add comment to test

Author: lcnr

tests/coverage/generic-unused-impl.cov-map

@@ -1,18 +1,18 @@
 Function name: <generic_unused_impl::W<_> as core::convert::From<[<_ as generic_unused_impl::Foo>::Assoc; 1]>>::from (unused)
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 0a, 05, 03, 06]
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 0b, 05, 03, 06]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Zero) at (prev + 10, 5) to (start + 3, 6)
+- Code(Zero) at (prev + 11, 5) to (start + 3, 6)
 Highest counter ID seen: (none)
 
 Function name: generic_unused_impl::main
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 10, 01, 00, 0d]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 11, 01, 00, 0d]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 16, 1) to (start + 0, 13)
+- Code(Counter(0)) at (prev + 17, 1) to (start + 0, 13)
 Highest counter ID seen: c0
 

tests/coverage/generic-unused-impl.coverage

@@ -1,3 +1,4 @@
+   LL|       |// Regression test for #135235.
    LL|       |trait Foo {
    LL|       |    type Assoc;
    LL|       |

tests/coverage/generic-unused-impl.rs

@@ -1,3 +1,4 @@
+// Regression test for #135235.
 trait Foo {
     type Assoc;