Adds a dev guide section on Rust Coverage

Author: richkadel

src/SUMMARY.md

@@ -147,6 +147,7 @@
     - [Backend Agnostic Codegen](./backend/backend-agnostic.md)
     - [Implicit Caller Location](./backend/implicit-caller-location.md)
 - [Profile-guided Optimization](./profile-guided-optimization.md)
+- [LLVM Source-Based Code Coverage](./llvm-coverage-instrumentation.md)
 - [Sanitizers Support](./sanitizers.md)
 - [Debugging Support in the Rust Compiler](./debugging-support-in-rustc.md)
 

src/img/coverage-branch-counting-01.png

@@ -29,7 +29,8 @@ LLVM actually [supports multiple forms][clang-pgo] of PGO:
 - GCOV-based profiling, where code coverage infrastructure is used to collect
   profiling information.
 - Front-end based instrumentation, where the compiler front-end (e.g. Clang)
-  inserts instrumentation intrinsics into the LLVM IR it generates.
+  inserts instrumentation intrinsics into the LLVM IR it generates (but see the
+  [^note-instrument-coverage]"Note").
 - IR-level instrumentation, where LLVM inserts the instrumentation intrinsics
   itself during optimization passes.
 
@@ -44,6 +45,11 @@ optimized. Instrumentation-based PGO has two components: a compile-time
 component and run-time component, and one needs to understand the overall
 workflow to see how they interact.
 
+[^note-instrument-coverage]: Note: `rustc` now supports front-end-based coverage
+instrumentation, via the experimental option
+[`-Z instrument-coverage`](../llvm-coverage-instrumentation), but using these
+coverage results for PGO has not been attempted at this time.
+
 ### Overall Workflow
 
 Generating a PGO-optimized program involves the following four steps: