This is step 2 towards fixing #77548.
In the codegen and codegen-units test suites, the `//` comment markers
were kept in order not to affect any source locations. This is because
these tests cannot be automatically `--bless`ed.
Unfortunately, this can't currently be tested. The problem is that we
need the file to be compiled first to then be used as dependency, which
cannot be done currently unfortunately in the rustdoc test suites.
Example:
```rust
// name this file "foo.rs"
/// ```
/// let x = foo::foo();
/// ```
pub fn foo() {}
```
If you run `rustdoc --test foo.rs`, you'll get:
```
running 1 test
test foo.rs - foo (line 1) ... FAILED
failures:
---- foo.rs - foo (line 1) stdout ----
error[E0463]: can't find crate for `foo`
--> foo.rs:0:1
|
2 | extern crate foo;
| ^^^^^^^^^^^^^^^^^ can't find crate
```
If a test were possible, it would look something like
````rust
#![crate_name = "mod"]
#![crate_type = "lib"]
//! ```
//! // NOTE: requires that the literal string 'mod' appears in the doctest for
//! // the bug to appear
//! assert_eq!(1, 1);
//! ```
````
