Fix post-monomorphization error note race in the parallel frontend

compiler/rustc_errors/src/lib.rs

@@ -461,6 +461,19 @@ impl Drop for DiagCtxtInner {
     }
 }
 
+thread_local! {
+    /// Number of errors emitted on the current thread.
+    ///
+    /// The shared error count ([`DiagCtxt::err_count`]) is bumped by every
+    /// thread, so a before/after delta taken around one unit of work can be
+    /// polluted by errors emitted concurrently on other threads. The
+    /// monomorphization collector relies on such a delta to attribute
+    /// post-monomorphization errors to the right item, and got confused when
+    /// collecting in parallel (see #154260). This per-thread count lets it take
+    /// a delta that is not affected by other threads.
+    static THREAD_ERR_COUNT: Cell<usize> = const { Cell::new(0) };
+}
+
 impl DiagCtxt {
     pub fn disable_warnings(mut self) -> Self {
         self.inner.get_mut().flags.can_emit_warnings = false;
@@ -714,6 +727,19 @@ impl<'a> DiagCtxtHandle<'a> {
                 .sum::<usize>()
     }
 
+    /// The number of errors that have been emitted on the *current thread*.
+    ///
+    /// Unlike [`DiagCtxtHandle::err_count`], which counts errors emitted by all
+    /// threads, this only counts errors emitted by the calling thread. It is
+    /// meant for code that takes a before/after delta to detect whether a unit
+    /// of work emitted an error, and must not be misled by errors emitted on
+    /// other threads at the same time (#154260). In serial compilation it is
+    /// equivalent to taking a delta of `err_count`.
+    #[inline]
+    pub fn err_count_on_current_thread(&self) -> usize {
+        THREAD_ERR_COUNT.with(Cell::get)
+    }
+
     /// This excludes lint errors and delayed bugs. Unless absolutely
     /// necessary, prefer `has_errors` to this method.
     pub fn has_errors_excluding_lint_errors(&self) -> Option<ErrorGuaranteed> {
@@ -1333,6 +1359,12 @@ impl DiagCtxtInner {
             }
 
             if is_error {
+                // Mirror the bump of the shared error count (`err_guars` /
+                // `lint_err_guars` below) on a per-thread counter, so a single
+                // thread can tell whether *it* emitted an error without being
+                // affected by other threads (see `err_count_on_current_thread`).
+                THREAD_ERR_COUNT.with(|c| c.set(c.get() + 1));
+
                 // If we have any delayed bugs recorded, we can discard them
                 // because they won't be used. (This should only occur if there
                 // have been no errors previously emitted, because we don't add

compiler/rustc_monomorphize/src/collector.rs

@@ -412,8 +412,14 @@ fn collect_items_rec<'tcx>(
     // error count. If it has changed, a PME occurred, and we trigger some diagnostics about the
     // current step of mono items collection.
     //
+    // We use the per-thread error count rather than the global one: the mono item graph is walked
+    // in parallel (see the `par_for_each_in` in `collect_crate_mono_items`), so the global count
+    // can be bumped by an unrelated error emitted on another thread between the two reads, which
+    // would then wrongly blame this item (#154260). The per-thread count is not affected by errors
+    // emitted on other threads, so the delta reflects work done on this thread.
+    //
     // FIXME: don't rely on global state, instead bubble up errors. Note: this is very hard to do.
-    let error_count = tcx.dcx().err_count();
+    let error_count = tcx.dcx().err_count_on_current_thread();
 
     // In `mentioned_items` we collect items that were mentioned in this MIR but possibly do not
     // need to be monomorphized. This is done to ensure that optimizing away function calls does not
@@ -538,7 +544,7 @@ fn collect_items_rec<'tcx>(
 
     // Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
     // mono item graph.
-    if tcx.dcx().err_count() > error_count
+    if tcx.dcx().err_count_on_current_thread() > error_count
         && starting_item.node.is_generic_fn()
         && starting_item.node.is_user_defined()
     {

tests/ui/abi/simd-abi-checks-avx.rs

@@ -1,7 +1,6 @@
 //@ only-x86_64
 //@ build-fail
 //@ compile-flags: -C target-feature=-avx
-//@ ignore-parallel-frontend post-monomorphization errors
 #![feature(portable_simd)]
 #![feature(simd_ffi)]
 #![allow(improper_ctypes_definitions)]

tests/ui/abi/simd-abi-checks-avx.stderr

@@ -1,93 +1,93 @@
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:59:11
+  --> $DIR/simd-abi-checks-avx.rs:58:11
    |
 LL |         f(g());
    |           ^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:59:9
+  --> $DIR/simd-abi-checks-avx.rs:58:9
    |
 LL |         f(g());
    |         ^^^^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:65:14
+  --> $DIR/simd-abi-checks-avx.rs:64:14
    |
 LL |         gavx(favx());
    |              ^^^^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:65:9
+  --> $DIR/simd-abi-checks-avx.rs:64:9
    |
 LL |         gavx(favx());
    |         ^^^^^^^^^^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:75:19
+  --> $DIR/simd-abi-checks-avx.rs:74:19
    |
 LL |         w(Wrapper(g()));
    |                   ^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `Wrapper` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:75:9
+  --> $DIR/simd-abi-checks-avx.rs:74:9
    |
 LL |         w(Wrapper(g()));
    |         ^^^^^^^^^^^^^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:89:9
+  --> $DIR/simd-abi-checks-avx.rs:88:9
    |
 LL |         some_extern();
    |         ^^^^^^^^^^^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function definition uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled
-  --> $DIR/simd-abi-checks-avx.rs:24:1
+  --> $DIR/simd-abi-checks-avx.rs:23:1
    |
 LL | unsafe extern "C" fn g() -> __m256 {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function defined here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function definition uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled
-  --> $DIR/simd-abi-checks-avx.rs:19:1
+  --> $DIR/simd-abi-checks-avx.rs:18:1
    |
 LL | unsafe extern "C" fn f(_: __m256) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function defined here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function definition uses SIMD vector type `Wrapper` which (with the chosen ABI) requires the `avx` target feature, which is not enabled
-  --> $DIR/simd-abi-checks-avx.rs:14:1
+  --> $DIR/simd-abi-checks-avx.rs:13:1
    |
 LL | unsafe extern "C" fn w(_: Wrapper) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function defined here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 error: this function call uses SIMD vector type `std::arch::x86_64::__m256` which (with the chosen ABI) requires the `avx` target feature, which is not enabled in the caller
-  --> $DIR/simd-abi-checks-avx.rs:53:8
+  --> $DIR/simd-abi-checks-avx.rs:52:8
    |
 LL |     || g()
    |        ^^^ function called here
    |
    = help: consider enabling it globally (`-C target-feature=+avx`) or locally (`#[target_feature(enable="avx")]`)
 
 note: the above error was encountered while instantiating `fn in_closure::{closure#0}`
-  --> $DIR/simd-abi-checks-avx.rs:81:9
+  --> $DIR/simd-abi-checks-avx.rs:80:9
    |
 LL |         in_closure()();
    |         ^^^^^^^^^^^^^^

tests/ui/consts/const-eval/index-out-of-bounds-never-type.rs

@@ -1,5 +1,4 @@
 //@ build-fail
-//@ ignore-parallel-frontend post-monomorphization errors
 // Regression test for #66975
 #![warn(unconditional_panic)]
 #![feature(never_type)]

tests/ui/consts/const-eval/index-out-of-bounds-never-type.stderr

@@ -1,17 +1,17 @@
 error[E0080]: index out of bounds: the length is 0 but the index is 0
-  --> $DIR/index-out-of-bounds-never-type.rs:10:61
+  --> $DIR/index-out-of-bounds-never-type.rs:9:61
    |
 LL |     const VOID: ! = { let x = 0 * std::mem::size_of::<T>(); [][x] };
    |                                                             ^^^^^ evaluation of `PrintName::<()>::VOID` failed here
 
 note: erroneous constant encountered
-  --> $DIR/index-out-of-bounds-never-type.rs:16:13
+  --> $DIR/index-out-of-bounds-never-type.rs:15:13
    |
 LL |     let _ = PrintName::<T>::VOID;
    |             ^^^^^^^^^^^^^^^^^^^^
 
 note: the above error was encountered while instantiating `fn f::<()>`
-  --> $DIR/index-out-of-bounds-never-type.rs:20:5
+  --> $DIR/index-out-of-bounds-never-type.rs:19:5
    |
 LL |     f::<()>();
    |     ^^^^^^^^^

tests/ui/consts/const-eval/issue-50814-2.mir-opt.stderr

@@ -1,55 +1,55 @@
 error[E0080]: index out of bounds: the length is 3 but the index is 42
-  --> $DIR/issue-50814-2.rs:17:24
+  --> $DIR/issue-50814-2.rs:16:24
    |
 LL |     const BAR: usize = [5, 6, 7][T::BOO];
    |                        ^^^^^^^^^^^^^^^^^ evaluation of `<A<()> as Foo<()>>::BAR` failed here
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:5
+  --> $DIR/issue-50814-2.rs:20:5
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:5
+  --> $DIR/issue-50814-2.rs:20:5
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:5
+  --> $DIR/issue-50814-2.rs:20:5
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:5
+  --> $DIR/issue-50814-2.rs:20:5
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^

tests/ui/consts/const-eval/issue-50814-2.normal.stderr

@@ -1,31 +1,31 @@
 error[E0080]: index out of bounds: the length is 3 but the index is 42
-  --> $DIR/issue-50814-2.rs:17:24
+  --> $DIR/issue-50814-2.rs:16:24
    |
 LL |     const BAR: usize = [5, 6, 7][T::BOO];
    |                        ^^^^^^^^^^^^^^^^^ evaluation of `<A<()> as Foo<()>>::BAR` failed here
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:5
+  --> $DIR/issue-50814-2.rs:20:5
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814-2.rs:21:6
+  --> $DIR/issue-50814-2.rs:20:6
    |
 LL |     &<A<T> as Foo<T>>::BAR
    |      ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: the above error was encountered while instantiating `fn foo::<()>`
-  --> $DIR/issue-50814-2.rs:33:22
+  --> $DIR/issue-50814-2.rs:32:22
    |
 LL |     println!("{:x}", foo::<()>() as *const usize as usize);
    |                      ^^^^^^^^^^^

tests/ui/consts/const-eval/issue-50814-2.rs

@@ -2,7 +2,6 @@
 //@ revisions: normal mir-opt
 //@ [mir-opt]compile-flags: -Zmir-opt-level=4
 //@ dont-require-annotations: NOTE
-//@ ignore-parallel-frontend post-monomorphization errors
 trait C {
     const BOO: usize;
 }

tests/ui/consts/const-eval/issue-50814.rs

@@ -1,6 +1,5 @@
 //@ build-fail
 //@ dont-require-annotations: NOTE
-//@ ignore-parallel-frontend post-monomorphization errors
 trait Unsigned {
     const MAX: u8;
 }

tests/ui/consts/const-eval/issue-50814.stderr

@@ -1,47 +1,47 @@
 error[E0080]: attempt to compute `u8::MAX + u8::MAX`, which would overflow
-  --> $DIR/issue-50814.rs:16:21
+  --> $DIR/issue-50814.rs:15:21
    |
 LL |     const MAX: u8 = A::MAX + B::MAX;
    |                     ^^^^^^^^^^^^^^^ evaluation of `<Sum<U8, U8> as Unsigned>::MAX` failed here
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814.rs:22:6
+  --> $DIR/issue-50814.rs:21:6
    |
 LL |     &Sum::<U8, U8>::MAX
    |      ^^^^^^^^^^^^^^^^^^
 
 error[E0080]: attempt to compute `u8::MAX + u8::MAX`, which would overflow
-  --> $DIR/issue-50814.rs:16:21
+  --> $DIR/issue-50814.rs:15:21
    |
 LL |     const MAX: u8 = A::MAX + B::MAX;
    |                     ^^^^^^^^^^^^^^^ evaluation of `<Sum<U8, U8> as Unsigned>::MAX` failed here
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814.rs:22:6
+  --> $DIR/issue-50814.rs:21:6
    |
 LL |     &Sum::<U8, U8>::MAX
    |      ^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814.rs:22:5
+  --> $DIR/issue-50814.rs:21:5
    |
 LL |     &Sum::<U8, U8>::MAX
    |     ^^^^^^^^^^^^^^^^^^^
 
 note: erroneous constant encountered
-  --> $DIR/issue-50814.rs:22:6
+  --> $DIR/issue-50814.rs:21:6
    |
 LL |     &Sum::<U8, U8>::MAX
    |      ^^^^^^^^^^^^^^^^^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 note: the above error was encountered while instantiating `fn foo::<i32>`
-  --> $DIR/issue-50814.rs:27:5
+  --> $DIR/issue-50814.rs:26:5
    |
 LL |     foo(0);
    |     ^^^^^^

tests/ui/consts/mono-reachable-invalid-const.rs

@@ -1,5 +1,4 @@
 //@ build-fail
-//@ ignore-parallel-frontend post-monomorphization errors
 struct Bar<const BITS: usize>;
 
 impl<const BITS: usize> Bar<BITS> {