fails to traverse from mod foo to foo.rs in some cases

[sourcefrog]

In 9617adc5ddad3603d730dc3306870364624625bf of https://github.com/openmls/openmls, cargo-mutants 765fca9 says

   0.561492717s  WARN traits/src/traits.rs:6: referent of mod "crypto" not found: tried ["/home/mbp/src/openmls/traits/src/traits/crypto.rs", "/home/mbp/src/openmls/traits/src/traits/crypto/mod.rs"]
   0.561519127s  WARN traits/src/traits.rs:7: referent of mod "key_store" not found: tried ["/home/mbp/src/openmls/traits/src/traits/key_store.rs", "/home/mbp/src/openmls/traits/src/traits/key_store/mod.rs"]
   0.561535347s  WARN traits/src/traits.rs:8: referent of mod "random" not found: tried ["/home/mbp/src/openmls/traits/src/traits/random.rs", "/home/mbp/src/openmls/traits/src/traits/random/mod.rs"]
   0.561550568s  WARN traits/src/traits.rs:9: referent of mod "signatures" not found: tried ["/home/mbp/src/openmls/traits/src/traits/signatures.rs", "/home/mbp/src/openmls/traits/src/traits/signatures/mod.rs"]
   0.561570058s  WARN traits/src/traits.rs:10: referent of mod "types" not found: tried ["/home/mbp/src/openmls/traits/src/traits/types.rs", "/home/mbp/src/openmls/traits/src/traits/types/mod.rs"]
Found 1287 mutants to test
Unmutated baseline ... ok in 40.8s build + 46.8s test

It still carries on with testing but is perhaps failing to generate some mutants.

This crate has

.
├── Cargo.toml
├── CHANGELOG.md
├── README.md
└── src
    ├── crypto.rs
    ├── key_store.rs
    ├── random.rs
    ├── signatures.rs
    ├── traits.rs
    └── types.rs

So it should find mod traits in src/traits.rs. I think it's confused because the top-level file is traits.rs, not lib.rs, due to configuration in Cargo.toml.

Specifically, openmls/traits/Cargo.toml has

[lib]
path = "src/traits.rs"

This code

cargo-mutants/src/visit.rs

Lines 250 to 298 in 765fca9

	// If there's no content in braces, then this is a `mod foo;`
	// statement referring to an external file. We find the file name
	// then remember to visit it later.
	//
	// Both the current module and the included sub-module can be in
	// either style: `.../foo.rs` or `.../foo/mod.rs`.
	//
	// If the current file ends with `/mod.rs`, then sub-modules
	// will be in the same directory as this file. Otherwise, this is
	// `/foo.rs` and sub-modules will be in `foo/`.
	//
	// Having determined the directory then we can look for either
	// `foo.rs` or `foo/mod.rs`.
	if node.content.is_none() {
	let my_path: &Utf8Path = self.source_file.tree_relative_path().as_ref();
	// Maybe matching on the name here is no the right approach and
	// we should instead remember how this file was found?
	let dir = if my_path.ends_with("mod.rs")
	|| my_path.ends_with("lib.rs")
	|| my_path.ends_with("main.rs")
	{
	my_path.parent().expect("mod path has no parent").to_owned()
	} else {
	my_path.with_extension("")
	};
	let mut found = false;
	let mut tried_paths = Vec::new();
	for &ext in &[".rs", "/mod.rs"] {
	let relative_path = TreeRelativePathBuf::new(dir.join(format!("{mod_name}{ext}")));
	let full_path = relative_path.within(&self.root);
	if full_path.is_file() {
	trace!("found submodule in {full_path}");
	self.more_files.push(relative_path);
	found = true;
	break;
	} else {
	tried_paths.push(full_path);
	}
	}
	if !found {
	warn!(
	"{path}:{line}: referent of mod {mod_name:#?} not found: tried {tried_paths:?}",
	path = self.source_file.tree_relative_path,
	line = node.mod_token.span.start().line,
	);
	}
	}
	self.in_namespace(mod_name, |v| syn::visit::visit_item_mod(v, node));
	}

assumes the top level file will always be lib.rs or main.rs but that's not reliable.

Maybe we ought to remember if the file is a top source file for the target and use that instead: see https://doc.rust-lang.org/cargo/reference/cargo-targets.html. We probably already have this in the cargo metadata.