Auto merge of #124507 - Zalathar:coverage-level, r=compiler-errors

coverage: Replace boolean options with a `CoverageLevel` enum

After #123409, and some discussion at https://github.com/rust-lang/rust/issues/79649#issuecomment-2042093553 and #124120, it became clear to me that we should have a unified concept of “coverage level”, instead of having several separate boolean flags that aren't actually independent.

This PR therefore introduces a `CoverageLevel` enum, to replace the existing boolean flags for `branch` and `mcdc`.

The `no-branch` value (for `-Zcoverage-options`) has been renamed to `block`, instructing the compiler to only instrument for block coverage, with no branch coverage or MD/DC instrumentation.

`@rustbot` label +A-code-coverage
cc `@ZhuUx` `@Lambdaris` `@RenjiSann`

3 files changed, 30 insertions, 18 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 4f259960ac3..a3978cc14a4 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -146,10 +146,26 @@ pub enum InstrumentCoverage {
 /// Individual flag values controlled by `-Z coverage-options`.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default)]
 pub struct CoverageOptions {
-    /// Add branch coverage instrumentation.
-    pub branch: bool,
-    /// Add mcdc coverage instrumentation.
-    pub mcdc: bool,
+    pub level: CoverageLevel,
+    // Other boolean or enum-valued options might be added here.
+}
+
+/// Controls whether branch coverage or MC/DC coverage is enabled.
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+pub enum CoverageLevel {
+    /// Instrument for coverage at the MIR block level.
+    Block,
+    /// Also instrument branch points (includes block coverage).
+    Branch,
+    /// Instrument for MC/DC. Mostly a superset of branch coverage, but might
+    /// differ in some corner cases.
+    Mcdc,
+}
+
+impl Default for CoverageLevel {
+    fn default() -> Self {
+        Self::Block
+    }
 }
 
 /// Settings for `-Z instrument-xray` flag.
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index e1474ee365b..7355e5b6953 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -394,7 +394,7 @@ 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 = "either  `no-branch`, `branch` or `mcdc`";
+    pub const parse_coverage_options: &str = "`block` | `branch` | `mcdc`";
     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";
@@ -946,15 +946,9 @@ mod parse {
 
         for option in v.split(',') {
             match option {
-                "no-branch" => {
-                    slot.branch = false;
-                    slot.mcdc = false;
-                }
-                "branch" => slot.branch = true,
-                "mcdc" => {
-                    slot.branch = true;
-                    slot.mcdc = true;
-                }
+                "block" => slot.level = CoverageLevel::Block,
+                "branch" => slot.level = CoverageLevel::Branch,
+                "mcdc" => slot.level = CoverageLevel::Mcdc,
                 _ => return false,
             }
         }
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 2bc14b43234..db01bb90d6f 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1,8 +1,8 @@
 use crate::code_stats::CodeStats;
 pub use crate::code_stats::{DataTypeKind, FieldInfo, FieldKind, SizeKind, VariantInfo};
 use crate::config::{
-    self, CrateType, FunctionReturn, InstrumentCoverage, OptLevel, OutFileName, OutputType,
-    RemapPathScopeComponents, SwitchWithOptPath,
+    self, CoverageLevel, CrateType, FunctionReturn, InstrumentCoverage, OptLevel, OutFileName,
+    OutputType, RemapPathScopeComponents, SwitchWithOptPath,
 };
 use crate::config::{ErrorOutputType, Input};
 use crate::errors;
@@ -349,11 +349,13 @@ impl Session {
     }
 
     pub fn instrument_coverage_branch(&self) -> bool {
-        self.instrument_coverage() && self.opts.unstable_opts.coverage_options.branch
+        self.instrument_coverage()
+            && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Branch
     }
 
     pub fn instrument_coverage_mcdc(&self) -> bool {
-        self.instrument_coverage() && self.opts.unstable_opts.coverage_options.mcdc
+        self.instrument_coverage()
+            && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Mcdc
     }
 
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {