Rollup merge of #143092 - RalfJung:const-check-lifetime-ext, r=oli-obk

const checks for lifetime-extended temporaries: avoid 'top-level scope' terminology

This error recently got changed in #140942 to use the terminology of "top-level scope", but after further discussion in rust-lang/reference#1865 it seems the reference will not be using that terminology after all. So let's also remove it from the compiler again, and let's focus on what actually happens with these temporaries: their lifetime is extended until the end of the program.

r? `@oli-obk` `@traviscross`

Author: compiler-errors

compiler/rustc_const_eval/messages.ftl

@@ -125,16 +125,11 @@ const_eval_incompatible_types =
     calling a function with argument of type {$callee_ty} passing data of type {$caller_ty}
 
 const_eval_interior_mutable_borrow_escaping =
-    interior mutable shared borrows of lifetime-extended temporaries in the top-level scope of a {const_eval_const_context} are not allowed
-    .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 =
-        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.
-
-
-        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
+    interior mutable shared borrows of temporaries that have their lifetime extended until the end of the program are not allowed
+    .label = this borrow of an interior mutable value refers to such a temporary
+    .note = Temporaries in constants and statics can have their lifetime extended until the end of the program
+    .note2 = To avoid accidentally creating global mutable state, such temporaries must be immutable
+    .help = If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 const_eval_intern_kind = {$kind ->
     [static] static
@@ -215,14 +210,11 @@ const_eval_modified_global =
     modifying a static's initial value from another static's initializer
 
 const_eval_mutable_borrow_escaping =
-    mutable borrows of lifetime-extended temporaries in the top-level scope of a {const_eval_const_context} are not allowed
-    .teach_note =
-        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.
-
-
-        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
+    mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
+    .label = this mutable borrow refers to such a temporary
+    .note = Temporaries in constants and statics can have their lifetime extended until the end of the program
+    .note2 = To avoid accidentally creating global mutable state, such temporaries must be immutable
+    .help = If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 const_eval_mutable_ptr_in_final = encountered mutable pointer in final value of {const_eval_intern_kind}
 

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -567,12 +567,7 @@ impl<'tcx> NonConstOp<'tcx> for EscapingCellBorrow {
         DiagImportance::Secondary
     }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::InteriorMutableBorrowEscaping {
-            span,
-            opt_help: matches!(ccx.const_kind(), hir::ConstContext::Static(_)),
-            kind: ccx.const_kind(),
-            teach: ccx.tcx.sess.teach(E0492),
-        })
+        ccx.dcx().create_err(errors::InteriorMutableBorrowEscaping { span, kind: ccx.const_kind() })
     }
 }
 
@@ -594,11 +589,7 @@ impl<'tcx> NonConstOp<'tcx> for EscapingMutBorrow {
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::MutableBorrowEscaping {
-            span,
-            kind: ccx.const_kind(),
-            teach: ccx.tcx.sess.teach(E0764),
-        })
+        ccx.dcx().create_err(errors::MutableBorrowEscaping { span, kind: ccx.const_kind() })
     }
 }
 

compiler/rustc_const_eval/src/errors.rs

@@ -151,12 +151,14 @@ pub(crate) struct UnmarkedIntrinsicExposed {
 
 #[derive(Diagnostic)]
 #[diag(const_eval_mutable_borrow_escaping, code = E0764)]
+#[note]
+#[note(const_eval_note2)]
+#[help]
 pub(crate) struct MutableBorrowEscaping {
     #[primary_span]
+    #[label]
     pub span: Span,
     pub kind: ConstContext,
-    #[note(const_eval_teach_note)]
-    pub teach: bool,
 }
 
 #[derive(Diagnostic)]
@@ -217,15 +219,14 @@ pub(crate) struct UnallowedInlineAsm {
 
 #[derive(Diagnostic)]
 #[diag(const_eval_interior_mutable_borrow_escaping, code = E0492)]
+#[note]
+#[note(const_eval_note2)]
+#[help]
 pub(crate) struct InteriorMutableBorrowEscaping {
     #[primary_span]
     #[label]
     pub span: Span,
-    #[help]
-    pub opt_help: bool,
     pub kind: ConstContext,
-    #[note(const_eval_teach_note)]
-    pub teach: bool,
 }
 
 #[derive(LintDiagnostic)]

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 borrows of lifetime-extended temporaries
+//~^ ERROR: mutable borrows of temporaries
 
 const fn trigger() -> [(); unsafe {
         let s = transmute::<(*const u8, usize), &ManuallyDrop<str>>((S.as_ptr(), 3));

tests/ui/consts/const-mut-refs/issue-76510.stderr

@@ -1,8 +1,12 @@
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/issue-76510.rs:3:29
    |
 LL | const S: &'static mut str = &mut " hello ";
-   |                             ^^^^^^^^^^^^^^
+   |                             ^^^^^^^^^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 error: aborting due to 1 previous error
 

tests/ui/consts/const-mut-refs/mut_ref_in_final.rs

@@ -12,13 +12,13 @@ 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 borrows of lifetime-extended temporaries
+const B: *mut i32 = &mut 4; //~ ERROR mutable borrows of temporaries
 
 // Ok, no actual mutable allocation exists
 const B2: Option<&mut i32> = None;
 
 // Not ok, can't prove that no mutable allocation ends up in final value
-const B3: Option<&mut i32> = Some(&mut 42); //~ ERROR mutable borrows of lifetime-extended temporaries
+const B3: Option<&mut i32> = Some(&mut 42); //~ ERROR mutable borrows of temporaries
 
 const fn helper(x: &mut i32) -> Option<&mut i32> { Some(x) }
 const B4: Option<&mut i32> = helper(&mut 42); //~ ERROR temporary value dropped while borrowed
@@ -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 borrows of lifetime-extended temporaries
+//~^ ERROR mutable borrows of temporaries
 static RAW_MUT_COERCE_S: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable borrows of lifetime-extended temporaries
+//~^ ERROR mutable borrows of temporaries
 const RAW_MUT_CAST_C: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
-//~^ ERROR mutable borrows of lifetime-extended temporaries
+//~^ ERROR mutable borrows of temporaries
 const RAW_MUT_COERCE_C: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable borrows of lifetime-extended temporaries
+//~^ ERROR mutable borrows of temporaries
 
 fn main() {
     println!("{}", unsafe { *A });

tests/ui/consts/const-mut-refs/mut_ref_in_final.stderr

@@ -1,14 +1,22 @@
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:15:21
    |
 LL | const B: *mut i32 = &mut 4;
-   |                     ^^^^^^
+   |                     ^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:21:35
    |
 LL | const B3: Option<&mut i32> = Some(&mut 42);
-   |                                   ^^^^^^^
+   |                                   ^^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/mut_ref_in_final.rs:24:42
@@ -72,29 +80,45 @@ 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 borrows of lifetime-extended temporaries in the top-level scope of a static are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:71:53
    |
 LL | static RAW_MUT_CAST_S: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
-   |                                                     ^^^^^^^
+   |                                                     ^^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a static are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:73:54
    |
 LL | static RAW_MUT_COERCE_S: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-   |                                                      ^^^^^^
+   |                                                      ^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:75:52
    |
 LL | const RAW_MUT_CAST_C: SyncPtr<i32> = SyncPtr { x : &mut 42 as *mut _ as *const _ };
-   |                                                    ^^^^^^^
+   |                                                    ^^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
-error[E0764]: mutable borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0764]: mutable borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/mut_ref_in_final.rs:77:53
    |
 LL | const RAW_MUT_COERCE_C: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-   |                                                     ^^^^^^
+   |                                                     ^^^^^^ this mutable borrow refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 error: aborting due to 12 previous errors
 

tests/ui/consts/const-promoted-opaque.atomic.stderr

@@ -7,11 +7,15 @@ LL |
 LL | };
    | - value is dropped here
 
-error[E0492]: interior mutable shared borrows of lifetime-extended temporaries in the top-level scope of a constant are not allowed
+error[E0492]: interior mutable shared borrows of temporaries that have their lifetime extended until the end of the program are not allowed
   --> $DIR/const-promoted-opaque.rs:36:19
    |
 LL | const BAZ: &Foo = &FOO;
-   |                   ^^^^ this borrow of an interior mutable value refers to a lifetime-extended temporary
+   |                   ^^^^ this borrow of an interior mutable value refers to such a temporary
+   |
+   = note: Temporaries in constants and statics can have their lifetime extended until the end of the program
+   = note: To avoid accidentally creating global mutable state, such temporaries must be immutable
+   = help: If you really want global mutable state, try replacing the temporary by an interior mutable `static` or a `static mut`
 
 error[E0716]: temporary value dropped while borrowed
   --> $DIR/const-promoted-opaque.rs:40:26

tests/ui/consts/const-promoted-opaque.rs

@@ -34,7 +34,7 @@ const BAR: () = {
 };
 
 const BAZ: &Foo = &FOO;
-//[atomic]~^ ERROR: interior mutable shared borrows of lifetime-extended temporaries
+//[atomic]~^ ERROR: interior mutable shared borrows of temporaries
 
 fn main() {
     let _: &'static _ = &FOO;

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 borrows of lifetime-extended temporaries
+//~^ ERROR: mutable borrows of temporaries
 
 static mut S: i32 = 3;
 const C2: &'static mut i32 = unsafe { &mut S };