Auto merge of #76004 - richkadel:llvm-coverage-map-gen-6b.5, r=tmandry

Tools, tests, and experimenting with MIR-derived coverage counters

Leverages the new mir_dump output file in HTML+CSS (from #76074) to visualize coverage code regions
and the MIR features that they came from (including overlapping spans).

See example below.

The `run-make-fulldeps/instrument-coverage` test has been refactored to maximize test coverage and reduce code duplication. The new tests support testing with and without `-Clink-dead-code`, so Rust coverage can be tested on MSVC (which, currently, only works with `link-dead-code` _disabled_).

New tests validate coverage region generation and coverage reports with multiple counters per function. Starting with a simple `if-else` branch tests, coverage tests for each additional syntax type can be added by simply dropping in a new Rust sample program.

Includes a basic, MIR-block-based implementation of coverage injection,
available via `-Zexperimental-coverage`. This implementation has known
flaws and omissions, but is simple enough to validate the new tools and
tests.

The existing `-Zinstrument-coverage` option currently enables
function-level coverage only, which at least appears to generate
accurate coverage reports at that level.

Experimental coverage is not accurate at this time. When branch coverage
works as intended, the `-Zexperimental-coverage` option should be
removed.

This PR replaces the bulk of PR #75828, with the remaining parts of
that PR distributed among other separate and indentpent PRs.

This PR depends on two of those other PRs: #76002, #76003 and #76074

Rust compiler MCP rust-lang/compiler-team#278

Relevant issue: #34701 - Implement support for LLVMs code coverage
instrumentation

![Screen-Recording-2020-08-21-at-2](https://user-images.githubusercontent.com/3827298/90972923-ff417880-e4d1-11ea-92bb-8713c6198f6d.gif)

r? @tmandry
FYI: @wesleywiser

2 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 65377b08820..19cd2385992 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1742,6 +1742,10 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         );
     }
 
+    if debugging_opts.experimental_coverage {
+        debugging_opts.instrument_coverage = true;
+    }
+
     if debugging_opts.instrument_coverage {
         if cg.profile_generate.enabled() || cg.profile_use.is_some() {
             early_error(
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index f502d43a0e0..ad36fa76986 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -893,6 +893,11 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         all statements)."),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
         "emit a section containing stack size metadata (default: no)"),
+    experimental_coverage: bool = (false, parse_bool, [TRACKED],
+        "enable and extend the `-Z instrument-coverage` function-level coverage \
+        feature, adding additional experimental (likely inaccurate) counters and \
+        code regions (used by `rustc` compiler developers to test new coverage \
+        counter placements) (default: no)"),
     fewer_names: bool = (false, parse_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),