coverage: Add debugging flag `-Zcoverage-options=no-mir-spans`

When set, this flag skips the code that normally extracts coverage spans from
MIR statements and terminators. That sometimes makes it easier to debug branch
coverage and MC/DC coverage, because the coverage output is less noisy.

For internal debugging only. If other code changes would make it hard to keep
supporting this flag, remove it.

3 files changed, 16 insertions, 2 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index a622f1b577d..5f9c3a14d60 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -149,7 +149,14 @@ pub enum InstrumentCoverage {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default)]
 pub struct CoverageOptions {
     pub level: CoverageLevel,
-    // Other boolean or enum-valued options might be added here.
+
+    /// `-Z coverage-options=no-mir-spans`: Don't extract block coverage spans
+    /// from MIR statements/terminators, making it easier to inspect/debug
+    /// branch and MC/DC coverage mappings.
+    ///
+    /// For internal debugging only. If other code changes would make it hard
+    /// to keep supporting this flag, remove it.
+    pub no_mir_spans: bool,
 }
 
 /// Controls whether branch coverage or MC/DC coverage is enabled.
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index fd4a3a9e6ce..145af50117c 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -395,7 +395,8 @@ mod desc {
     pub const parse_optimization_fuel: &str = "crate=integer";
     pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str = parse_bool;
-    pub const parse_coverage_options: &str = "`block` | `branch` | `condition` | `mcdc`";
+    pub const parse_coverage_options: &str =
+        "`block` | `branch` | `condition` | `mcdc` | `no-mir-spans`";
     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";
@@ -963,6 +964,7 @@ mod parse {
                 "branch" => slot.level = CoverageLevel::Branch,
                 "condition" => slot.level = CoverageLevel::Condition,
                 "mcdc" => slot.level = CoverageLevel::Mcdc,
+                "no-mir-spans" => slot.no_mir_spans = true,
                 _ => return false,
             }
         }
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 87bbfcf07c8..20e50ef1b4a 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -363,6 +363,11 @@ impl Session {
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Mcdc
     }
 
+    /// True if `-Zcoverage-options=no-mir-spans` was passed.
+    pub fn coverage_no_mir_spans(&self) -> bool {
+        self.opts.unstable_opts.coverage_options.no_mir_spans
+    }
+
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {
         self.opts.unstable_opts.sanitizer.contains(SanitizerSet::CFI)
     }