clarify and unify 'transient mutable borrow' errors

RalfJung · RalfJung · commit 1e43cc2eee9b · 2025-06-18T11:16:17.000+09:00
diff --git a/compiler/rustc_const_eval/messages.ftl b/compiler/rustc_const_eval/messages.ftl
@@ -124,12 +124,13 @@ const_eval_incompatible_return_types =
 const_eval_incompatible_types =
     calling a function with argument of type {$callee_ty} passing data of type {$caller_ty}
 
-const_eval_interior_mutable_ref_escaping =
-    {const_eval_const_context}s cannot refer to interior mutable data
-    .label = this borrow of an interior mutable value may end up in the final value
+const_eval_interior_mutable_borrow_escaping =
+    shared borrows of lifetime-extended temporaries with interior mutability are not allowed in {const_eval_const_context}s
+    .label = this borrow of an interior mutable value refers to a lifetime-extended temporary
     .help = to fix this, the value can be extracted to a separate `static` item and then referenced
     .teach_note =
-        References that escape into the final value of a constant or static must be immutable.
+        This creates a raw pointer to a temporary that has its lifetime extended to last for the entire program.
+        Lifetime-extended temporaries in constants and statics must be immutable.
         This is to avoid accidentally creating shared mutable state.
 
 
@@ -215,19 +216,11 @@ const_eval_modified_global =
 
 const_eval_mutable_ptr_in_final = encountered mutable pointer in final value of {const_eval_intern_kind}
 
-const_eval_mutable_raw_escaping =
-    raw mutable pointers are not allowed in the final value of {const_eval_const_context}s
+const_eval_mutable_borrow_escaping =
+    mutable borrows of lifetime-extended temporaries are not allowed in {const_eval_const_context}s
     .teach_note =
-        Pointers that escape into the final value of a constant or static must be immutable.
-        This is to avoid accidentally creating shared mutable state.
-
-
-        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
-
-const_eval_mutable_ref_escaping =
-    mutable references are not allowed in the final value of {const_eval_const_context}s
-    .teach_note =
-        References that escape into the final value of a constant or static must be immutable.
+        This creates a reference to a temporary that has its lifetime extended to last for the entire program.
+        Lifetime-extended temporaries in constants and statics must be immutable.
         This is to avoid accidentally creating shared mutable state.
 
 
diff --git a/compiler/rustc_const_eval/src/check_consts/check.rs b/compiler/rustc_const_eval/src/check_consts/check.rs
@@ -602,11 +602,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut);
 
                 if !is_allowed && self.place_may_escape(place) {
-                    self.check_op(ops::EscapingMutBorrow(if matches!(rvalue, Rvalue::Ref(..)) {
-                        hir::BorrowKind::Ref
-                    } else {
-                        hir::BorrowKind::Raw
-                    }));
+                    self.check_op(ops::EscapingMutBorrow);
                 }
             }
 
diff --git a/compiler/rustc_const_eval/src/check_consts/ops.rs b/compiler/rustc_const_eval/src/check_consts/ops.rs
@@ -583,7 +583,7 @@ impl<'tcx> NonConstOp<'tcx> for EscapingCellBorrow {
         DiagImportance::Secondary
     }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::InteriorMutableRefEscaping {
+        ccx.dcx().create_err(errors::InteriorMutableBorrowEscaping {
             span,
             opt_help: matches!(ccx.const_kind(), hir::ConstContext::Static(_)),
             kind: ccx.const_kind(),
@@ -596,7 +596,7 @@ impl<'tcx> NonConstOp<'tcx> for EscapingCellBorrow {
 /// This op is for `&mut` borrows in the trailing expression of a constant
 /// which uses the "enclosing scopes rule" to leak its locals into anonymous
 /// static or const items.
-pub(crate) struct EscapingMutBorrow(pub hir::BorrowKind);
+pub(crate) struct EscapingMutBorrow;
 
 impl<'tcx> NonConstOp<'tcx> for EscapingMutBorrow {
     fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
@@ -610,18 +610,11 @@ impl<'tcx> NonConstOp<'tcx> for EscapingMutBorrow {
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        match self.0 {
-            hir::BorrowKind::Raw => ccx.tcx.dcx().create_err(errors::MutableRawEscaping {
-                span,
-                kind: ccx.const_kind(),
-                teach: ccx.tcx.sess.teach(E0764),
-            }),
-            hir::BorrowKind::Ref => ccx.dcx().create_err(errors::MutableRefEscaping {
-                span,
-                kind: ccx.const_kind(),
-                teach: ccx.tcx.sess.teach(E0764),
-            }),
-        }
+        ccx.dcx().create_err(errors::MutableBorrowEscaping {
+            span,
+            kind: ccx.const_kind(),
+            teach: ccx.tcx.sess.teach(E0764),
+        })
     }
 }
 
diff --git a/compiler/rustc_const_eval/src/errors.rs b/compiler/rustc_const_eval/src/errors.rs
@@ -152,24 +152,15 @@ pub(crate) struct UnmarkedIntrinsicExposed {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_mutable_ref_escaping, code = E0764)]
-pub(crate) struct MutableRefEscaping {
+#[diag(const_eval_mutable_borrow_escaping, code = E0764)]
+pub(crate) struct MutableBorrowEscaping {
     #[primary_span]
     pub span: Span,
     pub kind: ConstContext,
     #[note(const_eval_teach_note)]
     pub teach: bool,
 }
 
-#[derive(Diagnostic)]
-#[diag(const_eval_mutable_raw_escaping, code = E0764)]
-pub(crate) struct MutableRawEscaping {
-    #[primary_span]
-    pub span: Span,
-    pub kind: ConstContext,
-    #[note(const_eval_teach_note)]
-    pub teach: bool,
-}
 #[derive(Diagnostic)]
 #[diag(const_eval_non_const_fmt_macro_call, code = E0015)]
 pub(crate) struct NonConstFmtMacroCall {
@@ -227,8 +218,8 @@ pub(crate) struct UnallowedInlineAsm {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_interior_mutable_ref_escaping, code = E0492)]
-pub(crate) struct InteriorMutableRefEscaping {
+#[diag(const_eval_interior_mutable_borrow_escaping, code = E0492)]
+pub(crate) struct InteriorMutableBorrowEscaping {
     #[primary_span]
     #[label]
     pub span: Span,
diff --git a/tests/ui/consts/const-mut-refs/issue-76510.rs b/tests/ui/consts/const-mut-refs/issue-76510.rs
@@ -1,7 +1,7 @@
 use std::mem::{transmute, ManuallyDrop};
 
 const S: &'static mut str = &mut " hello ";
-//~^ ERROR: mutable references are not allowed in the final value of constants
+//~^ ERROR: mutable borrows of lifetime-extended temporaries
 
 const fn trigger() -> [(); unsafe {
         let s = transmute::<(*const u8, usize), &ManuallyDrop<str>>((S.as_ptr(), 3));
diff --git a/tests/ui/consts/const-mut-refs/issue-76510.stderr b/tests/ui/consts/const-mut-refs/issue-76510.stderr
@@ -1,4 +1,4 @@
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants
   --> $DIR/issue-76510.rs:3:29
    |
 LL | const S: &'static mut str = &mut " hello ";
diff --git a/tests/ui/consts/const-mut-refs/mut_ref_in_final.rs b/tests/ui/consts/const-mut-refs/mut_ref_in_final.rs
@@ -12,7 +12,7 @@ const A: *const i32 = &4;
 // It could be made sound to allow it to compile,
 // but we do not want to allow this to compile,
 // as that would be an enormous footgun in oli-obk's opinion.
-const B: *mut i32 = &mut 4; //~ ERROR mutable references are not allowed
+const B: *mut i32 = &mut 4; //~ ERROR mutable borrows of lifetime-extended temporaries
 
 // Ok, no actual mutable allocation exists
 const B2: Option<&mut i32> = None;
@@ -29,7 +29,7 @@ const IMMUT_MUT_REF: &mut u16 = unsafe { mem::transmute(&13) };
 static IMMUT_MUT_REF_STATIC: &mut u16 = unsafe { mem::transmute(&13) };
 //~^ ERROR pointing to read-only memory
 
-// Ok, because no references to mutable data exist here, since the `{}` moves
+// Ok, because no borrows of mutable data exist here, since the `{}` moves
 // its value and then takes a reference to that.
 const C: *const i32 = &{
     let mut x = 42;
@@ -69,13 +69,13 @@ unsafe impl<T> Sync for SyncPtr<T> {}
 // (This relies on `SyncPtr` being a curly brace struct.)
 // However, we intern the inner memory as read-only, so this must be rejected.
 static RAW_MUT_CAST_S: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable borrows of lifetime-extended temporaries
 static RAW_MUT_COERCE_S: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable borrows of lifetime-extended temporaries
 const RAW_MUT_CAST_C: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable borrows of lifetime-extended temporaries
 const RAW_MUT_COERCE_C: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable borrows of lifetime-extended temporaries
 
 fn main() {
     println!("{}", unsafe { *A });
diff --git a/tests/ui/consts/const-mut-refs/mut_ref_in_final.stderr b/tests/ui/consts/const-mut-refs/mut_ref_in_final.stderr
@@ -1,4 +1,4 @@
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants
   --> $DIR/mut_ref_in_final.rs:15:21
    |
 LL | const B: *mut i32 = &mut 4;
@@ -76,25 +76,25 @@ LL | static mut FOO3: NotAMutex<&mut i32> = NotAMutex(UnsafeCell::new(&mut 42));
    |                                        |                              creates a temporary value which is freed while still in use
    |                                        using this value as a static requires that borrow lasts for `'static`
 
-error[E0764]: mutable references are not allowed in the final value of statics
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in statics
   --> $DIR/mut_ref_in_final.rs:71:53
    |
 LL | static RAW_MUT_CAST_S: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
    |                                                     ^^^^^^^
 
-error[E0764]: mutable references are not allowed in the final value of statics
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in statics
   --> $DIR/mut_ref_in_final.rs:73:54
    |
 LL | static RAW_MUT_COERCE_S: SyncPtr<i32> = SyncPtr { x: &mut 0 };
    |                                                      ^^^^^^
 
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants
   --> $DIR/mut_ref_in_final.rs:75:52
    |
 LL | const RAW_MUT_CAST_C: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
    |                                                    ^^^^^^^
 
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants
   --> $DIR/mut_ref_in_final.rs:77:53
    |
 LL | const RAW_MUT_COERCE_C: SyncPtr<i32> = SyncPtr { x: &mut 0 };
diff --git a/tests/ui/consts/const-promoted-opaque.atomic.stderr b/tests/ui/consts/const-promoted-opaque.atomic.stderr
@@ -7,11 +7,11 @@ LL |
 LL | };
    | - value is dropped here
 
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/const-promoted-opaque.rs:36:19
    |
 LL | const BAZ: &Foo = &FOO;
-   |                   ^^^^ this borrow of an interior mutable value may end up in the final value
+   |                   ^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/const-promoted-opaque.rs:40:26
diff --git a/tests/ui/consts/const-promoted-opaque.rs b/tests/ui/consts/const-promoted-opaque.rs
@@ -34,7 +34,7 @@ const BAR: () = {
 };
 
 const BAZ: &Foo = &FOO;
-//[atomic]~^ ERROR: constants cannot refer to interior mutable data
+//[atomic]~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 
 fn main() {
     let _: &'static _ = &FOO;
diff --git a/tests/ui/consts/issue-17718-const-bad-values.rs b/tests/ui/consts/issue-17718-const-bad-values.rs
@@ -5,7 +5,7 @@
 #![allow(static_mut_refs)]
 
 const C1: &'static mut [usize] = &mut [];
-//~^ ERROR: mutable references are not allowed
+//~^ ERROR: mutable borrows of lifetime-extended temporaries
 
 static mut S: i32 = 3;
 const C2: &'static mut i32 = unsafe { &mut S };
diff --git a/tests/ui/consts/issue-17718-const-bad-values.stderr b/tests/ui/consts/issue-17718-const-bad-values.stderr
@@ -1,4 +1,4 @@
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants
   --> $DIR/issue-17718-const-bad-values.rs:7:34
    |
 LL | const C1: &'static mut [usize] = &mut [];
diff --git a/tests/ui/consts/issue-17718-const-borrow.rs b/tests/ui/consts/issue-17718-const-borrow.rs
@@ -2,13 +2,13 @@ use std::cell::UnsafeCell;
 
 const A: UnsafeCell<usize> = UnsafeCell::new(1);
 const B: &'static UnsafeCell<usize> = &A;
-//~^ ERROR: cannot refer to interior mutable
+//~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 
 struct C { a: UnsafeCell<usize> }
 const D: C = C { a: UnsafeCell::new(1) };
 const E: &'static UnsafeCell<usize> = &D.a;
-//~^ ERROR: cannot refer to interior mutable
+//~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 const F: &'static C = &D;
-//~^ ERROR: cannot refer to interior mutable
+//~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 
 fn main() {}
diff --git a/tests/ui/consts/issue-17718-const-borrow.stderr b/tests/ui/consts/issue-17718-const-borrow.stderr
@@ -1,20 +1,20 @@
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/issue-17718-const-borrow.rs:4:39
    |
 LL | const B: &'static UnsafeCell<usize> = &A;
-   |                                       ^^ this borrow of an interior mutable value may end up in the final value
+   |                                       ^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/issue-17718-const-borrow.rs:9:39
    |
 LL | const E: &'static UnsafeCell<usize> = &D.a;
-   |                                       ^^^^ this borrow of an interior mutable value may end up in the final value
+   |                                       ^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/issue-17718-const-borrow.rs:11:23
    |
 LL | const F: &'static C = &D;
-   |                       ^^ this borrow of an interior mutable value may end up in the final value
+   |                       ^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/consts/partial_qualif.rs b/tests/ui/consts/partial_qualif.rs
@@ -3,7 +3,7 @@ use std::cell::Cell;
 const FOO: &(Cell<usize>, bool) = {
     let mut a = (Cell::new(0), false);
     a.1 = true; // sets `qualif(a)` to `qualif(a) | qualif(true)`
-    &{a} //~ ERROR cannot refer to interior mutable
+    &{a} //~ ERROR shared borrows of lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}
diff --git a/tests/ui/consts/partial_qualif.stderr b/tests/ui/consts/partial_qualif.stderr
@@ -1,8 +1,8 @@
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/partial_qualif.rs:6:5
    |
 LL |     &{a}
-   |     ^^^^ this borrow of an interior mutable value may end up in the final value
+   |     ^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/qualif_overwrite.rs b/tests/ui/consts/qualif_overwrite.rs
@@ -7,7 +7,7 @@ use std::cell::Cell;
 const FOO: &Option<Cell<usize>> = {
     let mut a = Some(Cell::new(0));
     a = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
-    &{a} //~ ERROR cannot refer to interior mutable
+    &{a} //~ ERROR shared borrows of lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}
diff --git a/tests/ui/consts/qualif_overwrite.stderr b/tests/ui/consts/qualif_overwrite.stderr
@@ -1,8 +1,8 @@
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/qualif_overwrite.rs:10:5
    |
 LL |     &{a}
-   |     ^^^^ this borrow of an interior mutable value may end up in the final value
+   |     ^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/qualif_overwrite_2.rs b/tests/ui/consts/qualif_overwrite_2.rs
@@ -5,7 +5,7 @@ use std::cell::Cell;
 const FOO: &Option<Cell<usize>> = {
     let mut a = (Some(Cell::new(0)),);
     a.0 = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
-    &{a.0} //~ ERROR cannot refer to interior mutable
+    &{a.0} //~ ERROR shared borrows of lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}
diff --git a/tests/ui/consts/qualif_overwrite_2.stderr b/tests/ui/consts/qualif_overwrite_2.stderr
@@ -1,8 +1,8 @@
-error[E0492]: constants cannot refer to interior mutable data
+error[E0492]: shared borrows of lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/qualif_overwrite_2.rs:8:5
    |
 LL |     &{a.0}
-   |     ^^^^^^ this borrow of an interior mutable value may end up in the final value
+   |     ^^^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/refs-to-cell-in-final.rs b/tests/ui/consts/refs-to-cell-in-final.rs
@@ -11,9 +11,9 @@ unsafe impl<T> Sync for SyncPtr<T> {}
 // The resulting constant would pass all validation checks, so it is crucial that this gets rejected
 // by static const checks!
 static RAW_SYNC_S: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
-//~^ ERROR: cannot refer to interior mutable data
+//~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
-//~^ ERROR: cannot refer to interior mutable data
+//~^ ERROR: shared borrows of lifetime-extended temporaries with interior mutability
 
 // This one does not get promoted because of `Drop`, and then enters interesting codepaths because
 // as a value it has no interior mutability, but as a type it does. See
@@ -39,7 +39,7 @@ const NONE_EXPLICIT_PROMOTED: &'static Option<Cell<i32>> = {
 
 // Not okay, since we are borrowing something with interior mutability.
 const INTERIOR_MUT_VARIANT: &Option<UnsafeCell<bool>> = &{
-    //~^ERROR: cannot refer to interior mutable data
+    //~^ERROR: shared borrows of lifetime-extended temporaries with interior mutability
     let mut x = None;
     assert!(x.is_none());
     x = Some(UnsafeCell::new(false));
diff --git a/tests/ui/consts/refs-to-cell-in-final.stderr b/tests/ui/consts/refs-to-cell-in-final.stderr
diff --git a/tests/ui/consts/write_to_static_via_mut_ref.rs b/tests/ui/consts/write_to_static_via_mut_ref.rs
diff --git a/tests/ui/consts/write_to_static_via_mut_ref.stderr b/tests/ui/consts/write_to_static_via_mut_ref.stderr
diff --git a/tests/ui/error-codes/E0017.rs b/tests/ui/error-codes/E0017.rs
diff --git a/tests/ui/error-codes/E0017.stderr b/tests/ui/error-codes/E0017.stderr
diff --git a/tests/ui/error-codes/E0492.stderr b/tests/ui/error-codes/E0492.stderr
diff --git a/tests/ui/issues/issue-46604.rs b/tests/ui/issues/issue-46604.rs
diff --git a/tests/ui/issues/issue-46604.stderr b/tests/ui/issues/issue-46604.stderr
diff --git a/tests/ui/statics/check-immutable-mut-slices.rs b/tests/ui/statics/check-immutable-mut-slices.rs
diff --git a/tests/ui/statics/check-immutable-mut-slices.stderr b/tests/ui/statics/check-immutable-mut-slices.stderr

Original file line number	Diff line number	Diff line change
`@@ -602,11 +602,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {`
`602`	`602`	`self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut);`
`603`	`603`
`604`	`604`	`if !is_allowed && self.place_may_escape(place) {`
`605`		`- self.check_op(ops::EscapingMutBorrow(if matches!(rvalue, Rvalue::Ref(..)) {`
`606`		`- hir::BorrowKind::Ref`
`607`		`- } else {`
`608`		`- hir::BorrowKind::Raw`
`609`		`- }));`
	`605`	`+ self.check_op(ops::EscapingMutBorrow);`
`610`	`606`	`}`
`611`	`607`	`}`
`612`	`608`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-error[E0764]: mutable references are not allowed in the final value of constants`
	`1`	`+error[E0764]: mutable borrows of lifetime-extended temporaries are not allowed in constants`
`2`	`2`	`--> $DIR/issue-76510.rs:3:29`
`3`	`3`	`\|`
`4`	`4`	`LL \| const S: &'static mut str = &mut " hello ";`