"the main function must be defined at the crate level" for doc test regression 1.81->1.82

[joshka]

The tokio.rs website tests all the code blocks in markdown files by building up a rust file with code included as doc comments on generated functions. The code is generally wrapped in a function to avoid any main functions being interpreted as a test main function. In 1.81, this worked fine, but 1.82 sees the internal main function and treats it as one which prevents the default test main being created.

Code

I tried this code:

/// ```
/// # fn dox() {
/// fn main() {}
/// # }
/// ```
fn foo() {}

I expected to see this happen: cargo test should succeed

Instead, this happened: cargo test failed

Version it worked on

It most recently worked on: 1.81

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: aarch64-apple-darwin
release: 1.82.0
LLVM version: 19.1.1

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[apiraino]

@joshka can you provide the exact file configuration (files and directories) that reproduces the issue? I'm sorry, I'm probably missing the obvious. thanks

[Darksonn]

Pasting it into the playground and choosing "test" hits the error, so a new project with the above in src/lib.rs should trigger it.

[apiraino]

ok, if my bisection is correct, it points to e5b3e68 (though I'm not completely sure how it's relevant)

cc @GuillaumeGomez

[joshka]

The function for checking for functions named main() previously only looked at the top level functions, but now looks recursively for any functions named main.

rust/src/librustdoc/doctest/make.rs

Lines 288 to 291 in e5b3e68

	// Recurse through functions body. It is necessary because the doctest source code is
	// wrapped in a function to limit the number of AST errors. If we don't recurse into
	// functions, we would thing all top-level items (so basically nothing).
	fn check_item(item: &ast::Item, info: &mut ParseSourceInfo, crate_name: &Option<&str>) {

It likely instead needs to stop the recursion level at the automatically generated doctest function. That new function wrapper was created so that the tests can be merged into a single file and compiled once for speed rather than a file per doctest.

In addition to the change here, the line numbers in errors in the new approach make it difficult to correlate multiple failures into a source document. This seems to have always been the case, but to have gotten worse with the doc test merging in 1.82. I haven't checked whether there's another bug raised for this, but it will be related the the same code change here.