add a type alias for the pattern bindings stack

I'll be modifying it in future commits, so I think it's cleanest to
abstract it out. Possibly a newtype would be ideal, but for now this is
least disruptive.

Author: dianne

compiler/rustc_resolve/src/late.rs

@@ -111,6 +111,17 @@ enum PatBoundCtx {
 Or,
 }
 
+/// Tracks bindings resolved within a pattern. This serves two purposes:
+///
+/// - This tracks when identifiers are bound multiple times within a pattern. In a product context,
+/// this is an error. In an or-pattern, this lets us reuse the same resolution for each instance.
+/// See `fresh_binding` and `resolve_pattern_inner` for more information.
+///
+/// - The guard expression of a guard pattern may use bindings from within the guard pattern, but
+/// not from elsewhere in the pattern containing it. This allows us to isolate the bindings in the
+/// subpattern to construct the scope for the guard.
+typePatternBindings = SmallVec<[(PatBoundCtx,FxHashSet<Ident>);1]>;
+
 /// Does this the item (from the item rib scope) allow generic parameters?
 #[derive(Copy,Clone,Debug)]
 pub(crate)enumHasGenericParams{
@@ -3857,7 +3868,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 &mutself,
 pat:&'astPat,
 pat_src:PatternSource,
-bindings:&mutSmallVec<[(PatBoundCtx,FxHashSet<Ident>);1]>,
+bindings:&mutPatternBindings,
 ){
 // We walk the pattern before declaring the pattern's inner bindings,
 // so that we avoid resolving a literal expression to a binding defined
@@ -3892,7 +3903,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 &mutself,
 pat:&Pat,
 pat_src:PatternSource,
-bindings:&mutSmallVec<[(PatBoundCtx,FxHashSet<Ident>);1]>,
+bindings:&mutPatternBindings,
 ){
 // Visit all direct subpatterns of this pattern.
  pat.walk(&mut |pat| {
@@ -3988,7 +3999,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 ident:Ident,
 pat_id:NodeId,
 pat_src:PatternSource,
-bindings:&mutSmallVec<[(PatBoundCtx,FxHashSet<Ident>);1]>,
+bindings:&mutPatternBindings,
 ) -> Res{
 // Add the binding to the local ribs, if it doesn't already exist in the bindings map.
 // (We must not add it if it's in the bindings map because that breaks the assumptions