cargo build --dependencies-only

[nagisa]

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cargo team notes:

• Summary

This is primarily targeted at docker layer caching for dependencies. For limiting docker layer caching of workspace members, see #14566.

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There should be an option to only build dependencies.

[nagisa]

I do not remember exactly why, but I do remember that I ended just running rustc manually.

[mcarton]

Sometimes you add a bunch of dependencies to your project, know it will take a while to compile next time you cargo build, but want your computer to do that as you start coding so the next cargo build is actually fast.
But I guess I got here searching for a cargo doc --dependencies-only, which allows you to get the doc of your dependencies while your project does not compile because you'd need the doc to know how exactly to fix that compilation error you've had for a half hour 😄

[gregwebs]

As described in #3615 this is useful with build to setup a cache of all dependencies.

[alexcrichton]

@gregwebs out of curiosity do you want to cache compiled dependencies or just downloaded dependencies? Caching compiled dependencies isn't implemented today (but would be with a command such as this) but downloading dependencies is available via cargo fetch.

[gregwebs]

Generally, as with my caching use case, the dependencies change infrequently and it makes sense to cache the compilation of them.

The Haskell tool stack went through all this and they seemed to generally decided to merge things into a single command where possible. For fetch they did end up with something kinda confusing though: build --dry-run --prefetch. For build --dependencies-only mentioned here they do have the same: build --only-dependencies

[alexcrichton]

@gregwebs ok thanks for the info!

[KalitaAlexey]

@alexcrichton,
It looks like I should continue my work on the PR.
Will Cargo's team accept it?

[alexcrichton]

@KalitaAlexey I personally wouldn't be convinced just yet, but it'd be good to canvas opinions from others on @rust-lang/tools as well

[KalitaAlexey]

@alexcrichton,
Anyway I have no time right now)

[nrc]

I don't see much of a use case - you can just do cargo build and ignore the output for the last crate. If you really need to do this (for efficiency) then there is API you can use.

[gregwebs]

What's the API?

[nrc]

Implement an Executor. That lets you intercept every call to rustc and you can do nothing if it is the last crate.