Combination of commits

Fixes multiple issue with counters, with simplification

  Includes a change to the implicit else span in ast_lowering, so coverage
  of the implicit else no longer spans the `then` block.

  Adds coverage for unused closures and async function bodies.

  Fixes: #78542

Adding unreachable regions for known MIR missing from coverage map

Cleaned up PR commits, and removed link-dead-code requirement and tests

  Coverage no longer depends on Issue #76038 (`-C link-dead-code` is
  no longer needed or enforced, so MSVC can use the same tests as
  Linux and MacOS now)

Restrict adding unreachable regions to covered files

  Improved the code that adds coverage for uncalled functions (with MIR
  but not-codegenned) to avoid generating coverage in files not already
  included in the files with covered functions.

Resolved last known issue requiring --emit llvm-ir workaround

  Fixed bugs in how unreachable code spans were added.

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

@@ -118,18 +118,8 @@ impl CoverageGraph {
 
             match term.kind {
                 TerminatorKind::Return { .. }
-                // FIXME(richkadel): Add test(s) for `Abort` coverage.
                 | TerminatorKind::Abort
-                // FIXME(richkadel): Add test(s) for `Assert` coverage.
-                // Should `Assert` be handled like `FalseUnwind` instead? Since we filter out unwind
-                // branches when creating the BCB CFG, aren't `Assert`s (without unwinds) just like
-                // `FalseUnwinds` (which are kind of like `Goto`s)?
-                | TerminatorKind::Assert { .. }
-                // FIXME(richkadel): Add test(s) for `Yield` coverage, and confirm coverage is
-                // sensible for code using the `yield` keyword.
                 | TerminatorKind::Yield { .. }
-                // FIXME(richkadel): Also add coverage tests using async/await, and threading.
-
                 | TerminatorKind::SwitchInt { .. } => {
                     // The `bb` has more than one _outgoing_ edge, or exits the function. Save the
                     // current sequence of `basic_blocks` gathered to this point, as a new
@@ -147,6 +137,14 @@ impl CoverageGraph {
                     // `Terminator`s `successors()` list) checking the number of successors won't
                     // work.
                 }
+
+                // The following `TerminatorKind`s are either not expected outside an unwind branch,
+                // or they should not (under normal circumstances) branch. Coverage graphs are
+                // simplified by assuring coverage results are accurate for well-behaved programs.
+                // Programs that panic and unwind may record slightly inaccurate coverage results
+                // for a coverage region containing the `Terminator` that began the panic. This
+                // is as intended. (See Issue #78544 for a possible future option to support
+                // coverage in test programs that panic.)
                 TerminatorKind::Goto { .. }
                 | TerminatorKind::Resume
                 | TerminatorKind::Unreachable
@@ -154,6 +152,7 @@ impl CoverageGraph {
                 | TerminatorKind::DropAndReplace { .. }
                 | TerminatorKind::Call { .. }
                 | TerminatorKind::GeneratorDrop
+                | TerminatorKind::Assert { .. }
                 | TerminatorKind::FalseEdge { .. }
                 | TerminatorKind::FalseUnwind { .. }
                 | TerminatorKind::InlineAsm { .. } => {}
@@ -278,6 +277,7 @@ rustc_index::newtype_index! {
     /// A node in the [control-flow graph][CFG] of CoverageGraph.
     pub(super) struct BasicCoverageBlock {
         DEBUG_FORMAT = "bcb{}",
+        const START_BCB = 0,
     }
 }