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() {}

tests/ui/consts/issue-17718-const-borrow.stderr

@@ -1,20 +1,20 @@
-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/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 references to 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 references to 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
 

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 references to lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}

tests/ui/consts/partial_qualif.stderr

@@ -1,8 +1,8 @@
-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/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
 

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 references to lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}

tests/ui/consts/qualif_overwrite.stderr

@@ -1,8 +1,8 @@
-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/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
 

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 references to lifetime-extended temporaries with interior mutability
 };
 
 fn main() {}

tests/ui/consts/qualif_overwrite_2.stderr

@@ -1,8 +1,8 @@
-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/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
 

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 references to 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 references to 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 references to lifetime-extended temporaries with interior mutability
     let mut x = None;
     assert!(x.is_none());
     x = Some(UnsafeCell::new(false));

tests/ui/consts/refs-to-cell-in-final.stderr

@@ -1,18 +1,18 @@
-error[E0492]: statics cannot refer to interior mutable data
+error[E0492]: shared references to lifetime-extended temporaries with interior mutability are not allowed in statics
   --> $DIR/refs-to-cell-in-final.rs:13:54
    |
 LL | static RAW_SYNC_S: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
-   |                                                      ^^^^^^^^^^^^^^ 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
    |
    = help: to fix this, the value can be extracted to a separate `static` item and then referenced
 
-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/refs-to-cell-in-final.rs:15:53
    |
 LL | const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
-   |                                                     ^^^^^^^^^^^^^^ 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 references to lifetime-extended temporaries with interior mutability are not allowed in constants
   --> $DIR/refs-to-cell-in-final.rs:41:57
    |
 LL |   const INTERIOR_MUT_VARIANT: &Option<UnsafeCell<bool>> = &{
@@ -23,7 +23,7 @@ LL | |     assert!(x.is_none());
 LL | |     x = Some(UnsafeCell::new(false));
 LL | |     x
 LL | | };
-   | |_^ 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
 

tests/ui/consts/write_to_static_via_mut_ref.rs

@@ -1,4 +1,4 @@
-static OH_NO: &mut i32 = &mut 42; //~ ERROR mutable references are not allowed
+static OH_NO: &mut i32 = &mut 42; //~ ERROR mutable references to lifetime-extended temporaries
 fn main() {
     assert_eq!(*OH_NO, 42);
     *OH_NO = 43; //~ ERROR cannot assign to `*OH_NO`, as `OH_NO` is an immutable static

tests/ui/consts/write_to_static_via_mut_ref.stderr

@@ -1,4 +1,4 @@
-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/write_to_static_via_mut_ref.rs:1:26
    |
 LL | static OH_NO: &mut i32 = &mut 42;

tests/ui/error-codes/E0017.rs

@@ -5,12 +5,12 @@ static X: i32 = 1;
 const C: i32 = 2;
 static mut M: i32 = 3;
 
-const CR: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
+const CR: &'static mut i32 = &mut C; //~ ERROR mutable references to lifetime-extended temporaries
 //~| WARN taking a mutable
 
 static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR cannot borrow immutable static item `X` as mutable
 
-static CONST_REF: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
+static CONST_REF: &'static mut i32 = &mut C; //~ ERROR mutable references to lifetime-extended temporaries
 //~| WARN taking a mutable
 
 fn main() {}

tests/ui/error-codes/E0017.stderr

@@ -13,7 +13,7 @@ LL | const C: i32 = 2;
    | ^^^^^^^^^^^^
    = note: `#[warn(const_item_mutation)]` on by default
 
-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/E0017.rs:8:30
    |
 LL | const CR: &'static mut i32 = &mut C;
@@ -39,7 +39,7 @@ note: `const` item defined here
 LL | const C: i32 = 2;
    | ^^^^^^^^^^^^
 
-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/E0017.rs:13:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C;

tests/ui/error-codes/E0492.stderr

@@ -1,14 +1,14 @@
-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/E0492.rs:4:33
    |
 LL | const B: &'static AtomicUsize = &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]: statics cannot refer to interior mutable data
+error[E0492]: shared references to lifetime-extended temporaries with interior mutability are not allowed in statics
   --> $DIR/E0492.rs:5:34
    |
 LL | static C: &'static AtomicUsize = &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
    |
    = help: to fix this, the value can be extracted to a separate `static` item and then referenced
 

tests/ui/issues/issue-46604.rs

@@ -1,4 +1,4 @@
-static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR mutable references are not allowed
+static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR mutable references to lifetime-extended temporaries
 fn write<T: AsRef<[u8]>>(buffer: T) { }
 
 fn main() {

tests/ui/issues/issue-46604.stderr

@@ -1,4 +1,4 @@
-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/issue-46604.rs:1:25
    |
 LL | static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];

tests/ui/statics/check-immutable-mut-slices.rs

@@ -1,6 +1,6 @@
 // Checks that immutable static items can't have mutable slices
 
 static TEST: &'static mut [isize] = &mut [];
-//~^ ERROR mutable references are not allowed
+//~^ ERROR mutable references to lifetime-extended temporaries
 
 pub fn main() { }