Provide more context when mutably borrowing an imutably borrowed value

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -213,7 +213,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             AccessKind::Mutate => {
                 err = self.cannot_assign(span, &(item_msg + &reason));
                 act = "assign";
-                acted_on = "written";
+                acted_on = "written to";
                 span
             }
             AccessKind::MutableBorrow => {
@@ -518,8 +518,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         err.span_label(
                             span,
                             format!(
-                                "`{name}` is a `{pointer_sigil}` {pointer_desc}, \
-                                 so the data it refers to cannot be {acted_on}",
+                                "`{name}` is a `{pointer_sigil}` {pointer_desc}, so it cannot be \
+                                 {acted_on}",
                             ),
                         );
 
@@ -542,7 +542,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 self.expected_fn_found_fn_mut_call(&mut err, span, act);
             }
 
-            PlaceRef { local: _, projection: [.., ProjectionElem::Deref] } => {
+            PlaceRef { local, projection: [.., ProjectionElem::Deref] } => {
                 err.span_label(span, format!("cannot {act}"));
 
                 match opt_source {
@@ -559,11 +559,36 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         ));
                         self.suggest_map_index_mut_alternatives(ty, &mut err, span);
                     }
-                    _ => (),
+                    _ => {
+                        let local = &self.body.local_decls[local];
+                        match local.local_info() {
+                            LocalInfo::StaticRef { def_id, .. } => {
+                                let span = self.infcx.tcx.def_span(def_id);
+                                err.span_label(span, format!("this `static` cannot be {acted_on}"));
+                            }
+                            LocalInfo::ConstRef { def_id } => {
+                                let span = self.infcx.tcx.def_span(def_id);
+                                err.span_label(span, format!("this `const` cannot be {acted_on}"));
+                            }
+                            LocalInfo::BlockTailTemp(_) | LocalInfo::Boring
+                                if !local.source_info.span.overlaps(span) =>
+                            {
+                                err.span_label(
+                                    local.source_info.span,
+                                    format!("this cannot be {acted_on}"),
+                                );
+                            }
+                            _ => {}
+                        }
+                    }
                 }
             }
 
-            _ => {
+            PlaceRef { local, .. } => {
+                let local = &self.body.local_decls[local];
+                if !local.source_info.span.overlaps(span) {
+                    err.span_label(local.source_info.span, format!("this cannot be {acted_on}"));
+                }
                 err.span_label(span, format!("cannot {act}"));
             }
         }

tests/ui/array-slice-vec/slice-mut-2.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
   --> $DIR/slice-mut-2.rs:7:18
    |
 LL |     let _ = &mut x[2..4];
-   |                  ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                  ^ `x` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this binding's type
    |

tests/ui/borrowck/accidentally-cloning-ref-borrow-error.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `sm.x` as mutable, as it is behind a `&` reference
   --> $DIR/accidentally-cloning-ref-borrow-error.rs:32:9
    |
 LL |     foo(&mut sm.x);
-   |         ^^^^^^^^^ `sm` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |         ^^^^^^^^^ `sm` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: `Str` doesn't implement `Clone`, so this call clones the reference `&Str`
   --> $DIR/accidentally-cloning-ref-borrow-error.rs:31:21

tests/ui/borrowck/argument_number_mismatch_ice.stderr

@@ -11,7 +11,7 @@ error[E0594]: cannot assign to `*input`, which is behind a `&` reference
   --> $DIR/argument_number_mismatch_ice.rs:10:9
    |
 LL |         *input = self.0;
-   |         ^^^^^^^^^^^^^^^ `input` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^^^^^^^^^^ `input` is a `&` reference, so it cannot be written to
    |
 help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
    |

tests/ui/borrowck/borrow-raw-address-of-deref-mutability.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
   --> $DIR/borrow-raw-address-of-deref-mutability.rs:6:13
    |
 LL |     let q = &raw mut *x;
-   |             ^^^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |             ^^^^^^^^^^^ `x` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this to be a mutable reference
    |
@@ -13,7 +13,7 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `*const` pointer
   --> $DIR/borrow-raw-address-of-deref-mutability.rs:12:13
    |
 LL |     let q = &raw mut *x;
-   |             ^^^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable
+   |             ^^^^^^^^^^^ `x` is a `*const` pointer, so it cannot be borrowed as mutable
    |
 help: consider specifying this binding's type
    |

tests/ui/borrowck/borrowck-access-permissions.stderr

@@ -12,6 +12,9 @@ LL |     let mut x = 1;
 error[E0596]: cannot borrow immutable static item `static_x` as mutable
   --> $DIR/borrowck-access-permissions.rs:16:19
    |
+LL | static static_x: i32 = 1;
+   | -------------------- this `static` cannot be borrowed as mutable
+...
 LL |         let _y1 = &mut static_x;
    |                   ^^^^^^^^^^^^^ cannot borrow as mutable
 
@@ -30,7 +33,7 @@ error[E0596]: cannot borrow `*ref_x` as mutable, as it is behind a `&` reference
   --> $DIR/borrowck-access-permissions.rs:36:19
    |
 LL |         let _y1 = &mut *ref_x;
-   |                   ^^^^^^^^^^^ `ref_x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                   ^^^^^^^^^^^ `ref_x` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this to be a mutable reference
    |
@@ -41,7 +44,7 @@ error[E0596]: cannot borrow `*ptr_x` as mutable, as it is behind a `*const` poin
   --> $DIR/borrowck-access-permissions.rs:46:23
    |
 LL |             let _y1 = &mut *ptr_x;
-   |                       ^^^^^^^^^^^ `ptr_x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable
+   |                       ^^^^^^^^^^^ `ptr_x` is a `*const` pointer, so it cannot be borrowed as mutable
    |
 help: consider changing this binding's type
    |
@@ -53,7 +56,7 @@ error[E0596]: cannot borrow `*foo_ref.f` as mutable, as it is behind a `&` refer
   --> $DIR/borrowck-access-permissions.rs:59:18
    |
 LL |         let _y = &mut *foo_ref.f;
-   |                  ^^^^^^^^^^^^^^^ `foo_ref` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                  ^^^^^^^^^^^^^^^ `foo_ref` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/borrowck-assign-to-andmut-in-aliasable-loc.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `*s.pointer`, which is behind a `&` reference
   --> $DIR/borrowck-assign-to-andmut-in-aliasable-loc.rs:9:5
    |
 LL |     *s.pointer += 1;
-   |     ^^^^^^^^^^^^^^^ `s` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^^^^^^^ `s` is a `&` reference, so it cannot be written to
    |
 help: consider changing this to be a mutable reference
    |
@@ -13,7 +13,7 @@ error[E0594]: cannot assign to `*s.pointer`, which is behind a `&` reference
   --> $DIR/borrowck-assign-to-andmut-in-aliasable-loc.rs:17:5
    |
 LL |     *s.pointer += 1;
-   |     ^^^^^^^^^^^^^^^ `s` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^^^^^^^ `s` is a `&` reference, so it cannot be written to
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/borrowck-assign-to-constants.stderr

@@ -1,6 +1,9 @@
 error[E0594]: cannot assign to immutable static item `foo`
   --> $DIR/borrowck-assign-to-constants.rs:5:5
    |
+LL | static foo: isize = 5;
+   | ----------------- this `static` cannot be written to
+...
 LL |     foo = 6;
    |     ^^^^^^^ cannot assign
 

tests/ui/borrowck/borrowck-borrow-mut-base-ptr-in-aliasable-loc.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `**t1`, which is behind a `&` reference
   --> $DIR/borrowck-borrow-mut-base-ptr-in-aliasable-loc.rs:9:5
    |
 LL |     **t1 = 22;
-   |     ^^^^^^^^^ `t1` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^ `t1` is a `&` reference, so it cannot be written to
    |
 help: consider specifying this binding's type
    |
@@ -23,7 +23,7 @@ error[E0596]: cannot borrow `**t0` as mutable, as it is behind a `&` reference
   --> $DIR/borrowck-borrow-mut-base-ptr-in-aliasable-loc.rs:19:26
    |
 LL |     let x:  &mut isize = &mut **t0;
-   |                          ^^^^^^^^^ `t0` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                          ^^^^^^^^^ `t0` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/borrowck-issue-14498.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `***p`, which is behind a `&` reference
   --> $DIR/borrowck-issue-14498.rs:16:5
    |
 LL |     ***p = 2;
-   |     ^^^^^^^^ `p` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^ `p` is a `&` reference, so it cannot be written to
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/borrowck-reborrow-from-mut.stderr

@@ -106,7 +106,7 @@ error[E0596]: cannot borrow `foo.bar1` as mutable, as it is behind a `&` referen
   --> $DIR/borrowck-reborrow-from-mut.rs:88:17
    |
 LL |     let _bar1 = &mut foo.bar1;
-   |                 ^^^^^^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                 ^^^^^^^^^^^^^ `foo` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/issue-115259-suggest-iter-mut.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` referenc
   --> $DIR/issue-115259-suggest-iter-mut.rs:15:65
    |
 LL |         self.layers.iter().fold(0, |result, mut layer| result + layer.process())
-   |                                             ---------           ^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                             ---------           ^^^^^ `layer` is a `&` reference, so it cannot be borrowed as mutable
    |                                             |
    |                                             consider changing this binding's type to be: `&mut Box<dyn Layer>`
    |

tests/ui/borrowck/issue-42344.stderr

@@ -1,6 +1,9 @@
 error[E0596]: cannot borrow `*TAB[_]` as mutable, as `TAB` is an immutable static item
   --> $DIR/issue-42344.rs:4:5
    |
+LL | static TAB: [&mut [u8]; 0] = [];
+   | -------------------------- this `static` cannot be borrowed as mutable
+...
 LL |     TAB[0].iter_mut();
    |     ^^^^^^ cannot borrow as mutable
 

tests/ui/borrowck/issue-62387-suggest-iter-mut-2.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*container` as mutable, as it is behind a `&` refer
   --> $DIR/issue-62387-suggest-iter-mut-2.rs:30:45
    |
 LL |             vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
-   |                                  ---------  ^^^^^^^^^ `container` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                  ---------  ^^^^^^^^^ `container` is a `&` reference, so it cannot be borrowed as mutable
    |                                  |
    |                                  consider changing this binding's type to be: `&mut Container`
    |

tests/ui/borrowck/issue-62387-suggest-iter-mut.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
   --> $DIR/issue-62387-suggest-iter-mut.rs:18:27
    |
 LL |     v.iter().for_each(|a| a.double());
-   |                        -  ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                        -  ^ `a` is a `&` reference, so it cannot be borrowed as mutable
    |                        |
    |                        consider changing this binding's type to be: `&mut A`
    |
@@ -15,7 +15,7 @@ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
   --> $DIR/issue-62387-suggest-iter-mut.rs:25:39
    |
 LL |     v.iter().rev().rev().for_each(|a| a.double());
-   |                                    -  ^ `a` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |                                    -  ^ `a` is a `&` reference, so it cannot be borrowed as mutable
    |                                    |
    |                                    consider changing this binding's type to be: `&mut A`
    |

tests/ui/borrowck/issue-69789-iterator-mut-suggestion.stderr

@@ -5,7 +5,7 @@ LL |     for item in &mut std::iter::empty::<&'static ()>() {
    |                 -------------------------------------- this iterator yields `&` references
 LL |
 LL |         *item = ();
-   |         ^^^^^^^^^^ `item` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^^^^^ `item` is a `&` reference, so it cannot be written to
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/issue-82032.stderr

@@ -7,7 +7,7 @@ LL |         for v in self.0.values() {
    |                  |      help: use mutable method: `values_mut()`
    |                  this iterator yields `&` references
 LL |             v.flush();
-   |             ^ `v` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |             ^ `v` is a `&` reference, so it cannot be borrowed as mutable
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/issue-83309-ice-immut-in-for-loop.rs

@@ -10,7 +10,7 @@ fn main() {
         //~^ NOTE this iterator yields `&` references
         *v -= 1;
         //~^ ERROR cannot assign to `*v`, which is behind a `&` reference
-        //~| NOTE `v` is a `&` reference, so the data it refers to cannot be written
+        //~| NOTE `v` is a `&` reference, so it cannot be written to
     }
 }
 

tests/ui/borrowck/issue-83309-ice-immut-in-for-loop.stderr

@@ -5,7 +5,7 @@ LL |     for v in Query.iter_mut() {
    |              ---------------- this iterator yields `&` references
 LL |
 LL |         *v -= 1;
-   |         ^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^^ `v` is a `&` reference, so it cannot be written to
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/issue-85765-closure.rs

@@ -5,27 +5,27 @@ fn main() {
         //~^ HELP consider changing this binding's type
         rofl.push(Vec::new());
         //~^ ERROR cannot borrow `*rofl` as mutable, as it is behind a `&` reference
-        //~| NOTE `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+        //~| NOTE `rofl` is a `&` reference, so it cannot be borrowed as mutable
 
         let mut mutvar = 42;
         let r = &mutvar;
         //~^ HELP consider changing this to be a mutable reference
         *r = 0;
         //~^ ERROR cannot assign to `*r`, which is behind a `&` reference
-        //~| NOTE `r` is a `&` reference, so the data it refers to cannot be written
+        //~| NOTE `r` is a `&` reference, so it cannot be written to
 
         #[rustfmt::skip]
         let x: &usize = &mut{0};
         //~^ HELP consider changing this binding's type
         *x = 1;
         //~^ ERROR cannot assign to `*x`, which is behind a `&` reference
-        //~| NOTE `x` is a `&` reference, so the data it refers to cannot be written
+        //~| NOTE `x` is a `&` reference, so it cannot be written to
 
         #[rustfmt::skip]
         let y: &usize = &mut(0);
         //~^ HELP consider changing this binding's type
         *y = 1;
         //~^ ERROR cannot assign to `*y`, which is behind a `&` reference
-        //~| NOTE `y` is a `&` reference, so the data it refers to cannot be written
+        //~| NOTE `y` is a `&` reference, so it cannot be written to
     };
 }

tests/ui/borrowck/issue-85765-closure.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*rofl` as mutable, as it is behind a `&` reference
   --> $DIR/issue-85765-closure.rs:6:9
    |
 LL |         rofl.push(Vec::new());
-   |         ^^^^ `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |         ^^^^ `rofl` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: consider changing this binding's type
    |
@@ -13,7 +13,7 @@ error[E0594]: cannot assign to `*r`, which is behind a `&` reference
   --> $DIR/issue-85765-closure.rs:13:9
    |
 LL |         *r = 0;
-   |         ^^^^^^ `r` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^ `r` is a `&` reference, so it cannot be written to
    |
 help: consider changing this to be a mutable reference
    |
@@ -24,7 +24,7 @@ error[E0594]: cannot assign to `*x`, which is behind a `&` reference
   --> $DIR/issue-85765-closure.rs:20:9
    |
 LL |         *x = 1;
-   |         ^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^ `x` is a `&` reference, so it cannot be written to
    |
 help: consider changing this binding's type
    |
@@ -35,7 +35,7 @@ error[E0594]: cannot assign to `*y`, which is behind a `&` reference
   --> $DIR/issue-85765-closure.rs:27:9
    |
 LL |         *y = 1;
-   |         ^^^^^^ `y` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^ `y` is a `&` reference, so it cannot be written to
    |
 help: consider changing this binding's type
    |

tests/ui/borrowck/issue-85765.rs

@@ -4,26 +4,26 @@ fn main() {
     //~^ HELP consider changing this binding's type
     rofl.push(Vec::new());
     //~^ ERROR cannot borrow `*rofl` as mutable, as it is behind a `&` reference
-    //~| NOTE `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+    //~| NOTE `rofl` is a `&` reference, so it cannot be borrowed as mutable
 
     let mut mutvar = 42;
     let r = &mutvar;
     //~^ HELP consider changing this to be a mutable reference
     *r = 0;
     //~^ ERROR cannot assign to `*r`, which is behind a `&` reference
-    //~| NOTE `r` is a `&` reference, so the data it refers to cannot be written
+    //~| NOTE `r` is a `&` reference, so it cannot be written to
 
     #[rustfmt::skip]
     let x: &usize = &mut{0};
     //~^ HELP consider changing this binding's type
     *x = 1;
     //~^ ERROR cannot assign to `*x`, which is behind a `&` reference
-    //~| NOTE `x` is a `&` reference, so the data it refers to cannot be written
+    //~| NOTE `x` is a `&` reference, so it cannot be written to
 
     #[rustfmt::skip]
     let y: &usize = &mut(0);
     //~^ HELP consider changing this binding's type
     *y = 1;
     //~^ ERROR cannot assign to `*y`, which is behind a `&` reference
-    //~| NOTE `y` is a `&` reference, so the data it refers to cannot be written
+    //~| NOTE `y` is a `&` reference, so it cannot be written to
 }