Auto merge of #116094 - Swatinem:coverage-branch-gate, r=wesleywiser

Introduce `-C instrument-coverage=branch` to gate branch coverage

This was extracted from #115061 and can land independently from other coverage related work.

The flag is unused for now, but is added in advance of adding branch coverage support.
It is an unstable, nightly only flag that needs to be used in combination with `-Zunstable-options`, like so: `-Zunstable-options -C instrument-coverage=branch`.

The goal is to develop branch coverage as an unstable opt-in feature first, before it matures and can be turned on by default.

compiler/rustc_session/src/config.rs

@@ -169,6 +169,9 @@ pub enum MirSpanview {
 pub enum InstrumentCoverage {
     /// Default `-C instrument-coverage` or `-C instrument-coverage=statement`
     All,
+    /// Additionally, instrument branches and output branch coverage.
+    /// `-Zunstable-options -C instrument-coverage=branch`
+    Branch,
     /// `-Zunstable-options -C instrument-coverage=except-unused-generics`
     ExceptUnusedGenerics,
     /// `-Zunstable-options -C instrument-coverage=except-unused-functions`
@@ -2747,7 +2750,10 @@ pub fn build_session_options(
         }
         (Some(InstrumentCoverage::Off | InstrumentCoverage::All), _) => {}
         (Some(_), _) if !unstable_opts.unstable_options => {
-            handler.early_error("`-C instrument-coverage=except-*` requires `-Z unstable-options`");
+            handler.early_error(
+                "`-C instrument-coverage=branch` and `-C instrument-coverage=except-*` \
+                require `-Z unstable-options`",
+            );
         }
         (None, None) => {}
         (None, ic) => {

compiler/rustc_session/src/options.rs

@@ -389,7 +389,7 @@ mod desc {
     pub const parse_mir_spanview: &str = "`statement` (default), `terminator`, or `block`";
     pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str =
-        "`all` (default), `except-unused-generics`, `except-unused-functions`, or `off`";
+        "`all` (default), `branch`, `except-unused-generics`, `except-unused-functions`, or `off`";
     pub const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
     pub const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
@@ -931,6 +931,7 @@ mod parse {
 
         *slot = Some(match v {
             "all" => InstrumentCoverage::All,
+            "branch" => InstrumentCoverage::Branch,
             "except-unused-generics" | "except_unused_generics" => {
                 InstrumentCoverage::ExceptUnusedGenerics
             }
@@ -1356,6 +1357,7 @@ options! {
         reports (note, the compiler build config must include `profiler = true`); \
         implies `-C symbol-mangling-version=v0`. Optional values are:
         `=all` (implicit value)
+        `=branch`
         `=except-unused-generics`
         `=except-unused-functions`
         `=off` (default)"),
@@ -1597,6 +1599,7 @@ options! {
         reports (note, the compiler build config must include `profiler = true`); \
         implies `-C symbol-mangling-version=v0`. Optional values are:
         `=all` (implicit value)
+        `=branch`
         `=except-unused-generics`
         `=except-unused-functions`
         `=off` (default)"),

compiler/rustc_session/src/session.rs

@@ -702,6 +702,10 @@ impl Session {
         self.opts.cg.instrument_coverage() != InstrumentCoverage::Off
     }
 
+    pub fn instrument_coverage_branch(&self) -> bool {
+        self.opts.cg.instrument_coverage() == InstrumentCoverage::Branch
+    }
+
     pub fn instrument_coverage_except_unused_generics(&self) -> bool {
         self.opts.cg.instrument_coverage() == InstrumentCoverage::ExceptUnusedGenerics
     }