Rollup merge of #122226 - Zalathar:zcoverage-options, r=nnethercote

coverage: Remove or migrate all unstable values of `-Cinstrument-coverage`

(This PR was substantially overhauled from its original version, which migrated all of the existing unstable values intact.)

This PR takes the three nightly-only values that are currently accepted by `-Cinstrument-coverage`, completely removes two of them (`except-unused-functions` and `except-unused-generics`), and migrates the third (`branch`) over to a newly-introduced unstable flag `-Zcoverage-options`.

I have a few motivations for wanting to do this:

- It's unclear whether anyone actually uses the `except-unused-*` values, so this serves as an opportunity to either remove them, or prompt existing users to object to their removal.
- After #117199, the stable values of `-Cinstrument-coverage` treat it as a boolean-valued flag, so having nightly-only extra values feels out-of-place.
  - Nightly-only values also require extra ad-hoc code to make sure they aren't accidentally exposed to stable users.
- The new system allows multiple different settings to be toggled independently, which isn't possible in the current single-value system.
- The new system makes it easier to introduce new behaviour behind an unstable toggle, and then gather nightly-user feedback before possibly making it the default behaviour for all users.
- The new system also gives us a convenient place to put relatively-narrow options that won't ever be the default, but that nightly users might still want access to.
- It's likely that we will eventually want to give stable users more fine-grained control over coverage instrumentation. The new flag serves as a prototype of what that stable UI might eventually look like.

The `branch` option is a placeholder that currently does nothing. It will be used by #122322 to opt into branch coverage instrumentation.

---

I see `-Zcoverage-options` as something that will exist more-or-less indefinitely, though individual sub-options might come and go as appropriate. I think there will always be some demand for nightly-only toggles, so I don't see `-Zcoverage-options` itself ever being stable, though we might eventually stabilize something similar to it.

compiler/rustc_session/src/config.rs

@@ -134,31 +134,26 @@ pub enum LtoCli {
 /// and higher). Nevertheless, there are many variables, depending on options
 /// selected, code structure, and enabled attributes. If errors are encountered,
 /// either while compiling or when generating `llvm-cov show` reports, consider
-/// lowering the optimization level, including or excluding `-C link-dead-code`,
-/// or using `-Zunstable-options -C instrument-coverage=except-unused-functions`
-/// or `-Zunstable-options -C instrument-coverage=except-unused-generics`.
-///
-/// Note that `ExceptUnusedFunctions` means: When `mapgen.rs` generates the
-/// coverage map, it will not attempt to generate synthetic functions for unused
-/// (and not code-generated) functions (whether they are generic or not). As a
-/// result, non-codegenned functions will not be included in the coverage map,
-/// and will not appear, as covered or uncovered, in coverage reports.
-///
-/// `ExceptUnusedGenerics` will add synthetic functions to the coverage map,
-/// unless the function has type parameters.
+/// lowering the optimization level, or including/excluding `-C link-dead-code`.
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum InstrumentCoverage {
     /// `-C instrument-coverage=no` (or `off`, `false` etc.)
     No,
     /// `-C instrument-coverage` or `-C instrument-coverage=yes`
     Yes,
-    /// 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`
-    ExceptUnusedFunctions,
+}
+
+/// 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.
@@ -2718,24 +2713,6 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         }
     }
 
-    // Check for unstable values of `-C instrument-coverage`.
-    // This is what prevents them from being used on stable compilers.
-    match cg.instrument_coverage {
-        // Stable values:
-        InstrumentCoverage::Yes | InstrumentCoverage::No => {}
-        // Unstable values:
-        InstrumentCoverage::Branch
-        | InstrumentCoverage::ExceptUnusedFunctions
-        | InstrumentCoverage::ExceptUnusedGenerics => {
-            if !unstable_opts.unstable_options {
-                early_dcx.early_fatal(
-                    "`-C instrument-coverage=branch` and `-C instrument-coverage=except-*` \
-                    require `-Z unstable-options`",
-                );
-            }
-        }
-    }
-
     if cg.instrument_coverage != InstrumentCoverage::No {
         if cg.profile_generate.enabled() || cg.profile_use.is_some() {
             early_dcx.early_fatal(
@@ -3204,12 +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;
@@ -3279,6 +3256,7 @@ pub(crate) mod dep_tracking {
         CodeModel,
         TlsModel,
         InstrumentCoverage,
+        CoverageOptions,
         InstrumentXRay,
         CrateType,
         MergeFunctions,

compiler/rustc_session/src/options.rs

@@ -395,7 +395,8 @@ mod desc {
     pub const parse_linker_flavor: &str = ::rustc_target::spec::LinkerFlavorCli::one_of();
     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 = "either a boolean (`yes`, `no`, `on`, `off`, etc) or (unstable) one of `branch`, `except-unused-generics`, `except-unused-functions`";
+    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";
@@ -928,21 +929,34 @@ mod parse {
             return true;
         };
 
+        // Parse values that have historically been accepted by stable compilers,
+        // even though they're currently just aliases for boolean values.
         *slot = match v {
             "all" => InstrumentCoverage::Yes,
-            "branch" => InstrumentCoverage::Branch,
-            "except-unused-generics" | "except_unused_generics" => {
-                InstrumentCoverage::ExceptUnusedGenerics
-            }
-            "except-unused-functions" | "except_unused_functions" => {
-                InstrumentCoverage::ExceptUnusedFunctions
-            }
             "0" => InstrumentCoverage::No,
             _ => return false,
         };
         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>,
@@ -1445,14 +1459,9 @@ options! {
         "set the threshold for inlining a function"),
     #[rustc_lint_opt_deny_field_access("use `Session::instrument_coverage` instead of this field")]
     instrument_coverage: InstrumentCoverage = (InstrumentCoverage::No, parse_instrument_coverage, [TRACKED],
-        "instrument the generated code to support LLVM source-based code coverage \
-        reports (note, the compiler build config must include `profiler = true`); \
-        implies `-C symbol-mangling-version=v0`. Optional values are:
-        `=no` `=n` `=off` `=false` (default)
-        `=yes` `=y` `=on` `=true` (implicit value)
-        `=branch` (unstable)
-        `=except-unused-generics` (unstable)
-        `=except-unused-functions` (unstable)"),
+        "instrument the generated code to support LLVM source-based code coverage reports \
+        (note, the compiler build config must include `profiler = true`); \
+        implies `-C symbol-mangling-version=v0`"),
     link_arg: (/* redirected to link_args */) = ((), parse_string_push, [UNTRACKED],
         "a single extra argument to append to the linker invocation (can be used several times)"),
     link_args: Vec<String> = (Vec::new(), parse_list, [UNTRACKED],
@@ -1574,6 +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

@@ -353,15 +353,7 @@ impl Session {
     }
 
     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
-    }
-
-    pub fn instrument_coverage_except_unused_functions(&self) -> bool {
-        self.opts.cg.instrument_coverage() == InstrumentCoverage::ExceptUnusedFunctions
+        self.instrument_coverage() && self.opts.unstable_opts.coverage_options.branch
     }
 
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {