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_codegen_llvm/src/coverageinfo/mapgen.rs

@@ -355,21 +355,20 @@ fn add_unused_functions(cx: &CodegenCx<'_, '_>) {
 
     let tcx = cx.tcx;
 
-    let ignore_unused_generics = tcx.sess.instrument_coverage_except_unused_generics();
-
     let eligible_def_ids = tcx.mir_keys(()).iter().filter_map(|local_def_id| {
         let def_id = local_def_id.to_def_id();
         let kind = tcx.def_kind(def_id);
         // `mir_keys` will give us `DefId`s for all kinds of things, not
         // just "functions", like consts, statics, etc. Filter those out.
-        // If `ignore_unused_generics` was specified, filter out any
-        // generic functions from consideration as well.
         if !matches!(kind, DefKind::Fn | DefKind::AssocFn | DefKind::Closure) {
             return None;
         }
-        if ignore_unused_generics && tcx.generics_of(def_id).requires_monomorphization(tcx) {
-            return None;
-        }
+
+        // FIXME(79651): Consider trying to filter out dummy instantiations of
+        // unused generic functions from library crates, because they can produce
+        // "unused instantiation" in coverage reports even when they are actually
+        // used by some downstream crate in the same binary.
+
         Some(local_def_id.to_def_id())
     });
 

compiler/rustc_interface/src/tests.rs

@@ -4,11 +4,12 @@ use rustc_data_structures::profiling::TimePassesFormat;
 use rustc_errors::{emitter::HumanReadableErrorType, registry, ColorConfig};
 use rustc_session::config::{
     build_configuration, build_session_options, rustc_optgroups, BranchProtection, CFGuard, Cfg,
-    CollapseMacroDebuginfo, DebugInfo, DumpMonoStatsFormat, ErrorOutputType, ExternEntry,
-    ExternLocation, Externs, FunctionReturn, InliningThreshold, Input, InstrumentCoverage,
-    InstrumentXRay, LinkSelfContained, LinkerPluginLto, LocationDetail, LtoCli, NextSolverConfig,
-    OomStrategy, Options, OutFileName, OutputType, OutputTypes, PAuthKey, PacRet, Passes, Polonius,
-    ProcMacroExecutionStrategy, Strip, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
+    CollapseMacroDebuginfo, CoverageOptions, DebugInfo, DumpMonoStatsFormat, ErrorOutputType,
+    ExternEntry, ExternLocation, Externs, FunctionReturn, InliningThreshold, Input,
+    InstrumentCoverage, InstrumentXRay, LinkSelfContained, LinkerPluginLto, LocationDetail, LtoCli,
+    NextSolverConfig, OomStrategy, Options, OutFileName, OutputType, OutputTypes, PAuthKey, PacRet,
+    Passes, Polonius, ProcMacroExecutionStrategy, Strip, SwitchWithOptPath, SymbolManglingVersion,
+    WasiExecModel,
 };
 use rustc_session::lint::Level;
 use rustc_session::search_paths::SearchPath;
@@ -750,6 +751,7 @@ fn test_unstable_options_tracking_hash() {
     );
     tracked!(codegen_backend, Some("abc".to_string()));
     tracked!(collapse_macro_debuginfo, CollapseMacroDebuginfo::Yes);
+    tracked!(coverage_options, CoverageOptions { branch: true });
     tracked!(crate_attr, vec!["abc".to_string()]);
     tracked!(cross_crate_inline_threshold, InliningThreshold::Always);
     tracked!(debug_info_for_profiling, true);

compiler/rustc_monomorphize/src/partitioning.rs

@@ -175,9 +175,7 @@ where
     }
 
     // Mark one CGU for dead code, if necessary.
-    let instrument_dead_code =
-        tcx.sess.instrument_coverage() && !tcx.sess.instrument_coverage_except_unused_functions();
-    if instrument_dead_code {
+    if tcx.sess.instrument_coverage() {
         mark_code_coverage_dead_code_cgu(&mut codegen_units);
     }
 

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 {

src/doc/rustc/src/instrument-coverage.md

@@ -346,14 +346,13 @@ $ llvm-cov report \
   more fine-grained coverage options are added.
   Using this value is currently not recommended.
 
-### Unstable values
+## `-Z coverage-options=<options>`
 
-- `-Z unstable-options -C instrument-coverage=branch`:
-  Placeholder for potential branch coverage support in the future.
-- `-Z unstable-options -C instrument-coverage=except-unused-generics`:
-  Instrument all functions except unused generics.
-- `-Z unstable-options -C instrument-coverage=except-unused-functions`:
-  Instrument only used (called) functions and instantiated generic functions.
+This unstable option provides finer control over some aspects of coverage
+instrumentation. Pass one or more of the following values, separated by commas.
+
+- `branch` or `no-branch`
+  - Placeholder for potential branch coverage support in the future.
 
 ## Other references
 

src/doc/unstable-book/src/compiler-flags/coverage-options.md

@@ -0,0 +1,8 @@
+# `coverage-options`
+
+This option controls details of the coverage instrumentation performed by
+`-C instrument-coverage`.
+
+Multiple options can be passed, separated by commas. Valid options are:
+
+- `branch` or `no-branch`: Placeholder for future branch coverage support.

tests/coverage/auxiliary/used_crate.rs

@@ -76,13 +76,8 @@ fn use_this_lib_crate() {
 // ```
 //
 // The notice is triggered because the function is unused by the library itself,
-// and when the library is compiled, a synthetic function is generated, so
-// unused function coverage can be reported. Coverage can be skipped for unused
-// generic functions with:
-//
-// ```shell
-// $ `rustc -Zunstable-options -C instrument-coverage=except-unused-generics ...`
-// ```
+// so when the library is compiled, an "unused" set of mappings for that function
+// is included in the library's coverage metadata.
 //
 // Even though this function is used by `uses_crate.rs` (and
 // counted), with substitutions for `T`, those instantiations are only generated
@@ -98,6 +93,6 @@ fn use_this_lib_crate() {
 // another binary that never used this generic function, then it would be valid
 // to show the unused generic, with unknown substitution (`_`).
 //
-// The alternative is to exclude all generics from being included in the "unused
-// functions" list, which would then omit coverage results for
-// `unused_generic_function<T>()`, below.
+// The alternative would be to exclude all generics from being included in the
+// "unused functions" list, which would then omit coverage results for
+// `unused_generic_function<T>()`.

tests/coverage/uses_crate.coverage

@@ -124,13 +124,8 @@ $DIR/auxiliary/used_crate.rs:
    LL|       |// ```
    LL|       |//
    LL|       |// The notice is triggered because the function is unused by the library itself,
-   LL|       |// and when the library is compiled, a synthetic function is generated, so
-   LL|       |// unused function coverage can be reported. Coverage can be skipped for unused
-   LL|       |// generic functions with:
-   LL|       |//
-   LL|       |// ```shell
-   LL|       |// $ `rustc -Zunstable-options -C instrument-coverage=except-unused-generics ...`
-   LL|       |// ```
+   LL|       |// so when the library is compiled, an "unused" set of mappings for that function
+   LL|       |// is included in the library's coverage metadata.
    LL|       |//
    LL|       |// Even though this function is used by `uses_crate.rs` (and
    LL|       |// counted), with substitutions for `T`, those instantiations are only generated
@@ -146,9 +141,9 @@ $DIR/auxiliary/used_crate.rs:
    LL|       |// another binary that never used this generic function, then it would be valid
    LL|       |// to show the unused generic, with unknown substitution (`_`).
    LL|       |//
-   LL|       |// The alternative is to exclude all generics from being included in the "unused
-   LL|       |// functions" list, which would then omit coverage results for
-   LL|       |// `unused_generic_function<T>()`, below.
+   LL|       |// The alternative would be to exclude all generics from being included in the
+   LL|       |// "unused functions" list, which would then omit coverage results for
+   LL|       |// `unused_generic_function<T>()`.
 
 $DIR/uses_crate.rs:
    LL|       |// This test was failing on Linux for a while due to #110393 somehow making

tests/ui/instrument-coverage/bad-value.bad.stderr

@@ -1,2 +1,2 @@
-error: incorrect value `bad-value` for codegen option `instrument-coverage` - either a boolean (`yes`, `no`, `on`, `off`, etc) or (unstable) one of `branch`, `except-unused-generics`, `except-unused-functions` was expected
+error: incorrect value `bad-value` for codegen option `instrument-coverage` - one of: `y`, `yes`, `on`, `true`, `n`, `no`, `off` or `false` was expected
 

tests/ui/instrument-coverage/bad-value.blank.stderr

@@ -1,2 +1,2 @@
-error: incorrect value `` for codegen option `instrument-coverage` - either a boolean (`yes`, `no`, `on`, `off`, etc) or (unstable) one of `branch`, `except-unused-generics`, `except-unused-functions` was expected
+error: incorrect value `` for codegen option `instrument-coverage` - one of: `y`, `yes`, `on`, `true`, `n`, `no`, `off` or `false` was expected
 

tests/ui/instrument-coverage/coverage-options.bad.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `bad` for unstable option `coverage-options` - `branch` or `no-branch` was expected
+

tests/ui/instrument-coverage/coverage-options.rs

@@ -0,0 +1,14 @@
+//@ needs-profiler-support
+//@ revisions: branch no-branch bad
+//@ compile-flags -Cinstrument-coverage
+
+//@ [branch] check-pass
+//@ [branch] compile-flags: -Zcoverage-options=branch
+
+//@ [no-branch] check-pass
+//@ [no-branch] compile-flags: -Zcoverage-options=no-branch
+
+//@ [bad] check-fail
+//@ [bad] compile-flags: -Zcoverage-options=bad
+
+fn main() {}

tests/ui/instrument-coverage/unstable.branch.stderr

@@ -1,2 +0,0 @@
-error: `-C instrument-coverage=branch` and `-C instrument-coverage=except-*` require `-Z unstable-options`
-

tests/ui/instrument-coverage/unstable.except-unused-functions.stderr

@@ -1,2 +0,0 @@
-error: `-C instrument-coverage=branch` and `-C instrument-coverage=except-*` require `-Z unstable-options`
-

tests/ui/instrument-coverage/unstable.except-unused-generics.stderr

@@ -1,2 +0,0 @@
-error: `-C instrument-coverage=branch` and `-C instrument-coverage=except-*` require `-Z unstable-options`
-

tests/ui/instrument-coverage/unstable.rs

@@ -1,6 +0,0 @@
-//@ revisions: branch except-unused-functions except-unused-generics
-//@ [branch] compile-flags: -Cinstrument-coverage=branch
-//@ [except-unused-functions] compile-flags: -Cinstrument-coverage=except-unused-functions
-//@ [except-unused-generics] compile-flags: -Cinstrument-coverage=except-unused-generics
-
-fn main() {}