Merge multiple mutable borrows of immutable binding errors

Fix #53466.

3 files changed, 107 insertions, 35 deletions

diff --git a/compiler/rustc_borrowck/src/borrowck_errors.rs b/compiler/rustc_borrowck/src/borrowck_errors.rs
index 01be379120d..e4942f9b666 100644
--- a/compiler/rustc_borrowck/src/borrowck_errors.rs
+++ b/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",)
     }
 
diff --git a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
index 99ca4c637bd..b04b11e7c9b 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
+++ b/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 => {
@@ -194,15 +197,50 @@ 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);
-                borrow_spans.var_span_label(
-                    &mut err,
-                    format!(
-                        "mutable borrow occurs due to use of {} in closure",
-                        self.describe_any_place(access_place.as_ref()),
-                    ),
-                    "mutable",
-                );
+                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!(
+                            "mutable borrow occurs due to use of {} in closure",
+                            self.describe_any_place(access_place.as_ref()),
+                        ),
+                        "mutable",
+                    );
+                }
                 borrow_span
             }
         };
@@ -276,7 +314,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                                 pat_span: _,
                             },
                         )))) => {
-                            err.span_note(sp, "the binding is already a mutable borrow");
+                            if suggest {
+                                err.span_note(sp, "the binding is already a mutable borrow");
+                            }
                         }
                         _ => {
                             err.span_note(
@@ -333,16 +373,20 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let local_decl = &self.body.local_decls[local];
                 assert_eq!(local_decl.mutability, Mutability::Not);
 
-                err.span_label(span, format!("cannot {act}"));
-                err.span_suggestion(
-                    local_decl.source_info.span,
-                    "consider changing this to be mutable",
-                    format!("mut {}", self.local_names[local].unwrap()),
-                    Applicability::MachineApplicable,
-                );
-                let tcx = self.infcx.tcx;
-                if let ty::Closure(id, _) = *the_place_err.ty(self.body, tcx).ty.kind() {
-                    self.show_mutating_upvar(tcx, id.expect_local(), the_place_err, &mut err);
+                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",
+                        format!("mut {}", self.local_names[local].unwrap()),
+                        Applicability::MachineApplicable,
+                    );
+                    let tcx = self.infcx.tcx;
+                    if let ty::Closure(id, _) = *the_place_err.ty(self.body, tcx).ty.kind() {
+                        self.show_mutating_upvar(tcx, id.expect_local(), the_place_err, &mut err);
+                    }
                 }
             }
 
@@ -615,7 +659,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             }
         }
 
-        self.buffer_error(err);
+        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) {
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 168b798788b..ae1bea008b6 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/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);