Migrate multiple mut borrows diagnostic

compiler/rustc_error_messages/locales/en-US/mir_build.ftl

@@ -292,3 +292,9 @@ mir_build_borrow_of_moved_value = borrow of moved value
     .label = value moved into `{$name}` here
     .occurs_because_label = move occurs because `{$name}` has type `{$ty}` which does not implement the `Copy` trait
     .value_borrowed_label = value borrowed here after move
+
+mir_build_multiple_mut_borrows = cannot borrow value as mutable more than once at a time
+    .label = first mutable borrow, by `{$name}`, occurs here
+    .mutable_borrow = another mutable borrow, by `{$name_mut}`, occurs here
+    .immutable_borrow = also borrowed as immutable, by `{$name_immut}`, here
+    .moved = also moved into `{$name_moved}` here

compiler/rustc_mir_build/src/errors.rs

@@ -576,3 +576,37 @@ pub struct BorrowOfMovedValue<'tcx> {
     pub name: Ident,
     pub ty: Ty<'tcx>,
 }
+
+#[derive(SessionDiagnostic)]
+#[diag(mir_build::multiple_mut_borrows)]
+pub struct MultipleMutBorrows {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub binding_span: Span,
+    #[subdiagnostic]
+    pub occurences: Vec<MultipleMutBorrowOccurence>,
+    pub name: Ident,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum MultipleMutBorrowOccurence {
+    #[label(mir_build::mutable_borrow)]
+    Mutable {
+        #[primary_span]
+        span: Span,
+        name_mut: Ident,
+    },
+    #[label(mir_build::immutable_borrow)]
+    Immutable {
+        #[primary_span]
+        span: Span,
+        name_immut: Ident,
+    },
+    #[label(mir_build::moved)]
+    Moved {
+        #[primary_span]
+        span: Span,
+        name_moved: Ident,
+    },
+}

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -991,19 +991,19 @@ fn check_borrow_conflicts_in_at_patterns(cx: &MatchVisitor<'_, '_, '_>, pat: &Pa
     // Report errors if any.
     if !conflicts_mut_mut.is_empty() {
         // Report mutability conflicts for e.g. `ref mut x @ Some(ref mut y)`.
-        let mut err = sess
-            .struct_span_err(pat.span, "cannot borrow value as mutable more than once at a time");
-        err.span_label(binding_span, format!("first mutable borrow, by `{}`, occurs here", name));
+        let mut occurences = vec![];
+
         for (span, name) in conflicts_mut_mut {
-            err.span_label(span, format!("another mutable borrow, by `{}`, occurs here", name));
+            occurences.push(MultipleMutBorrowOccurence::Mutable { span, name_mut: name });
         }
         for (span, name) in conflicts_mut_ref {
-            err.span_label(span, format!("also borrowed as immutable, by `{}`, here", name));
+            occurences.push(MultipleMutBorrowOccurence::Immutable { span, name_immut: name });
         }
         for (span, name) in conflicts_move {
-            err.span_label(span, format!("also moved into `{}` here", name));
+            occurences.push(MultipleMutBorrowOccurence::Moved { span, name_moved: name });
         }
-        err.emit();
+
+        sess.emit_err(MultipleMutBorrows { span: pat.span, binding_span, occurences, name });
     } else if !conflicts_mut_ref.is_empty() {
         // Report mutability conflicts for e.g. `ref x @ Some(ref mut y)` or the converse.
         let (primary, also) = match mut_outer {