Cargo always builds cargo check

[Mark-Simulacrum]

I'm seeing the following locally in a new project (created via cargo new):

$ RUST_LOG=cargo::ops::cargo_rustc::fingerprint=debug cargo check
DEBUG 2018-03-23T23:02:00Z: cargo::ops::cargo_rustc::fingerprint: fingerprint at: /home/mark/Build/testing/fffff/target/debug/.fingerprint/fffff-d3ba5f321a9eb8a2/bin-fffff-d3ba5f321a9eb8a2
   Compiling fffff v0.1.0 (file:///home/mark/Build/testing/fffff)
DEBUG 2018-03-23T23:02:00Z: cargo::ops::cargo_rustc::fingerprint: write fingerprint: /home/mark/Build/testing/fffff/target/debug/.fingerprint/fffff-d3ba5f321a9eb8a2/bin-fffff-d3ba5f321a9eb8a2
    Finished dev [unoptimized + debuginfo] target(s) in 0.10 secs
$ RUST_LOG=cargo::ops::cargo_rustc::fingerprint=trace cargo check
DEBUG 2018-03-23T23:02:16Z: cargo::ops::cargo_rustc::fingerprint: fingerprint at: /home/mark/Build/testing/fffff/target/debug/.fingerprint/fffff-d3ba5f321a9eb8a2/bin-fffff-d3ba5f321a9eb8a2
   Compiling fffff v0.1.0 (file:///home/mark/Build/testing/fffff)
DEBUG 2018-03-23T23:02:16Z: cargo::ops::cargo_rustc::fingerprint: write fingerprint: /home/mark/Build/testing/fffff/target/debug/.fingerprint/fffff-d3ba5f321a9eb8a2/bin-fffff-d3ba5f321a9eb8a2
    Finished dev [unoptimized + debuginfo] target(s) in 0.11 secs

This isn't really too big of an issue, but it feels wrong that Cargo does work (and spawns rustc) despite nothing changing in the project to my knowledge. It's also somewhat concerning that I don't see anything with =info on the RUST_LOG which leads me to believe that this is perhaps unintentional?

[alexcrichton]

Hm this doesn't reproduce with a "hello world" instance, do you have a way to reproduce this locally though?

[Mark-Simulacrum]

Yeah, I see the following. I'm not sure what else I could provide; seems odd that you can't reproduce, and yet I've reproduce on two different machines now...

macbook: ~
$ cargo new always-build
     Created binary (application) `always-build` project

mark: ~
$ cd always-build/

mark: ~/always-build
$ cargo check
   Compiling always-build v0.1.0 (file:///Users/mark/always-build)
    Finished dev [unoptimized + debuginfo] target(s) in 0.15 secs

mark: ~/always-build
$ cargo check
   Compiling always-build v0.1.0 (file:///Users/mark/always-build)
    Finished dev [unoptimized + debuginfo] target(s) in 0.12 secs

$ cargo -vV
cargo 1.26.0-nightly (d10ec661b 2018-03-20)
release: 1.26.0
commit-hash: d10ec661b06420654bbc4ed0ccd32295698aa1dc
commit-date: 2018-03-20

[ehuss]

This was discussed a bit in #3624 and #4318. bin targets will always be re-checked because they do not generate .rmeta files, and this code always assumes it is checking a library:

cargo/src/cargo/ops/cargo_rustc/context/compilation_files.rs

Lines 231 to 237 in 3548057

	if unit.profile.check {
	let filename = out_dir.join(format!("lib{}.rmeta", stem));
	let link_dst = link_stem
	.clone()
	.map(|(ld, ls)| ld.join(format!("lib{}.rmeta", ls)));
	ret.push((filename, link_dst, TargetFileType::Linkable));
	} else {

The bigger question is should check be re-run each time, even if there are no changes. matklad indicated that cargo should instead cache the output from the compiler so that warnings can be redisplayed when the cache is fresh (for build or check). I feel like that would be somewhat tricky. It may need to somehow extend the fingerprint to include things like --message-format, --color, terminal settings (for "auto" color), etc.

[Mark-Simulacrum]

I'm personally okay with that; if we can't check bin targets then that's fine, though that's somewhat surprising -- I'd expect us to be able to produce metadata anyway. But that's largely unimportant; I'm going to close this as not a bug.