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.

2 files changed, 15 insertions, 8 deletions

diff --git a/src/doc/rustc/src/instrument-coverage.md b/src/doc/rustc/src/instrument-coverage.md
index 2f93252eddc..7780f2102ba 100644
--- a/src/doc/rustc/src/instrument-coverage.md
+++ b/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 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.
+## `-Z coverage-options=<options>`
+
+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
 
diff --git a/src/doc/unstable-book/src/compiler-flags/coverage-options.md b/src/doc/unstable-book/src/compiler-flags/coverage-options.md
new file mode 100644
index 00000000000..105dce61511
--- /dev/null
+++ b/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.