Merge multiple mutable borrows of immutable binding errors

Fix #53466.

compiler/rustc_borrowck/src/borrowck_errors.rs

@@ -12,7 +12,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         place: &str,
         borrow_place: &str,
         value_place: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         self.infcx.tcx.sess.create_err(crate::session_diagnostics::MoveBorrow {
             place,
             span,
@@ -28,7 +28,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         desc: &str,
         borrow_span: Span,
         borrow_desc: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -50,7 +50,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         old_loan_span: Span,
         old_opt_via: &str,
         old_load_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let via =
             |msg: &str| if msg.is_empty() { "".to_string() } else { format!(" (via {})", msg) };
         let mut err = struct_span_err!(
@@ -98,7 +98,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         desc: &str,
         old_loan_span: Span,
         old_load_end_span: Option<Span>,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             new_loan_span,
@@ -269,7 +269,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         span: Span,
         desc: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(self, span, E0594, "cannot assign to {}", desc)
     }
 
@@ -348,7 +348,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         span: Span,
         path: &str,
         reason: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(self, span, E0596, "cannot borrow {} as mutable{}", path, reason,)
     }
 
@@ -359,7 +359,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         immutable_place: &str,
         immutable_section: &str,
         action: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             mutate_span,
@@ -378,7 +378,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         span: Span,
         yield_span: Span,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -392,7 +392,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
     pub(crate) fn cannot_borrow_across_destructor(
         &self,
         borrow_span: Span,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(
             self,
             borrow_span,
@@ -405,7 +405,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         span: Span,
         path: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(self, span, E0597, "{} does not live long enough", path,)
     }
 
@@ -415,7 +415,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         return_kind: &str,
         reference_desc: &str,
         path_desc: &str,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             span,
@@ -440,7 +440,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         closure_kind: &str,
         borrowed_path: &str,
         capture_span: Span,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let mut err = struct_span_err!(
             self,
             closure_span,
@@ -458,14 +458,14 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
     pub(crate) fn thread_local_value_does_not_live_long_enough(
         &self,
         span: Span,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(self, span, E0712, "thread-local variable borrowed past end of function",)
     }
 
     pub(crate) fn temporary_value_borrowed_for_too_long(
         &self,
         span: Span,
-    ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         struct_span_err!(self, span, E0716, "temporary value dropped while borrowed",)
     }
 

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -180,6 +180,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         // the verbs used in some diagnostic messages.
         let act;
         let acted_on;
+        let mut suggest = true;
+        let mut mut_error = None;
+        let mut count = 1;
 
         let span = match error_access {
             AccessKind::Mutate => {
@@ -194,7 +197,41 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
                 let borrow_spans = self.borrow_spans(span, location);
                 let borrow_span = borrow_spans.args_or_use();
-                err = self.cannot_borrow_path_as_mutable_because(borrow_span, &item_msg, &reason);
+                match the_place_err {
+                    PlaceRef { local, projection: [] }
+                        if self.body.local_decls[local].can_be_made_mutable() =>
+                    {
+                        let span = self.body.local_decls[local].source_info.span;
+                        mut_error = Some(span);
+                        if let Some((buffer, c)) = self.get_buffered_mut_error(span) {
+                            // We've encountered a second (or more) attempt to mutably borrow an
+                            // immutable binding, so the likely problem is with the binding
+                            // declaration, not the use. We collect these in a single diagnostic
+                            // and make the binding the primary span of the error.
+                            err = buffer;
+                            count = c + 1;
+                            if count == 2 {
+                                err.replace_span_with(span, false);
+                                err.span_label(span, "not mutable");
+                            }
+                            suggest = false;
+                        } else {
+                            err = self.cannot_borrow_path_as_mutable_because(
+                                borrow_span,
+                                &item_msg,
+                                &reason,
+                            );
+                        }
+                    }
+                    _ => {
+                        err = self.cannot_borrow_path_as_mutable_because(
+                            borrow_span,
+                            &item_msg,
+                            &reason,
+                        );
+                    }
+                }
+                if suggest {
                     borrow_spans.var_span_label(
                         &mut err,
                         format!(
@@ -203,6 +240,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         ),
                         "mutable",
                     );
+                }
                 borrow_span
             }
         };
@@ -276,8 +314,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                                 pat_span: _,
                             },
                         )))) => {
+                            if suggest {
                                 err.span_note(sp, "the binding is already a mutable borrow");
                             }
+                        }
                         _ => {
                             err.span_note(
                                 decl.source_info.span,
@@ -333,7 +373,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let local_decl = &self.body.local_decls[local];
                 assert_eq!(local_decl.mutability, Mutability::Not);
 
+                if count < 10 {
                     err.span_label(span, format!("cannot {act}"));
+                }
+                if suggest {
                     err.span_suggestion(
                         local_decl.source_info.span,
                         "consider changing this to be mutable",
@@ -345,6 +388,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         self.show_mutating_upvar(tcx, id.expect_local(), the_place_err, &mut err);
                     }
                 }
+            }
 
             // Also suggest adding mut for upvars
             PlaceRef {
@@ -615,8 +659,12 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             }
         }
 
+        if let Some(span) = mut_error {
+            self.buffer_mut_error(span, err, count);
+        } else {
             self.buffer_error(err);
         }
+    }
 
     fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diagnostic, span: Span) {
         let Some(adt) = ty.ty_adt_def() else { return };

compiler/rustc_borrowck/src/lib.rs

@@ -2270,6 +2270,7 @@ mod error {
         /// same primary span come out in a consistent order.
         buffered_move_errors:
             BTreeMap<Vec<MoveOutIndex>, (PlaceRef<'tcx>, DiagnosticBuilder<'tcx, ErrorGuaranteed>)>,
+        buffered_mut_errors: FxHashMap<Span, (DiagnosticBuilder<'tcx, ErrorGuaranteed>, usize)>,
         /// Diagnostics to be reported buffer.
         buffered: Vec<Diagnostic>,
         /// Set to Some if we emit an error during borrowck
@@ -2281,6 +2282,7 @@ mod error {
             BorrowckErrors {
                 tcx,
                 buffered_move_errors: BTreeMap::new(),
+                buffered_mut_errors: Default::default(),
                 buffered: Default::default(),
                 tainted_by_errors: None,
             }
@@ -2331,12 +2333,34 @@ mod error {
             }
         }
 
+        pub fn get_buffered_mut_error(
+            &mut self,
+            span: Span,
+        ) -> Option<(DiagnosticBuilder<'tcx, ErrorGuaranteed>, usize)> {
+            self.errors.buffered_mut_errors.remove(&span)
+        }
+
+        pub fn buffer_mut_error(
+            &mut self,
+            span: Span,
+            t: DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+            count: usize,
+        ) {
+            self.errors.buffered_mut_errors.insert(span, (t, count));
+        }
+
         pub fn emit_errors(&mut self) -> Option<ErrorGuaranteed> {
             // Buffer any move errors that we collected and de-duplicated.
             for (_, (_, diag)) in std::mem::take(&mut self.errors.buffered_move_errors) {
                 // We have already set tainted for this error, so just buffer it.
                 diag.buffer(&mut self.errors.buffered);
             }
+            for (_, (mut diag, count)) in std::mem::take(&mut self.errors.buffered_mut_errors) {
+                if count > 10 {
+                    diag.note(&format!("...and {} other attempted mutable borrows", count - 10));
+                }
+                diag.buffer(&mut self.errors.buffered);
+            }
 
             if !self.errors.buffered.is_empty() {
                 self.errors.buffered.sort_by_key(|diag| diag.sort_span);

compiler/rustc_errors/src/diagnostic.rs

@@ -365,12 +365,12 @@ impl Diagnostic {
         self
     }
 
-    pub fn replace_span_with(&mut self, after: Span) -> &mut Self {
+    pub fn replace_span_with(&mut self, after: Span, keep_label: bool) -> &mut Self {
         let before = self.span.clone();
         self.set_span(after);
         for span_label in before.span_labels() {
             if let Some(label) = span_label.label {
-                if span_label.is_primary {
+                if span_label.is_primary && keep_label {
                     self.span.push_span_label(after, label);
                 } else {
                     self.span.push_span_label(span_label.span, label);

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -198,12 +198,12 @@ pub(super) fn emit_frag_parse_err(
         );
         if !e.span.is_dummy() {
             // early end of macro arm (#52866)
-            e.replace_span_with(parser.token.span.shrink_to_hi());
+            e.replace_span_with(parser.token.span.shrink_to_hi(), true);
         }
     }
     if e.span.is_dummy() {
         // Get around lack of span in error (#30128)
-        e.replace_span_with(site_span);
+        e.replace_span_with(site_span, true);
         if !parser.sess.source_map().is_imported(arm_span) {
             e.span_label(arm_span, "in this macro arm");
         }

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3237,7 +3237,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         })) = call_node
         {
             if Some(rcvr.span) == err.span.primary_span() {
-                err.replace_span_with(path.ident.span);
+                err.replace_span_with(path.ident.span, true);
             }
         }
         if let Some(Node::Expr(hir::Expr {

src/test/ui/asm/x86_64/type-check-5.rs

@@ -22,11 +22,10 @@ fn main() {
         // Outputs require mutable places
 
         let v: Vec<u64> = vec![0, 1, 2];
+        //~^ ERROR cannot borrow `v` as mutable, as it is not declared as mutable
         asm!("{}", in(reg) v[0]);
         asm!("{}", out(reg) v[0]);
-        //~^ ERROR cannot borrow `v` as mutable, as it is not declared as mutable
         asm!("{}", inout(reg) v[0]);
-        //~^ ERROR cannot borrow `v` as mutable, as it is not declared as mutable
 
         // Sym operands must point to a function or static
 

src/test/ui/asm/x86_64/type-check-5.stderr

@@ -25,24 +25,20 @@ LL |         let mut y: u64 = 0;
    |                        +++
 
 error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
-  --> $DIR/type-check-5.rs:26:29
+  --> $DIR/type-check-5.rs:24:13
    |
 LL |         let v: Vec<u64> = vec![0, 1, 2];
-   |             - help: consider changing this to be mutable: `mut v`
+   |             ^
-LL |         asm!("{}", in(reg) v[0]);
+   |             |
-LL |         asm!("{}", out(reg) v[0]);
+   |             not mutable
-   |                             ^ cannot borrow as mutable
+   |             help: consider changing this to be mutable: `mut v`
-
-error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
-  --> $DIR/type-check-5.rs:28:31
-   |
-LL |         let v: Vec<u64> = vec![0, 1, 2];
-   |             - help: consider changing this to be mutable: `mut v`
 ...
+LL |         asm!("{}", out(reg) v[0]);
+   |                             - cannot borrow as mutable
 LL |         asm!("{}", inout(reg) v[0]);
-   |                               ^ cannot borrow as mutable
+   |                               - cannot borrow as mutable
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0381, E0596.
 For more information about an error, try `rustc --explain E0381`.

src/test/ui/borrowck/many-mutable-borrows.rs

@@ -0,0 +1,18 @@
+fn main() {
+    let v = Vec::new(); //~ ERROR cannot borrow `v` as mutable, as it is not declared as mutable
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+    v.push(0);
+}

src/test/ui/borrowck/many-mutable-borrows.stderr

@@ -0,0 +1,32 @@
+error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable
+  --> $DIR/many-mutable-borrows.rs:2:9
+   |
+LL |     let v = Vec::new();
+   |         ^
+   |         |
+   |         not mutable
+   |         help: consider changing this to be mutable: `mut v`
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+LL |     v.push(0);
+   |     --------- cannot borrow as mutable
+   |
+   = note: ...and 5 other attempted mutable borrows
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/borrowck/mut-borrow-of-mut-ref.rs

@@ -2,15 +2,14 @@
 #![crate_type = "rlib"]
 
 pub fn f(b: &mut i32) {
-    //~^ NOTE the binding is already a mutable borrow
+    //~^ ERROR cannot borrow
+    //~| NOTE not mutable
     //~| NOTE the binding is already a mutable borrow
     h(&mut b);
-    //~^ ERROR cannot borrow
+    //~^ NOTE cannot borrow as mutable
-    //~| NOTE cannot borrow as mutable
     //~| HELP try removing `&mut` here
     g(&mut &mut b);
-    //~^ ERROR cannot borrow
+    //~^ NOTE cannot borrow as mutable
-    //~| NOTE cannot borrow as mutable
     //~| HELP try removing `&mut` here
 }
 

src/test/ui/borrowck/mut-borrow-of-mut-ref.stderr

@@ -1,8 +1,14 @@
 error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
-  --> $DIR/mut-borrow-of-mut-ref.rs:7:7
+  --> $DIR/mut-borrow-of-mut-ref.rs:4:10
    |
+LL | pub fn f(b: &mut i32) {
+   |          ^ not mutable
+...
 LL |     h(&mut b);
-   |       ^^^^^^ cannot borrow as mutable
+   |       ------ cannot borrow as mutable
+...
+LL |     g(&mut &mut b);
+   |            ------ cannot borrow as mutable
    |
 note: the binding is already a mutable borrow
   --> $DIR/mut-borrow-of-mut-ref.rs:4:13
@@ -14,18 +20,6 @@ help: try removing `&mut` here
 LL -     h(&mut b);
 LL +     h(b);
    |
-
-error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
-  --> $DIR/mut-borrow-of-mut-ref.rs:11:12
-   |
-LL |     g(&mut &mut b);
-   |            ^^^^^^ cannot borrow as mutable
-   |
-note: the binding is already a mutable borrow
-  --> $DIR/mut-borrow-of-mut-ref.rs:4:13
-   |
-LL | pub fn f(b: &mut i32) {
-   |             ^^^^^^^^
 help: try removing `&mut` here
    |
 LL -     g(&mut &mut b);
@@ -33,13 +27,13 @@ LL +     g(&mut b);
    |
 
 error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
-  --> $DIR/mut-borrow-of-mut-ref.rs:18:12
+  --> $DIR/mut-borrow-of-mut-ref.rs:17:12
    |
 LL |     h(&mut &mut b);
    |            ^^^^^^ cannot borrow as mutable
    |
 note: the binding is already a mutable borrow
-  --> $DIR/mut-borrow-of-mut-ref.rs:17:13
+  --> $DIR/mut-borrow-of-mut-ref.rs:16:13
    |
 LL | pub fn g(b: &mut i32) {
    |             ^^^^^^^^
@@ -50,7 +44,7 @@ LL +     h(&mut b);
    |
 
 error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
-  --> $DIR/mut-borrow-of-mut-ref.rs:35:5
+  --> $DIR/mut-borrow-of-mut-ref.rs:34:5
    |
 LL |     f.bar();
    |     ^^^^^^^ cannot borrow as mutable
@@ -60,6 +54,6 @@ help: consider making the binding mutable
 LL | pub fn baz(mut f: &mut String) {
    |            +++
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0596`.

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

@@ -50,9 +50,9 @@ fn ref_closure(mut x: (i32,)) {
     });
 }
 
-fn imm_local(x: (i32,)) {
+fn imm_local(x: (i32,)) { //~ ERROR
-    &mut x; //~ ERROR
+    &mut x;
-    &mut x.0; //~ ERROR
+    &mut x.0;
 }
 
 fn imm_capture(x: (i32,)) {

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

@@ -245,21 +245,17 @@ LL |         &mut x.0;
    |         ^^^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
-  --> $DIR/mutability-errors.rs:54:5
+  --> $DIR/mutability-errors.rs:53:14
    |
 LL | fn imm_local(x: (i32,)) {
-   |              - help: consider changing this to be mutable: `mut x`
+   |              ^
-LL |     &mut x;
+   |              |
-   |     ^^^^^^ cannot borrow as mutable
+   |              not mutable
-
+   |              help: consider changing this to be mutable: `mut x`
-error[E0596]: cannot borrow `x.0` as mutable, as `x` is not declared as mutable
-  --> $DIR/mutability-errors.rs:55:5
-   |
-LL | fn imm_local(x: (i32,)) {
-   |              - help: consider changing this to be mutable: `mut x`
 LL |     &mut x;
+   |     ------ cannot borrow as mutable
 LL |     &mut x.0;
-   |     ^^^^^^^^ cannot borrow as mutable
+   |     -------- cannot borrow as mutable
 
 error[E0594]: cannot assign to `x`, as it is not declared as mutable
   --> $DIR/mutability-errors.rs:60:9
@@ -357,7 +353,7 @@ error[E0596]: cannot borrow `X.0` as mutable, as `X` is an immutable static item
 LL |     &mut X.0;
    |     ^^^^^^^^ cannot borrow as mutable
 
-error: aborting due to 38 previous errors
+error: aborting due to 37 previous errors
 
 Some errors have detailed explanations: E0594, E0596.
 For more information about an error, try `rustc --explain E0594`.