Fix typos “a”→“an”

compiler/rustc_mir/src/transform/coverage/counters.rs

@@ -100,7 +100,7 @@ impl CoverageCounters {
         CounterValueReference::from(next)
     }
 
-    /// Expression IDs start from u32::MAX and go down because a Expression can reference
+    /// Expression IDs start from u32::MAX and go down because an Expression can reference
     /// (add or subtract counts) of both Counter regions and Expression regions. The counter
     /// expression operand IDs must be unique across both types.
     fn next_expression(&mut self) -> InjectedExpressionId {

compiler/rustc_mir/src/transform/coverage/mod.rs

@@ -334,7 +334,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
     /// process (via `take_counter()`).
     ///
     /// Any other counter associated with a `BasicCoverageBlock`, or its incoming edge, but not
-    /// associated with a `CoverageSpan`, should only exist if the counter is a `Expression`
+    /// associated with a `CoverageSpan`, should only exist if the counter is an `Expression`
     /// dependency (one of the expression operands). Collect them, and inject the additional
     /// counters into the MIR, without a reportable coverage span.
     fn inject_indirect_counters(

compiler/rustc_mir/src/transform/rustc_peek.rs

@@ -88,7 +88,7 @@ impl<'tcx> MirPass<'tcx> for SanityCheck {
 /// For each such call, determines what the dataflow bit-state is for
 /// the L-value corresponding to `expr`; if the bit-state is a 1, then
 /// that call to `rustc_peek` is ignored by the sanity check. If the
-/// bit-state is a 0, then this pass emits a error message saying
+/// bit-state is a 0, then this pass emits an error message saying
 /// "rustc_peek: bit not set".
 ///
 /// The intention is that one can write unit tests for dataflow by