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_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);