Auto merge of #84562 - richkadel:issue-83601, r=tmandry

Adds feature-gated `#[no_coverage]` function attribute, to fix derived Eq `0` coverage issue #83601

Derived Eq no longer shows uncovered

The Eq trait has a special hidden function. MIR `InstrumentCoverage`
would add this function to the coverage map, but it is never called, so
the `Eq` trait would always appear uncovered.

Fixes: #83601

The fix required creating a new function attribute `no_coverage` to mark
functions that should be ignored by `InstrumentCoverage` and the
coverage `mapgen` (during codegen).

Adding a `no_coverage` feature gate with tracking issue #84605.

r? `@tmandry`
cc: `@wesleywiser`

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.issue-83601.txt

@@ -0,0 +1,22 @@
+    1|       |// Shows that rust-lang/rust/83601 is resolved
+    2|       |
+    3|      3|#[derive(Debug, PartialEq, Eq)]
+                              ^2
+  ------------------
+  | <issue_83601::Foo as core::cmp::PartialEq>::eq:
+  |    3|      2|#[derive(Debug, PartialEq, Eq)]
+  ------------------
+  | Unexecuted instantiation: <issue_83601::Foo as core::cmp::PartialEq>::ne
+  ------------------
+    4|       |struct Foo(u32);
+    5|       |
+    6|      1|fn main() {
+    7|      1|    let bar = Foo(1);
+    8|      0|    assert_eq!(bar, Foo(1));
+    9|      1|    let baz = Foo(0);
+   10|      0|    assert_ne!(baz, Foo(1));
+   11|      1|    println!("{:?}", Foo(1));
+   12|      1|    println!("{:?}", bar);
+   13|      1|    println!("{:?}", baz);
+   14|      1|}
+

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

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

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

@@ -0,0 +1,19 @@
+    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-reports/expected_show_coverage.partial_eq.txt

@@ -2,7 +2,7 @@
     2|       |// structure of this test.
     3|       |
     4|      2|#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
-                       ^0            ^0      ^0 ^0  ^1       ^1 ^0^0
+                       ^0            ^0      ^0     ^1       ^1 ^0^0
   ------------------
   | Unexecuted instantiation: <partial_eq::Version as core::cmp::PartialEq>::ne
   ------------------

src/test/run-make-fulldeps/coverage/issue-83601.rs

@@ -0,0 +1,14 @@
+// Shows that rust-lang/rust/83601 is resolved
+
+#[derive(Debug, PartialEq, Eq)]
+struct Foo(u32);
+
+fn main() {
+    let bar = Foo(1);
+    assert_eq!(bar, Foo(1));
+    let baz = Foo(0);
+    assert_ne!(baz, Foo(1));
+    println!("{:?}", Foo(1));
+    println!("{:?}", bar);
+    println!("{:?}", baz);
+}

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

@@ -0,0 +1,17 @@
+// Enables `no_coverage` on the entire crate
+#![feature(no_coverage)]
+
+#[no_coverage]
+fn do_not_add_coverage_1() {
+    println!("called but not covered");
+}
+
+#[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/run-make-fulldeps/coverage/no_cov_func.rs

@@ -0,0 +1,18 @@
+// 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

@@ -0,0 +1,8 @@
+#![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() {}
+
+#[no_coverage] //~ ERROR the `#[no_coverage]` attribute is an experimental feature
+fn requires_feature_no_coverage() {}

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

@@ -0,0 +1,13 @@
+error[E0658]: the `#[no_coverage]` attribute is an experimental feature
+  --> $DIR/feature-gate-no_coverage.rs:7:1
+   |
+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
+
+For more information about this error, try `rustc --explain E0658`.