Stop using DiagnosticBuilder::buffer in BorrowckErrors.

But we can't easily switch from `Vec<Diagnostic>` to
`Vec<DiagnosticBuilder<G>>` because there's a mix of errors and warnings
which result in different `G` types. So we must make
`DiagnosticBuilder::into_diagnostic` public, but that's ok, and it will
get more use in subsequent commits.

compiler/rustc_borrowck/src/lib.rs

@@ -2399,10 +2399,10 @@ mod error {
         /// and we want only the best of those errors.
         ///
         /// The `report_use_of_moved_or_uninitialized` function checks this map and replaces the
-        /// diagnostic (if there is one) if the `Place` of the error being reported is a prefix of the
-        /// `Place` of the previous most diagnostic. This happens instead of buffering the error. Once
-        /// all move errors have been reported, any diagnostics in this map are added to the buffer
-        /// to be emitted.
+        /// diagnostic (if there is one) if the `Place` of the error being reported is a prefix of
+        /// the `Place` of the previous most diagnostic. This happens instead of buffering the
+        /// error. Once all move errors have been reported, any diagnostics in this map are added
+        /// to the buffer to be emitted.
         ///
         /// `BTreeMap` is used to preserve the order of insertions when iterating. This is necessary
         /// when errors in the map are being re-added to the error buffer so that errors with the
@@ -2410,7 +2410,8 @@ mod error {
         buffered_move_errors:
             BTreeMap<Vec<MoveOutIndex>, (PlaceRef<'tcx>, DiagnosticBuilder<'tcx>)>,
         buffered_mut_errors: FxIndexMap<Span, (DiagnosticBuilder<'tcx>, usize)>,
-        /// Diagnostics to be reported buffer.
+        /// Buffer of diagnostics to be reported. Uses `Diagnostic` rather than `DiagnosticBuilder`
+        /// because it has a mixture of error diagnostics and non-error diagnostics.
         buffered: Vec<Diagnostic>,
         /// Set to Some if we emit an error during borrowck
         tainted_by_errors: Option<ErrorGuaranteed>,
@@ -2434,11 +2435,11 @@ mod error {
                     "diagnostic buffered but not emitted",
                 ))
             }
-            t.buffer(&mut self.buffered);
+            self.buffered.push(t.into_diagnostic());
         }
 
         pub fn buffer_non_error_diag(&mut self, t: DiagnosticBuilder<'_, ()>) {
-            t.buffer(&mut self.buffered);
+            self.buffered.push(t.into_diagnostic());
         }
 
         pub fn set_tainted_by_errors(&mut self, e: ErrorGuaranteed) {
@@ -2486,13 +2487,13 @@ mod error {
             // 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);
+                self.errors.buffered.push(diag.into_diagnostic());
             }
             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);
+                self.errors.buffered.push(diag.into_diagnostic());
             }
 
             if !self.errors.buffered.is_empty() {

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -260,7 +260,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     }
 
     /// Converts the builder to a `Diagnostic` for later emission.
-    fn into_diagnostic(mut self) -> Diagnostic {
+    pub fn into_diagnostic(mut self) -> Diagnostic {
         self.take_diag()
     }