libstd gets rebuilt unconditionally

[flo-l]

For reference, the reddit thread, where the discussion started.

My goal

I want to hack on rustc. So I want to make a change, recompile as fast as possible and test the new binary with some input.rs.

The Problem

make rebuilds libstd even if it's unchanged. This takes ages and can't be skipped.

How to reproduce

I'm on Fedora Linux 23, fully updated.

I started with a clone of the rust repo, in which I already compiled rustc more than once (so LLVM was already built etc.).

git reset --hard 298730e   # this is the commit I used for this test
git clean -df

./configure --enable-debug

output

make rustc-stage1 NO_REBUILD=1

output

touch src/librustc_mir/lib.rs

make rustc-stage1 NO_REBUILD=1

output

As you can see in the output of the second make command libstd, libcore etc. get rebuilt.

[flo-l]

cc @alexcrichton

[alexcrichton]

Ah sorry, I believe I have misunderstood! The rebuilding as part of the second make command is actually the stage 1 set of libraries. These are the ones which are produced by the stage0 artifacts. That is, the chain of dependencies is:

1. You modified src/librustc_mir/lib.rs
2. This file is a dependency of x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_mir
3. Transitively, that's a dependency of x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/bin/rustc, the final stage0 artifact.
4. That file is a dependency of x86_64-unknown-linux-gnu/stage1/bin/rustc, the stage1 compiler.

The stage1 compiler has changed (as part of librustc_mir changing), so its dependants need to be rebuilt as well. These include the stage1 libraries (e.g. libcore + libstd).

Does that make sense? I'm gonna close this as it's intended behavior, but I'll of course answer any questions you may have!

[flo-l]

Ah ok I understand, thanks for clarifying!

As mentioned above I'd like to build just rustc with some changes I made. Which make target corresponds to that and where will the binary be?

[alexcrichton]

If all you want is the compiler itself, then this should suffice:

make x86_64-unknown-linux-gnu/stage1/bin/rustc

There may be a more ergonomic way to type that, although I don't recall off the top of my head...

Note though that this compiler won't be too too useful as it won't have a standard library to link against.

[flo-l]

That does what it should! Thank you so much! 😁

And about that libstd thing: Could the --disable-inject-std-version option of the configure script help?

[alexcrichton]

Wow I had no idea that option even still existed... I think that's from an era long since passed and no longer actually does anything!

[flo-l]

I just checked, make x86_64-unknown-linux-gnu/stage1/bin/rustc seems to work, the produced binary reuses the previously built libstd. 🎉

Thank you very much, I was fighting with this for days!

[alexcrichton]

Glad I could help!