Rollup merge of #84871 - richkadel:no-coverage-unstable-only, r=nagisa

Disallows `#![feature(no_coverage)]` on stable and beta (using standard crate-level gating)

Fixes: #84836

Removes the function-level feature gating solution originally implemented, and solves the same problem using `allow_internal_unstable`, so normal crate-level feature gating mechanism can still be used (which disallows the feature on stable and beta).

I tested this, building the compiler with and without `CFG_DISABLE_UNSTABLE_FEATURES=1`

With unstable features disabled, I get the expected result as shown here:

```shell
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc     src/test/run-make-fulldeps/coverage/no_cov_crate.rs
error[E0554]: `#![feature]` may not be used on the dev release channel
 --> src/test/run-make-fulldeps/coverage/no_cov_crate.rs:2:1
  |
2 | #![feature(no_coverage)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
```

r? ````@Mark-Simulacrum````
cc: ````@tmandry```` ````@wesleywiser````

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.no_cov_func.txt

@@ -1,19 +0,0 @@
-    1|       |// Enables `no_coverage` on individual functions
-    2|       |
-    3|       |#[feature(no_coverage)]
-    4|       |#[no_coverage]
-    5|       |fn do_not_add_coverage_1() {
-    6|       |    println!("called but not covered");
-    7|       |}
-    8|       |
-    9|       |#[no_coverage]
-   10|       |#[feature(no_coverage)]
-   11|       |fn do_not_add_coverage_2() {
-   12|       |    println!("called but not covered");
-   13|       |}
-   14|       |
-   15|      1|fn main() {
-   16|      1|    do_not_add_coverage_1();
-   17|      1|    do_not_add_coverage_2();
-   18|      1|}
-

src/test/run-make-fulldeps/coverage/no_cov_func.rs

@@ -1,18 +0,0 @@
-// Enables `no_coverage` on individual functions
-
-#[feature(no_coverage)]
-#[no_coverage]
-fn do_not_add_coverage_1() {
-    println!("called but not covered");
-}
-
-#[no_coverage]
-#[feature(no_coverage)]
-fn do_not_add_coverage_2() {
-    println!("called but not covered");
-}
-
-fn main() {
-    do_not_add_coverage_1();
-    do_not_add_coverage_2();
-}

src/test/ui/feature-gates/feature-gate-no_coverage.rs

@@ -1,8 +1,13 @@
 #![crate_type = "lib"]
 
-#[no_coverage]
-#[feature(no_coverage)] // does not have to be enabled before `#[no_coverage]`
-fn no_coverage_is_enabled_on_this_function() {}
+#[derive(PartialEq, Eq)] // ensure deriving `Eq` does not enable `feature(no_coverage)`
+struct Foo {
+    a: u8,
+    b: u32,
+}
 
 #[no_coverage] //~ ERROR the `#[no_coverage]` attribute is an experimental feature
-fn requires_feature_no_coverage() {}
+fn requires_feature_no_coverage() -> bool {
+    let bar = Foo { a: 0, b: 0 };
+    bar == Foo { a: 0, b: 0 }
+}

src/test/ui/feature-gates/feature-gate-no_coverage.stderr

@@ -1,12 +1,11 @@
 error[E0658]: the `#[no_coverage]` attribute is an experimental feature
-  --> $DIR/feature-gate-no_coverage.rs:7:1
+  --> $DIR/feature-gate-no_coverage.rs:9:1
    |
 LL | #[no_coverage]
    | ^^^^^^^^^^^^^^
    |
    = note: see issue #84605 <https://github.com/rust-lang/rust/issues/84605> for more information
    = help: add `#![feature(no_coverage)]` to the crate attributes to enable
-   = help: or, alternatively, add `#[feature(no_coverage)]` to the function
 
 error: aborting due to previous error