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.

1 files changed, 1 insertions, 3 deletions

diff --git a/compiler/rustc_monomorphize/src/partitioning.rs b/compiler/rustc_monomorphize/src/partitioning.rs
index 8bebc30e435..296eb3120ee 100644
--- a/compiler/rustc_monomorphize/src/partitioning.rs
+++ b/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);
     }