Fix typos “a”→“an”

compiler/rustc_middle/src/traits/mod.rs

@@ -509,7 +509,7 @@ pub enum ImplSource<'tcx, N> {
     TraitUpcasting(ImplSourceTraitUpcastingData<'tcx, N>),
 
     /// ImplSource automatically generated for a closure. The `DefId` is the ID
-    /// of the closure expression. This is a `ImplSource::UserDefined` in spirit, but the
+    /// of the closure expression. This is an `ImplSource::UserDefined` in spirit, but the
     /// impl is generated by the compiler and does not appear in the source.
     Closure(ImplSourceClosureData<'tcx, N>),
 

compiler/rustc_middle/src/traits/select.rs

@@ -111,7 +111,7 @@ pub enum SelectionCandidate<'tcx> {
     ProjectionCandidate(usize),
 
     /// Implementation of a `Fn`-family trait by one of the anonymous types
-    /// generated for a `||` expression.
+    /// generated for an `||` expression.
     ClosureCandidate,
 
     /// Implementation of a `Generator` trait by one of the anonymous types