Add --emit=nothing to tell rustc not to emit any files (even if --crate-type=lib)

[LukasKalbertodt]

In order to only check if a program is well-formed (think cargo check), one would like to call rustc without emitting any files. cargo check uses --emit=metadata -Z no-codegen because the metadata is needed to check dependent crates.

However, it seems like it's impossible to tell the compiler to not write any files in the case of --crate-type=lib. For the default --crate-type=bin it works: rustc -Z no-codegen foo.rs does not emit any files. But again, for rustc -Z no-codegen --crate-type=lib foo.rs it emits an .rlib file.

In case this isn't a bug, would it be possible to add an --emit=nothing or equivalent option?

_{(Related question on StackOverflow)}

[leonardo-m]

See also Issue #50289

[nagisa]

What would happen of one specified --emit=nothing,link,llvm-ir? What about --emit=link,llvm-ir,nothing?

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The rlib file emitted with rustc -Z no-codegen --crate-type=lib foo.rs does not contain any machine code, only metadata (and so is equivalent to rmeta) which, again, would be necessary to check dependent crates.

[dtolnay]

I would love to use this for build.rs feature detection, for example the stuff in https://github.com/dtolnay/anyhow/blob/d2c83e20a3b341946c502380e4230ff4e820ef32/build.rs.

Currently I use --emit=metadata but that does too much work. Testing on the serde crate, time rustc --crate-name serde --crate-type lib --emit=metadata serde/src/lib.rs takes 1.45 seconds while time rustc --crate-name serde --crate-type lib --emit=dep-info serde/src/lib.rs takes just 0.25 seconds, 6× faster, and still appears to do all the useful typechecking I need. However I am not comfortable switching build.rs from --emit=metadata over to --emit=dep-info because there is no reason --emit=dep-info would always need to do typechecking / trait solving. In the future a rustc optimization could make it not do what I need (while still emitting a dep-info file).

A typecheck-only emit mode, with no outputs other than diagnostics or json to stderr, would be great.

[dtolnay]

@nagisa:

What would happen if one specified --emit=nothing,link,llvm-ir? What about --emit=link,llvm-ir,nothing?

Counterproposal: instead of --emit=nothing, we should spell this as --emit=. The emit flag already takes a comma-separated list of outputs to produce. Today that list cannot be empty, or else it'll fail with the error message below. We just need to make the list allowed to be empty. That cleanly avoids needing to answer the meaning of nothing,link / link,nothing.

$ rustc --emit= src/main.rs

error: unknown emission type: `` - expected one of: `llvm-bc`, `asm`, `llvm-ir`, `mir`, `obj`, `metadata`, `link`, `dep-info`