clarify and unify 'transient mutable borrow' errors

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -125,11 +125,12 @@ 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
+    shared references to 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.
 
 
@@ -216,18 +217,20 @@ 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
+    raw mutable pointers to 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 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`.
 
 const_eval_mutable_ref_escaping =
-    mutable references are not allowed in the final value of {const_eval_const_context}s
+    mutable references to lifetime-extended temporaries are not allowed in {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.
 
 

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 references to lifetime-extended 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

@@ -1,4 +1,4 @@
-error[E0764]: mutable references are not allowed in the final value of constants
+error[E0764]: mutable references to lifetime-extended temporaries are not allowed in constants
   --> $DIR/issue-76510.rs:3:29
    |
 LL | const S: &'static mut str = &mut " hello ";

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 references to lifetime-extended temporaries
 
 // Ok, no actual mutable allocation exists
 const B2: Option<&mut i32> = None;
@@ -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 references to lifetime-extended temporaries
 static RAW_MUT_COERCE_S: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable references to 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 references to lifetime-extended temporaries
 const RAW_MUT_COERCE_C: SyncPtr<i32> = SyncPtr { x: &mut 0 };
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable references to lifetime-extended temporaries
 
 fn main() {
     println!("{}", unsafe { *A });

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 references to 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 references to 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 references to 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 references to 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 references to 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 };

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 references to 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

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 references to lifetime-extended temporaries with interior mutability
 
 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 references are not allowed
+//~^ ERROR: mutable references to lifetime-extended temporaries
 
 static mut S: i32 = 3;
 const C2: &'static mut i32 = unsafe { &mut S };

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 references to lifetime-extended temporaries are not allowed in constants
   --> $DIR/issue-17718-const-bad-values.rs:7:34
    |
 LL | const C1: &'static mut [usize] = &mut [];

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 references to 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 references to lifetime-extended temporaries with interior mutability
 const F: &'static C = &D;
-//~^ ERROR: cannot refer to interior mutable
+//~^ ERROR: shared references to lifetime-extended temporaries with interior mutability
 
 fn main() {}