coverage: Add -Zcoverage-options for fine control of coverage

This new nightly-only flag can be used to toggle fine-grained flags that
control the details of coverage instrumentation.

Currently the only supported flag value is `branch` (or `no-branch`), which is
a placeholder for upcoming support for branch coverage. Other flag values can
be added in the future, to prototype proposed new behaviour, or to enable
special non-default behaviour.

compiler/rustc_session/src/config.rs

@@ -143,6 +143,19 @@ pub enum InstrumentCoverage {
     Yes,
 }
 
+/// Individual flag values controlled by `-Z coverage-options`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct CoverageOptions {
+    /// Add branch coverage instrumentation (placeholder flag; not yet implemented).
+    pub branch: bool,
+}
+
+impl Default for CoverageOptions {
+    fn default() -> Self {
+        Self { branch: false }
+    }
+}
+
 /// Settings for `-Z instrument-xray` flag.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
 pub struct InstrumentXRay {
@@ -3168,12 +3181,12 @@ pub enum WasiExecModel {
 /// how the hash should be calculated when adding a new command-line argument.
 pub(crate) mod dep_tracking {
     use super::{
-        BranchProtection, CFGuard, CFProtection, CollapseMacroDebuginfo, CrateType, DebugInfo,
-        DebugInfoCompression, ErrorOutputType, FunctionReturn, InliningThreshold,
-        InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail, LtoCli,
-        NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes, Polonius,
-        RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm, SplitDwarfKind,
-        SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
+        BranchProtection, CFGuard, CFProtection, CollapseMacroDebuginfo, CoverageOptions,
+        CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FunctionReturn,
+        InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
+        LtoCli, NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes,
+        Polonius, RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm,
+        SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
     };
     use crate::lint;
     use crate::utils::NativeLib;
@@ -3243,6 +3256,7 @@ pub(crate) mod dep_tracking {
         CodeModel,
         TlsModel,
         InstrumentCoverage,
+        CoverageOptions,
         InstrumentXRay,
         CrateType,
         MergeFunctions,

compiler/rustc_session/src/options.rs

@@ -396,6 +396,7 @@ 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 = "`branch` or `no-branch`";
     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";
@@ -938,6 +939,24 @@ mod parse {
         true
     }
 
+    pub(crate) fn parse_coverage_options(slot: &mut CoverageOptions, v: Option<&str>) -> bool {
+        let Some(v) = v else { return true };
+
+        for option in v.split(',') {
+            let (option, enabled) = match option.strip_prefix("no-") {
+                Some(without_no) => (without_no, false),
+                None => (option, true),
+            };
+            let slot = match option {
+                "branch" => &mut slot.branch,
+                _ => return false,
+            };
+            *slot = enabled;
+        }
+
+        true
+    }
+
     pub(crate) fn parse_instrument_xray(
         slot: &mut Option<InstrumentXRay>,
         v: Option<&str>,
@@ -1564,6 +1583,8 @@ options! {
         "set option to collapse debuginfo for macros"),
     combine_cgu: bool = (false, parse_bool, [TRACKED],
         "combine CGUs into a single one"),
+    coverage_options: CoverageOptions = (CoverageOptions::default(), parse_coverage_options, [TRACKED],
+        "control details of coverage instrumentation"),
     crate_attr: Vec<String> = (Vec::new(), parse_string_push, [TRACKED],
         "inject the given attribute in the crate"),
     cross_crate_inline_threshold: InliningThreshold = (InliningThreshold::Sometimes(100), parse_inlining_threshold, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -352,6 +352,10 @@ impl Session {
         self.opts.cg.instrument_coverage() != InstrumentCoverage::No
     }
 
+    pub fn instrument_coverage_branch(&self) -> bool {
+        self.instrument_coverage() && self.opts.unstable_opts.coverage_options.branch
+    }
+
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {
         self.opts.unstable_opts.sanitizer.contains(SanitizerSet::CFI)
     }