No unleashing of legal code

src/librustc_mir/transform/check_consts/validation.rs

@@ -247,11 +247,17 @@ impl Validator<'mir, 'tcx> {
             return;
         }
 
-        // If an operation is supported in miri (and is not already controlled by a feature gate) it
+        // If an operation is supported in miri it
         // can be turned on with `-Zunleash-the-miri-inside-of-you`.
-        let is_unleashable = O::IS_SUPPORTED_IN_MIRI && O::feature_gate().is_none();
+        let is_unleashable = O::IS_SUPPORTED_IN_MIRI;
 
         if is_unleashable && self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
+            // If all skipped operations could also be turned on by a feature gate, then we want
+            // to emit an error. So when we see a non-feature gate operation being skipped, disable
+            // that error reporting.
+            if O::feature_gate().is_none() {
+                self.tcx.sess.register_non_feature_unleash_skip();
+            }
             self.tcx.sess.span_warn(span, "skipping const checks");
             return;
         }

src/librustc_session/session.rs

@@ -142,6 +142,10 @@ pub struct Session {
     /// and immediately printing the backtrace to stderr.
     pub ctfe_backtrace: Lock<CtfeBacktrace>,
 
+    /// This is used to ensure that not all "skipping const checks" from
+    /// `-Zunleash-the-miri-inside-of-you` are because of feature gates being skipped.
+    seen_non_feature_skip: Lock<bool>,
+
     /// Base directory containing the `src/` for the Rust standard library, and
     /// potentially `rustc` as well, if we can can find it. Right now it's always
     /// `$sysroot/lib/rustlib/src/rust` (i.e. the `rustup` `rust-src` component).
@@ -188,7 +192,29 @@ impl From<&'static lint::Lint> for DiagnosticMessageId {
     }
 }
 
+impl Drop for Session {
+    fn drop(&mut self) {
+        // We've only seen feature gate related skips or no skips at all
+        if !*self.seen_non_feature_skip.get_mut() {
+            // And we have unleash enabled.
+            if self.opts.debugging_opts.unleash_the_miri_inside_of_you {
+                // Then that unleash is not necessary
+                panic!(
+                    "`-Zunleash-the-miri-inside-of-you` unnecessary because it only unleashes
+                    feature gates that could have been enabled via attributes.
+                    Remove `-Zunleash-the-miri-inside-of-you` in order to let rustc tell you which
+                    features you need.",
+                );
+            }
+        }
+    }
+}
+
 impl Session {
+    pub fn register_non_feature_unleash_skip(&self) {
+        *self.seen_non_feature_skip.lock() = true;
+    }
+
     pub fn local_crate_disambiguator(&self) -> CrateDisambiguator {
         *self.crate_disambiguator.get()
     }
@@ -1081,6 +1107,8 @@ pub fn build_session_with_source_map(
         _ => CtfeBacktrace::Disabled,
     });
 
+    let seen_non_feature_skip = Lock::new(false);
+
     // Try to find a directory containing the Rust `src`, for more details see
     // the doc comment on the `real_rust_source_base_dir` field.
     let real_rust_source_base_dir = {
@@ -1139,6 +1167,7 @@ pub fn build_session_with_source_map(
         confused_type_with_std_module: Lock::new(Default::default()),
         system_library_path: OneThread::new(RefCell::new(Default::default())),
         ctfe_backtrace,
+        seen_non_feature_skip,
         real_rust_source_base_dir,
     };
 

src/librustdoc/test.rs

@@ -98,7 +98,7 @@ pub fn run(options: Options) -> i32 {
         externs: options.externs.clone(),
         unstable_features: UnstableFeatures::from_environment(),
         actually_rustdoc: true,
-        debugging_opts: config::DebuggingOptions { ..config::basic_debugging_options() },
+        debugging_opts: config::basic_debugging_options(),
         edition: options.edition,
         target_triple: options.target.clone(),
         ..config::Options::default()

src/test/ui/consts/miri_unleashed/mutating_global.rs

@@ -1,4 +1,3 @@
-// compile-flags: -Zunleash-the-miri-inside-of-you
 #![allow(const_err)]
 
 // Make sure we cannot mutate globals.

src/test/ui/consts/miri_unleashed/mutating_global.stderr

@@ -1,5 +1,5 @@
 error[E0080]: could not evaluate static initializer
-  --> $DIR/mutating_global.rs:10:9
+  --> $DIR/mutating_global.rs:9:9
    |
 LL |         GLOBAL = 99
    |         ^^^^^^^^^^^ modifying a static's initial value from another static's initializer

src/test/ui/consts/miri_unleashed/read_from_static.rs

@@ -1,5 +1,4 @@
 // run-pass
-// compile-flags: -Zunleash-the-miri-inside-of-you
 #![feature(const_mut_refs)]
 #![allow(const_err)]
 

src/test/ui/consts/recursive-zst-static.default.stderr

@@ -1,17 +1,17 @@
 error[E0391]: cycle detected when const-evaluating `FOO`
-  --> $DIR/recursive-zst-static.rs:10:18
+  --> $DIR/recursive-zst-static.rs:9:18
    |
 LL | static FOO: () = FOO;
    |                  ^^^
    |
 note: ...which requires const-evaluating `FOO`...
-  --> $DIR/recursive-zst-static.rs:10:1
+  --> $DIR/recursive-zst-static.rs:9:1
    |
 LL | static FOO: () = FOO;
    | ^^^^^^^^^^^^^^^^^^^^^
    = note: ...which again requires const-evaluating `FOO`, completing the cycle
 note: cycle used when const-evaluating + checking `FOO`
-  --> $DIR/recursive-zst-static.rs:10:1
+  --> $DIR/recursive-zst-static.rs:9:1
    |
 LL | static FOO: () = FOO;
    | ^^^^^^^^^^^^^^^^^^^^^

src/test/ui/consts/recursive-zst-static.rs

@@ -1,5 +1,4 @@
-// revisions: default unleash
-//[unleash]compile-flags: -Zunleash-the-miri-inside-of-you
+// revisions: default
 
 // This test ensures that we do not allow ZST statics to initialize themselves without ever
 // actually creating a value of that type. This is important, as the ZST may have private fields

src/test/ui/mir-dataflow/indirect-mutation-offset.rs

@@ -1,5 +1,3 @@
-// compile-flags: -Zunleash-the-miri-inside-of-you
-
 // This test demonstrates a shortcoming of the `MaybeMutBorrowedLocals` analysis. It does not
 // handle code that takes a reference to one field of a struct, then use pointer arithmetic to
 // transform it to another field of that same struct that may have interior mutability. For now,
@@ -18,14 +16,11 @@ struct PartialInteriorMut {
     cell: UnsafeCell<i32>,
 }
 
-#[rustc_mir(rustc_peek_indirectly_mutable,stop_after_dataflow)]
+#[rustc_mir(rustc_peek_indirectly_mutable, stop_after_dataflow)]
 const BOO: i32 = {
-    let x = PartialInteriorMut {
-        zst: [],
-        cell: UnsafeCell::new(0),
-    };
+    let x = PartialInteriorMut { zst: [], cell: UnsafeCell::new(0) };
 
-    let p_zst: *const _ = &x.zst ; // Doesn't cause `x` to get marked as indirectly mutable.
+    let p_zst: *const _ = &x.zst; // Doesn't cause `x` to get marked as indirectly mutable.
 
     let rmut_cell = unsafe {
         // Take advantage of the fact that `zst` and `cell` are at the same location in memory.
@@ -36,7 +31,7 @@ const BOO: i32 = {
         &mut *pmut_cell
     };
 
-    *rmut_cell = 42;  // Mutates `x` indirectly even though `x` is not marked indirectly mutable!!!
+    *rmut_cell = 42; // Mutates `x` indirectly even though `x` is not marked indirectly mutable!!!
     let val = *rmut_cell;
     unsafe { rustc_peek(x) }; //~ ERROR rustc_peek: bit not set
 

src/test/ui/mir-dataflow/indirect-mutation-offset.stderr

@@ -1,5 +1,5 @@
 error: rustc_peek: bit not set
-  --> $DIR/indirect-mutation-offset.rs:41:14
+  --> $DIR/indirect-mutation-offset.rs:36:14
    |
 LL |     unsafe { rustc_peek(x) };
    |              ^^^^^^^^^^^^^