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 => {
@@ -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/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 the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
    |

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
 

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 the data it refers to 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 the data it refers to 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 the data it refers to cannot be written to
    |
 help: consider specifying this binding's type
    |

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 the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |

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-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 the data it refers to cannot be written to
 
 error: aborting due to 1 previous error
 

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 the data it refers to cannot be written to
 
 error: aborting due to 1 previous error
 

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

@@ -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 the data it refers to 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 the data it refers to 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 the data it refers to cannot be written to
    |
 help: consider changing this binding's type
    |

tests/ui/borrowck/issue-85765.stderr

@@ -13,7 +13,7 @@ error[E0594]: cannot assign to `*r`, which is behind a `&` reference
   --> $DIR/issue-85765.rs:12:5
    |
 LL |     *r = 0;
-   |     ^^^^^^ `r` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^ `r` is a `&` reference, so the data it refers to 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.rs:19:5
    |
 LL |     *x = 1;
-   |     ^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^ `x` is a `&` reference, so the data it refers to 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.rs:26:5
    |
 LL |     *y = 1;
-   |     ^^^^^^ `y` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^ `y` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this binding's type
    |

tests/ui/borrowck/issue-92015.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `*foo`, which is behind a `&` reference
   --> $DIR/issue-92015.rs:6:5
    |
 LL |     *foo = 1;
-   |     ^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider specifying this binding's type
    |

tests/ui/borrowck/issue-93093.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `self.foo`, which is behind a `&` reference
   --> $DIR/issue-93093.rs:8:9
    |
 LL |         self.foo += 1;
-   |         ^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written
+   |         ^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/borrowck/mutability-errors.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `*x`, which is behind a `&` reference
   --> $DIR/mutability-errors.rs:9:5
    |
 LL |     *x = (1,);
-   |     ^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |
@@ -13,7 +13,7 @@ error[E0594]: cannot assign to `x.0`, which is behind a `&` reference
   --> $DIR/mutability-errors.rs:10:5
    |
 LL |     x.0 = 1;
-   |     ^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |
@@ -70,7 +70,7 @@ error[E0594]: cannot assign to `*x`, which is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:23:5
    |
 LL |     *x = (1,);
-   |     ^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be written
+   |     ^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable pointer
    |
@@ -81,7 +81,7 @@ error[E0594]: cannot assign to `x.0`, which is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:24:5
    |
 LL |     (*x).0 = 1;
-   |     ^^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be written
+   |     ^^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable pointer
    |
@@ -358,24 +358,36 @@ LL | fn imm_capture(mut x: (i32,)) {
 error[E0594]: cannot assign to immutable static item `X`
   --> $DIR/mutability-errors.rs:76:5
    |
+LL | static X: (i32,) = (0,);
+   | ---------------- this `static` cannot be written to
+...
 LL |     X = (1,);
    |     ^^^^^^^^ cannot assign
 
 error[E0594]: cannot assign to `X.0`, as `X` is an immutable static item
   --> $DIR/mutability-errors.rs:77:5
    |
+LL | static X: (i32,) = (0,);
+   | ---------------- this `static` cannot be written to
+...
 LL |     X.0 = 1;
    |     ^^^^^^^ cannot assign
 
 error[E0596]: cannot borrow immutable static item `X` as mutable
   --> $DIR/mutability-errors.rs:78:5
    |
+LL | static X: (i32,) = (0,);
+   | ---------------- this `static` cannot be borrowed as mutable
+...
 LL |     &mut X;
    |     ^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow `X.0` as mutable, as `X` is an immutable static item
   --> $DIR/mutability-errors.rs:79:5
    |
+LL | static X: (i32,) = (0,);
+   | ---------------- this `static` cannot be borrowed as mutable
+...
 LL |     &mut X.0;
    |     ^^^^^^^^ cannot borrow as mutable
 

tests/ui/borrowck/no-invalid-mut-suggestion-for-raw-pointer-issue-127562.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `*ptr`, which is behind a `*const` pointer
   --> $DIR/no-invalid-mut-suggestion-for-raw-pointer-issue-127562.rs:7:14
    |
 LL |     unsafe { *ptr = 3; }
-   |              ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written
+   |              ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable pointer
    |

tests/ui/consts/miri_unleashed/mutable_references.stderr

@@ -131,6 +131,9 @@ LL | const RAW_MUT_COERCE: SyncPtr<i32> = SyncPtr { x: &mut 0 };
 error[E0594]: cannot assign to `*OH_YES`, as `OH_YES` is an immutable static item
   --> $DIR/mutable_references.rs:109:5
    |
+LL | static OH_YES: &mut i32 = &mut 42;
+   | ----------------------- this `static` cannot be written to
+...
 LL |     *OH_YES = 99;
    |     ^^^^^^^^^^^^ cannot assign
 

tests/ui/consts/write_to_static_via_mut_ref.stderr

@@ -11,6 +11,9 @@ LL | static OH_NO: &mut i32 = &mut 42;
 error[E0594]: cannot assign to `*OH_NO`, as `OH_NO` is an immutable static item
   --> $DIR/write_to_static_via_mut_ref.rs:4:5
    |
+LL | static OH_NO: &mut i32 = &mut 42;
+   | ---------------------- this `static` cannot be written to
+...
 LL |     *OH_NO = 43;
    |     ^^^^^^^^^^^ cannot assign
 

tests/ui/error-codes/E0017.stderr

@@ -26,6 +26,9 @@ LL | const CR: &'static mut i32 = &mut C;
 error[E0596]: cannot borrow immutable static item `X` as mutable
   --> $DIR/E0017.rs:11:39
    |
+LL | static X: i32 = 1;
+   | ------------- this `static` cannot be borrowed as mutable
+...
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ cannot borrow as mutable
 

tests/ui/error-codes/E0389.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `fancy_ref.num`, which is behind a `&` reference
   --> $DIR/E0389.rs:8:5
    |
 LL |     fancy_ref.num = 6;
-   |     ^^^^^^^^^^^^^^^^^ `fancy_ref` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^^^^^^^^^ `fancy_ref` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |

tests/ui/error-codes/E0594.stderr

@@ -1,6 +1,9 @@
 error[E0594]: cannot assign to immutable static item `NUM`
   --> $DIR/E0594.rs:4:5
    |
+LL | static NUM: i32 = 18;
+   | --------------- this `static` cannot be written to
+...
 LL |     NUM = 20;
    |     ^^^^^^^^ cannot assign
 

tests/ui/issues/issue-46604.stderr

@@ -11,6 +11,9 @@ LL | static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];
 error[E0594]: cannot assign to `buf[_]`, as `buf` is an immutable static item
   --> $DIR/issue-46604.rs:6:5
    |
+LL | static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];
+   | --------------------- this `static` cannot be written to
+...
 LL |     buf[0]=2;
    |     ^^^^^^^^ cannot assign
 

tests/ui/issues/issue-51515.stderr

@@ -2,7 +2,7 @@ error[E0594]: cannot assign to `*foo`, which is behind a `&` reference
   --> $DIR/issue-51515.rs:4:5
    |
 LL |     *foo = 32;
-   |     ^^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider changing this to be a mutable reference
    |
@@ -13,7 +13,7 @@ error[E0594]: cannot assign to `*bar`, which is behind a `&` reference
   --> $DIR/issue-51515.rs:8:5
    |
 LL |     *bar = 64;
-   |     ^^^^^^^^^ `bar` is a `&` reference, so the data it refers to cannot be written
+   |     ^^^^^^^^^ `bar` is a `&` reference, so the data it refers to cannot be written to
    |
 help: consider specifying this binding's type
    |