Avoid some clones.

`Builder::expr_into_pattern` has a single call site. Currently the
`pattern` argument at the call site is always cloned.

This commit changes things so that we instead do a clone within
`expr_into_pattern`, but only if the pattern has the
`PatKind::AscribeUserType` kind, and we only clone the annotation within
the pattern instead of the entire pattern.

Author: nnethercote

compiler/rustc_mir_build/src/build/block.rs

@@ -160,7 +160,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                                 ArmHasGuard(false),
                                                 Some((None, initializer_span)),
                                             );
-                                            this.expr_into_pattern(block, (**pattern).clone(), init) // irrefutable pattern
+                                            this.expr_into_pattern(block, pattern, init) // irrefutable pattern
                                         }
                                     })
                                 },

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -490,7 +490,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(super) fn expr_into_pattern(
         &mut self,
         mut block: BasicBlock,
-        irrefutable_pat: Pat<'tcx>,
+        irrefutable_pat: &Pat<'tcx>,
         initializer: &Expr<'tcx>,
     ) -> BlockAnd<()> {
         match irrefutable_pat.kind {
@@ -525,7 +525,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             },
                         ..
                     },
-                ascription: thir::Ascription { annotation, variance: _ },
+                ascription: thir::Ascription { ref annotation, variance: _ },
             } => {
                 let place =
                     self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true);
@@ -538,7 +538,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
                 let ty_source_info = self.source_info(annotation.span);
 
-                let base = self.canonical_user_type_annotations.push(annotation);
+                let base = self.canonical_user_type_annotations.push(annotation.clone());
                 self.cfg.push(
                     block,
                     Statement {
@@ -578,7 +578,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn place_into_pattern(
         &mut self,
         block: BasicBlock,
-        irrefutable_pat: Pat<'tcx>,
+        irrefutable_pat: &Pat<'tcx>,
         initializer: PlaceBuilder<'tcx>,
         set_match_place: bool,
     ) -> BlockAnd<()> {

compiler/rustc_mir_build/src/build/mod.rs

@@ -1052,7 +1052,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         Some((Some(&place), span)),
                     );
                     let place_builder = PlaceBuilder::from(local);
-                    unpack!(block = self.place_into_pattern(block, *pattern, place_builder, false));
+                    unpack!(
+                        block =
+                            self.place_into_pattern(block, pattern.as_ref(), place_builder, false)
+                    );
                 }
             }
             self.source_scope = original_source_scope;