Rollup merge of #122937 - Zalathar:unbox, r=oli-obk

Unbox and unwrap the contents of `StatementKind::Coverage`

The payload of coverage statements was historically a structure with several fields, so it was boxed to avoid bloating `StatementKind`.

Now that the payload is a single relatively-small enum, we can replace `Box<Coverage>` with just `CoverageKind`.

This patch also adds a size assertion for `StatementKind`, to avoid accidentally bloating it in the future.

``@rustbot`` label +A-code-coverage

compiler/rustc_middle/src/mir/syntax.rs

@@ -373,7 +373,7 @@ pub enum StatementKind<'tcx> {
     ///
     /// Interpreters and codegen backends that don't support coverage instrumentation
     /// can usually treat this as a no-op.
-    Coverage(Box<Coverage>),
+    Coverage(CoverageKind),
 
     /// Denotes a call to an intrinsic that does not require an unwind path and always returns.
     /// This avoids adding a new block and a terminator for simple intrinsics.
@@ -517,12 +517,6 @@ pub enum FakeReadCause {
     ForIndex,
 }
 
-#[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
-#[derive(TypeFoldable, TypeVisitable)]
-pub struct Coverage {
-    pub kind: CoverageKind,
-}
-
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
 #[derive(TypeFoldable, TypeVisitable)]
 pub struct CopyNonOverlapping<'tcx> {
@@ -1458,5 +1452,6 @@ mod size_asserts {
     static_assert_size!(Place<'_>, 16);
     static_assert_size!(PlaceElem<'_>, 24);
     static_assert_size!(Rvalue<'_>, 40);
+    static_assert_size!(StatementKind<'_>, 16);
     // tidy-alphabetical-end
 }