coverage: Attach an optional FunctionCoverageInfo to mir::Body

This allows coverage information to be attached to the function as a whole when
appropriate, instead of being smuggled through coverage statements in the
function's basic blocks.

As an example, this patch moves the `function_source_hash` value out of
individual `CoverageKind::Counter` statements and into the per-function info.

When synthesizing unused functions for coverage purposes, the absence of this
info is taken to indicate that a function was not eligible for coverage and
should not be synthesized.

compiler/rustc_middle/src/mir/coverage.rs

@@ -60,7 +60,6 @@ impl Debug for Operand {
 #[derive(Clone, PartialEq, TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
 pub enum CoverageKind {
     Counter {
-        function_source_hash: u64,
         /// ID of this counter within its enclosing function.
         /// Expressions in the same function can refer to it as an operand.
         id: CounterId,
@@ -80,7 +79,7 @@ impl Debug for CoverageKind {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
         use CoverageKind::*;
         match self {
-            Counter { id, .. } => write!(fmt, "Counter({:?})", id.index()),
+            Counter { id } => write!(fmt, "Counter({:?})", id.index()),
             Expression { id, lhs, op, rhs } => write!(
                 fmt,
                 "Expression({:?}) = {:?} {} {:?}",
@@ -133,3 +132,12 @@ impl Op {
         matches!(self, Self::Subtract)
     }
 }
+
+/// Stores per-function coverage information attached to a `mir::Body`,
+/// to be used in conjunction with the individual coverage statements injected
+/// into the function's basic blocks.
+#[derive(Clone, Debug)]
+#[derive(TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable, TypeVisitable)]
+pub struct FunctionCoverageInfo {
+    pub function_source_hash: u64,
+}