Improve error reporting for modifications behind & references

Author: FabianWolff

compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs

@@ -147,7 +147,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     if let Some(desc) = access_place_desc {
                         item_msg = format!("`{}`", desc);
                         reason = match error_access {
-                            AccessKind::Mutate => format!(" which is behind {}", pointer_type),
+                            AccessKind::Mutate => format!(", which is behind {}", pointer_type),
                             AccessKind::MutableBorrow => {
                                 format!(", as it is behind {}", pointer_type)
                             }
@@ -897,16 +897,32 @@ fn suggest_ampmut<'tcx>(
 ) -> (Span, String) {
     if let Some(assignment_rhs_span) = opt_assignment_rhs_span {
         if let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span) {
+            let is_mutbl = |ty: &str| -> bool {
+                if ty.starts_with("mut") {
+                    let rest = &ty[3..];
+                    match rest.chars().next() {
+                        // e.g. `&mut x`
+                        Some(c) if c.is_whitespace() => true,
+                        // e.g. `&mut(x)`
+                        Some('(') => true,
+                        // e.g. `&mutablevar`
+                        _ => false,
+                    }
+                } else {
+                    false
+                }
+            };
             if let (true, Some(ws_pos)) =
                 (src.starts_with("&'"), src.find(|c: char| -> bool { c.is_whitespace() }))
             {
                 let lt_name = &src[1..ws_pos];
-                let ty = &src[ws_pos..];
-                if !ty.trim_start().starts_with("mut") {
+                let ty = src[ws_pos..].trim_start();
+                if !is_mutbl(ty) {
                     return (assignment_rhs_span, format!("&{} mut {}", lt_name, ty));
                 }
             } else if let Some(stripped) = src.strip_prefix('&') {
-                if !stripped.trim_start().starts_with("mut") {
+                let stripped = stripped.trim_start();
+                if !is_mutbl(stripped) {
                     return (assignment_rhs_span, format!("&mut {}", stripped));
                 }
             }

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

@@ -1,12 +1,12 @@
-error[E0594]: cannot assign to `*s.pointer` which is behind a `&` reference
+error[E0594]: cannot assign to `*s.pointer`, which is behind a `&` reference
   --> $DIR/borrowck-assign-to-andmut-in-aliasable-loc.rs:9:5
    |
 LL | fn a(s: &S) {
    |         -- help: consider changing this to be a mutable reference: `&mut S<'_>`
 LL |     *s.pointer += 1;
    |     ^^^^^^^^^^^^^^^ `s` is a `&` reference, so the data it refers to cannot be written
 
-error[E0594]: cannot assign to `*s.pointer` which is behind a `&` reference
+error[E0594]: cannot assign to `*s.pointer`, which is behind a `&` reference
   --> $DIR/borrowck-assign-to-andmut-in-aliasable-loc.rs:17:5
    |
 LL | fn c(s: & &mut S) {

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

@@ -1,4 +1,4 @@
-error[E0594]: cannot assign to `**t1` which is behind a `&` reference
+error[E0594]: cannot assign to `**t1`, which is behind a `&` reference
   --> $DIR/borrowck-borrow-mut-base-ptr-in-aliasable-loc.rs:9:5
    |
 LL |     let t1 = t0;

src/test/ui/borrowck/borrowck-issue-14498.stderr

@@ -1,4 +1,4 @@
-error[E0594]: cannot assign to `***p` which is behind a `&` reference
+error[E0594]: cannot assign to `***p`, which is behind a `&` reference
   --> $DIR/borrowck-issue-14498.rs:16:5
    |
 LL |     let p = &y;

src/test/ui/borrowck/issue-69789-iterator-mut-suggestion.stderr

@@ -1,4 +1,4 @@
-error[E0594]: cannot assign to `*item` which is behind a `&` reference
+error[E0594]: cannot assign to `*item`, which is behind a `&` reference
   --> $DIR/issue-69789-iterator-mut-suggestion.rs:7:9
    |
 LL |     for item in &mut std::iter::empty::<&'static ()>() {

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

@@ -9,7 +9,7 @@ fn main() {
     for v in Query.iter_mut() {
         //~^ NOTE this iterator yields `&` references
         *v -= 1;
-        //~^ ERROR cannot assign to `*v` which is behind a `&` reference
+        //~^ ERROR cannot assign to `*v`, which is behind a `&` reference
         //~| NOTE `v` is a `&` reference, so the data it refers to cannot be written
     }
 }

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

@@ -1,4 +1,4 @@
-error[E0594]: cannot assign to `*v` which is behind a `&` reference
+error[E0594]: cannot assign to `*v`, which is behind a `&` reference
   --> $DIR/issue-83309-ice-immut-in-for-loop.rs:11:9
    |
 LL |     for v in Query.iter_mut() {

src/test/ui/borrowck/issue-85765.rs

@@ -5,4 +5,11 @@ fn main() {
     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
+
+    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
 }

src/test/ui/borrowck/issue-85765.stderr

@@ -7,6 +7,16 @@ LL |
 LL |     rofl.push(Vec::new());
    |     ^^^^ `rofl` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
-error: aborting due to previous error
+error[E0594]: cannot assign to `*r`, which is behind a `&` reference
+  --> $DIR/issue-85765.rs:12:5
+   |
+LL |     let r = &mutvar;
+   |             ------- help: consider changing this to be a mutable reference: `&mut mutvar`
+LL |
+LL |     *r = 0;
+   |     ^^^^^^ `r` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0596`.
+Some errors have detailed explanations: E0594, E0596.
+For more information about an error, try `rustc --explain E0594`.

src/test/ui/borrowck/mutability-errors.stderr

@@ -1,12 +1,12 @@
-error[E0594]: cannot assign to `*x` which is behind a `&` reference
+error[E0594]: cannot assign to `*x`, which is behind a `&` reference
   --> $DIR/mutability-errors.rs:9:5
    |
 LL | fn named_ref(x: &(i32,)) {
    |                 ------- help: consider changing this to be a mutable reference: `&mut (i32,)`
 LL |     *x = (1,);
    |     ^^^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
 
-error[E0594]: cannot assign to `x.0` which is behind a `&` reference
+error[E0594]: cannot assign to `x.0`, which is behind a `&` reference
   --> $DIR/mutability-errors.rs:10:5
    |
 LL | fn named_ref(x: &(i32,)) {
@@ -57,15 +57,15 @@ error[E0596]: cannot borrow data in a `&` reference as mutable
 LL |     &mut f().0;
    |     ^^^^^^^^^^ cannot borrow as mutable
 
-error[E0594]: cannot assign to `*x` which is behind a `*const` pointer
+error[E0594]: cannot assign to `*x`, which is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:23:5
    |
 LL | unsafe fn named_ptr(x: *const (i32,)) {
    |                        ------------- help: consider changing this to be a mutable pointer: `*mut (i32,)`
 LL |     *x = (1,);
    |     ^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be written
 
-error[E0594]: cannot assign to `x.0` which is behind a `*const` pointer
+error[E0594]: cannot assign to `x.0`, which is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:24:5
    |
 LL | unsafe fn named_ptr(x: *const (i32,)) {