coverage: Store all of a function's mappings in function coverage info

Previously, mappings were attached to individual coverage statements in MIR.
That necessitated special handling in MIR optimizations to avoid deleting those
statements, since otherwise codegen would be unable to reassemble the original
list of mappings.

With this change, a function's list of mappings is now attached to its MIR
body, and survives intact even if individual statements are deleted by
optimizations.

compiler/rustc_middle/src/mir/coverage.rs

@@ -61,11 +61,13 @@ impl Debug for CovTerm {
 
 #[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub enum CoverageKind {
-    Counter {
-        /// ID of this counter within its enclosing function.
-        /// Expressions in the same function can refer to it as an operand.
-        id: CounterId,
-    },
+    /// Marks the point in MIR control flow represented by a coverage counter.
+    ///
+    /// This is eventually lowered to `llvm.instrprof.increment` in LLVM IR.
+    ///
+    /// If this statement does not survive MIR optimizations, any mappings that
+    /// refer to this counter can have those references simplified to zero.
+    CounterIncrement { id: CounterId },
     Expression {
         /// ID of this coverage-counter expression within its enclosing function.
         /// Other expressions in the same function can refer to it as an operand.
@@ -74,14 +76,13 @@ pub enum CoverageKind {
         op: Op,
         rhs: CovTerm,
     },
-    Unreachable,
 }
 
 impl Debug for CoverageKind {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
         use CoverageKind::*;
         match self {
-            Counter { id } => write!(fmt, "Counter({:?})", id.index()),
+            CounterIncrement { id } => write!(fmt, "CounterIncrement({:?})", id.index()),
             Expression { id, lhs, op, rhs } => write!(
                 fmt,
                 "Expression({:?}) = {:?} {} {:?}",
@@ -93,7 +94,6 @@ impl Debug for CoverageKind {
                 },
                 rhs,
             ),
-            Unreachable => write!(fmt, "Unreachable"),
         }
     }
 }
@@ -158,4 +158,6 @@ pub struct FunctionCoverageInfo {
     pub function_source_hash: u64,
     pub num_counters: usize,
     pub num_expressions: usize,
+
+    pub mappings: Vec<Mapping>,
 }