Rustdoc creates doctests with `extern crate mod;`

[Swatinem]

In #79770 I tried to create a new rustdoc-ui testcase that runs doctests. However having the mod substring in a doctest comment injected an invalid extern crate mod; into the code generated for the doctest.

Turns out this here is a bit too aggressive:

rust/src/librustdoc/doctest.rs

Lines 533 to 534 in 0f6f2d6

	if s.contains(cratename) {
	prog.push_str(&format!("extern crate {};\n", cratename));

It looks like compiletest actually gave the crate the name mod, perhaps because it was defined in a issue-XXX/mod.rs file, and having a doctest such as this triggers the this auto-injection, resulting in a failed doctest run:

/// ```
/// // ^ mod.rs line 10
/// assert_eq!(1, 1);
/// ```

[jyn514]

I think rustdoc needs to check if the crate name is a keyword and add r# if so. Actually it's probably enough to unconditionally add r# since it works for things that aren't keywords.

[Swatinem]

Actually it's probably enough to unconditionally add r# since it works for things that aren't keywords.

there is actually two bugs here. mod not being properly escaped is one thing, the other is probably the way that rustdoc is invoked, or not quite sure. Basically it infers the crate name from the file name if not explicitly given, and then just assumes that a crate with that name exists, which it doesn’t.

[jyn514]

@Swatinem that's an issue with compiletest, not rustdoc - it needs to compile the crate itself before running rustdoc on it: #79775 (comment). I don't know how to fix that, I think compiletest needs to add support for this maybe?