coverage: Store the number of counters/expressions in function coverage info

Coverage codegen can now allocate arrays based on the number of
counters/expressions originally used by the instrumentor.

The existing query that inspects coverage statements is still used for
determining the number of counters passed to `llvm.instrprof.increment`. If
some high-numbered counters were removed by MIR optimizations, the instrumented
binary can potentially use less memory and disk space at runtime.

compiler/rustc_middle/src/mir/coverage.rs

@@ -140,4 +140,6 @@ impl Op {
 #[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub struct FunctionCoverageInfo {
     pub function_source_hash: u64,
+    pub num_counters: usize,
+    pub num_expressions: usize,
 }

compiler/rustc_middle/src/mir/query.rs

@@ -1,5 +1,6 @@
 //! Values computed by queries that use MIR.
 
+use crate::mir;
 use crate::ty::{self, OpaqueHiddenType, Ty, TyCtxt};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::unord::UnordSet;
@@ -445,14 +446,19 @@ pub struct DestructuredConstant<'tcx> {
     pub fields: &'tcx [(ConstValue<'tcx>, Ty<'tcx>)],
 }
 
-/// Coverage information summarized from a MIR if instrumented for source code coverage (see
-/// compiler option `-Cinstrument-coverage`). This information is generated by the
-/// `InstrumentCoverage` MIR pass and can be retrieved via the `coverageinfo` query.
+/// Summarizes coverage IDs inserted by the `InstrumentCoverage` MIR pass
+/// (for compiler option `-Cinstrument-coverage`), after MIR optimizations
+/// have had a chance to potentially remove some of them.
+///
+/// Used by the `coverage_ids_info` query.
 #[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
-pub struct CoverageInfo {
-    /// The total number of coverage region counters added to the MIR `Body`.
-    pub num_counters: u32,
-
-    /// The total number of coverage region counter expressions added to the MIR `Body`.
-    pub num_expressions: u32,
+pub struct CoverageIdsInfo {
+    /// Coverage codegen needs to know the highest counter ID that is ever
+    /// incremented within a function, so that it can set the `num-counters`
+    /// argument of the `llvm.instrprof.increment` intrinsic.
+    ///
+    /// This may be less than the highest counter ID emitted by the
+    /// InstrumentCoverage MIR pass, if the highest-numbered counter increments
+    /// were removed by MIR optimizations.
+    pub max_counter_id: mir::coverage::CounterId,
 }