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

This flag has to be used in combination with `-Zunstable-options`,
and is added in advance of adding branch coverage instrumentation.

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
     }