tweak "make mut" spans when assigning to locals

Author: Ezrashaw

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1202,36 +1202,42 @@ fn suggest_ampmut<'tcx>(
     opt_assignment_rhs_span: Option<Span>,
     opt_ty_info: Option<Span>,
 ) -> (bool, Span, String) {
+    // if there is a RHS and it starts with a `&` from it, then check if it is
+    // mutable, and if not, put suggest putting `mut ` to make it mutable.
+    // we don't have to worry about lifetime annotations here because they are
+    // not valid when taking a reference. For example, the following is not valid Rust:
+    //
+    // let x: &i32 = &'a 5;
+    //                ^^ lifetime annotation not allowed
+    //
     if let Some(assignment_rhs_span) = opt_assignment_rhs_span
         && let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span)
+        && let Some(stripped) = src.strip_prefix('&')
     {
-        let is_mutbl = |ty: &str| -> bool {
-            if let Some(rest) = ty.strip_prefix("mut") {
-                match rest.chars().next() {
-                    // e.g. `&mut x`
-                    Some(c) if c.is_whitespace() => true,
-                    // e.g. `&mut(x)`
-                    Some('(') => true,
-                    // e.g. `&mut{x}`
-                    Some('{') => true,
-                    // e.g. `&mutablevar`
-                    _ => false,
-                }
-            } else {
-                false
+        let is_mut = if let Some(rest) = stripped.trim_start().strip_prefix("mut") {
+            match rest.chars().next() {
+                // e.g. `&mut x`
+                Some(c) if c.is_whitespace() => true,
+                // e.g. `&mut(x)`
+                Some('(') => true,
+                // e.g. `&mut{x}`
+                Some('{') => true,
+                // e.g. `&mutablevar`
+                _ => false,
             }
+        } else {
+            false
         };
-        if let (true, Some(ws_pos)) = (src.starts_with("&'"), src.find(char::is_whitespace)) {
-            let lt_name = &src[1..ws_pos];
-            let ty = src[ws_pos..].trim_start();
-            if !is_mutbl(ty) {
-                return (true, assignment_rhs_span, format!("&{lt_name} mut {ty}"));
-            }
-        } else if let Some(stripped) = src.strip_prefix('&') {
-            let stripped = stripped.trim_start();
-            if !is_mutbl(stripped) {
-                return (true, assignment_rhs_span, format!("&mut {stripped}"));
-            }
+        // if the reference is already mutable then there is nothing we can do
+        // here.
+        if !is_mut {
+            let span = assignment_rhs_span;
+            // shrink the span to just after the `&` in `&variable`
+            let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();
+
+            // FIXME(Ezrashaw): returning is bad because we still might want to
+            // update the annotated type, see #106857.
+            return (true, span, "mut ".to_owned());
         }
     }
 

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

@@ -7,7 +7,7 @@ LL |     let _ = &mut x[2..4];
 help: consider changing this to be a mutable reference
    |
 LL |     let x: &[isize] = &mut [1, 2, 3, 4, 5];
-   |                       ~~~~~~~~~~~~~~~~~~~~
+   |                        +++
 
 error: aborting due to previous error
 

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

@@ -7,7 +7,7 @@ LL |     let q = &raw mut *x;
 help: consider changing this to be a mutable reference
    |
 LL |     let x = &mut 0;
-   |             ~~~~~~
+   |              +++
 
 error[E0596]: cannot borrow `*x` as mutable, as it is behind a `*const` pointer
   --> $DIR/borrow-raw-address-of-deref-mutability.rs:14:13
@@ -18,7 +18,7 @@ LL |     let q = &raw mut *x;
 help: consider changing this to be a mutable pointer
    |
 LL |     let x = &mut 0 as *const i32;
-   |             ~~~~~~
+   |              +++
 
 error: aborting due to 2 previous errors
 

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

@@ -35,7 +35,7 @@ LL |         let _y1 = &mut *ref_x;
 help: consider changing this to be a mutable reference
    |
 LL |         let ref_x = &mut x;
-   |                     ~~~~~~
+   |                      +++
 
 error[E0596]: cannot borrow `*ptr_x` as mutable, as it is behind a `*const` pointer
   --> $DIR/borrowck-access-permissions.rs:39:23
@@ -46,7 +46,7 @@ LL |             let _y1 = &mut *ptr_x;
 help: consider changing this to be a mutable pointer
    |
 LL |         let ptr_x : *const _ = &mut x;
-   |                                ~~~~~~
+   |                                 +++
 
 error[E0596]: cannot borrow `*foo_ref.f` as mutable, as it is behind a `&` reference
   --> $DIR/borrowck-access-permissions.rs:48:18
@@ -57,7 +57,7 @@ LL |         let _y = &mut *foo_ref.f;
 help: consider changing this to be a mutable reference
    |
 LL |         let foo_ref = &mut foo;
-   |                       ~~~~~~~~
+   |                        +++
 
 error: aborting due to 6 previous errors
 

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

@@ -7,7 +7,7 @@ LL |     ***p = 2;
 help: consider changing this to be a mutable reference
    |
 LL |     let p = &mut y;
-   |             ~~~~~~
+   |              +++
 
 error[E0506]: cannot assign to `**y` because it is borrowed
   --> $DIR/borrowck-issue-14498.rs:25:5

tests/ui/borrowck/issue-85765.stderr

@@ -18,7 +18,7 @@ LL |     *r = 0;
 help: consider changing this to be a mutable reference
    |
 LL |     let r = &mut mutvar;
-   |             ~~~~~~~~~~~
+   |              +++
 
 error[E0594]: cannot assign to `*x`, which is behind a `&` reference
   --> $DIR/issue-85765.rs:19:5

tests/ui/closures/2229_closure_analysis/diagnostics/mut_ref.stderr

@@ -10,7 +10,7 @@ LL |         **ref_mref_x = y;
 help: consider changing this to be a mutable reference
    |
 LL |     let ref_mref_x = &mut mref_x;
-   |                      ~~~~~~~~~~~
+   |                       +++
 
 error[E0596]: cannot borrow `**mref_ref_x` as mutable, as it is behind a `&` reference
   --> $DIR/mut_ref.rs:26:13

tests/ui/did_you_mean/issue-40823.stderr

@@ -7,7 +7,7 @@ LL |     buf.iter_mut();
 help: consider changing this to be a mutable reference
    |
 LL |     let mut buf = &mut [1, 2, 3, 4];
-   |                   ~~~~~~~~~~~~~~~~~
+   |                    +++
 
 error: aborting due to previous error
 

tests/ui/error-codes/E0389.stderr

@@ -7,7 +7,7 @@ LL |     fancy_ref.num = 6;
 help: consider changing this to be a mutable reference
    |
 LL |     let fancy_ref = &mut (&mut fancy);
-   |                     ~~~~~~~~~~~~~~~~~
+   |                      +++
 
 error: aborting due to previous error
 

tests/ui/issues/issue-51515.rs

@@ -1,7 +1,6 @@
 fn main() {
     let foo = &16;
     //~^ HELP consider changing this to be a mutable reference
-    //~| SUGGESTION &mut 16
     *foo = 32;
     //~^ ERROR cannot assign to `*foo`, which is behind a `&` reference
     let bar = foo;