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).

While testing, I also noticed two other issues:

* spanview debug file output ICEd on a function with no body. The
workaround for this is included in this PR.
* `assert_*!()` macro coverage can appear covered if followed by another
`assert_*!()` macro. Normally they appear uncovered. I submitted a new
Issue #84561, and added a coverage test to demonstrate this issue.

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.issue-84561.txt

@@ -0,0 +1,34 @@
+    1|       |// FIXME(#84561): function-like macros produce unintuitive coverage results.
+    2|       |// This test demonstrates some of the problems.
+    3|       |
+    4|      9|#[derive(Debug, PartialEq, Eq)]
+                       ^5
+  ------------------
+  | <issue_84561::Foo as core::cmp::PartialEq>::eq:
+  |    4|      9|#[derive(Debug, PartialEq, Eq)]
+  ------------------
+  | Unexecuted instantiation: <issue_84561::Foo as core::cmp::PartialEq>::ne
+  ------------------
+    5|       |struct Foo(u32);
+    6|       |
+    7|      1|fn main() {
+    8|      1|    let bar = Foo(1);
+    9|      0|    assert_eq!(bar, Foo(1));
+   10|      1|    let baz = Foo(0);
+   11|      0|    assert_ne!(baz, Foo(1));
+   12|      1|    println!("{:?}", Foo(1));
+   13|      1|    println!("{:?}", bar);
+   14|      1|    println!("{:?}", baz);
+   15|       |
+   16|      1|    assert_eq!(Foo(1), Foo(1));
+   17|      1|    assert_ne!(Foo(0), Foo(1));
+   18|      0|    assert_eq!(Foo(2), Foo(2));
+   19|      1|    let bar = Foo(1);
+   20|      1|    assert_ne!(Foo(0), Foo(3));
+   21|      1|    assert_ne!(Foo(0), Foo(4));
+   22|      1|    assert_eq!(Foo(3), Foo(3));
+   23|      0|    assert_ne!(Foo(0), Foo(5));
+   24|      1|    println!("{:?}", bar);
+   25|      1|    println!("{:?}", Foo(1));
+   26|      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/issue-84561.rs

@@ -0,0 +1,26 @@
+// FIXME(#84561): function-like macros produce unintuitive coverage results.
+// This test demonstrates some of the problems.
+
+#[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);
+
+    assert_eq!(Foo(1), Foo(1));
+    assert_ne!(Foo(0), Foo(1));
+    assert_eq!(Foo(2), Foo(2));
+    let bar = Foo(1);
+    assert_ne!(Foo(0), Foo(3));
+    assert_ne!(Foo(0), Foo(4));
+    assert_eq!(Foo(3), Foo(3));
+    assert_ne!(Foo(0), Foo(5));
+    println!("{:?}", bar);
+    println!("{:?}", Foo(1));
+}