Provide more context when mutably borrowing an imutable borrow

Point at statics and consts being mutable borrowed or written to:

```
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
```

Point at the expression that couldn't be mutably borrowed from a pattern:

```
error[E0596]: cannot borrow data in a `&` reference as mutable
  --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14
   |
LL |     match &arg.field {
   |           ---------- this cannot be borrowed as mutable
LL |         Some(ref mut s) => s.push('a'),
   |              ^^^^^^^^^ cannot borrow as mutable
```

Author: estebank

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