dep-info rmeta path is wrong

[ehuss]

The path to the rmeta file in the dep-info file is wrong. It is missing the lib prefix.

rustc --emit=dep-info,metadata --crate-type lib foo.rs

foo.d will contain foo.rmeta: foo.rs, but I think it should be libfoo.rmeta: foo.rs because the filename is libfoo.rmeta.

The crate type doesn't matter. A "bin" type will also emit foo.rmeta, but bin types (confusingly) also use libfoo.rmeta.

(This doesn't really affect Cargo, as it ignores these filenames.)

[contrun]

Can you please elaborate on why this does not affect cargo and how cargo set the output file name?

It seems the crate type is parsed from

rust/src/librustc_session/config.rs

Lines 1844 to 1865 in 1581278

	pub fn parse_crate_types_from_list(list_list: Vec<String>) -> Result<Vec<CrateType>, String> {
	let mut crate_types: Vec<CrateType> = Vec::new();
	for unparsed_crate_type in &list_list {
	for part in unparsed_crate_type.split(',') {
	let new_part = match part {
	"lib" => default_lib_output(),
	"rlib" => CrateType::Rlib,
	"staticlib" => CrateType::Staticlib,
	"dylib" => CrateType::Dylib,
	"cdylib" => CrateType::Cdylib,
	"bin" => CrateType::Executable,
	"proc-macro" => CrateType::ProcMacro,
	_ => return Err(format!("unknown crate type: `{}`", part)),
	};
	if !crate_types.contains(&new_part) {
	crate_types.push(new_part)
	}
	}
	}
	Ok(crate_types)
	}

It is a vector. I am not sure how to determine whether I need to add the prefix lib or not.

[ehuss]

I'm not sure I understand the relevance of the note about parsing crate types.

This is where Cargo parses the dep-info file. It only looks at the first line, and ignores the value before the :.

You can look at #49289 to start to investigate why binaries emit a filename with the lib prefix. That PR is what added rmeta output for binaries (the rmeta file is empty, it only exists to signal to Cargo that the compilation finished). I believe this is where the rmeta filename is determined. The dep-info filenames are computed around here. Somehow the two are out of sync.

[TheDreadedAndy]

According to the documentation for --emit, the default filename for an rmeta file should be CRATE_NAME.rmeta. That would suggest that the actual problem is the file name itself, which I believe is set here. Changing that would likely be breaking for anyone who doesn't use Cargo, though.

A workaround for this problem is to specify the output path for the metadata file. e.g. --emit=metadata=path/to/libfoo.rmeta. The dep-info is generated correctly when that is done.

[ehuss]

Oh, looks like the --emit docs are wrong. The rmeta file needs to start with lib to work with crate discovery, and to match the .rlib naming convention. I posted #95131 with a fix for the docs.

[Enselic]

PR with fix: #114750