run-make/type-mismatch-sysroot-crate/uses-hashbrown.rs

@@ -0,0 +1 @@
+pub fn foo(_: hashbrown::HashMap) {}