no separate caching for target/host

[davemilter]

Problem

I compile for PC and "Raspberry PI 3" from the same sources,
every time I change target it recomiples from scratch all dependicies what is very strange,
because of data in separate directories:
target/debug
and
target/armv7-unknown-linux-gnueabihf/debug

Steps

1. build/check two times
   myapp.zip
   crate in attachment.

cargo check
cargo check

2. Cross-compile it

cargo check --target=armv7-unknown-linux-gnueabihf
cargo check --target=armv7-unknown-linux-gnueabihf

3. Recompile for host again:

cargo check

and you see that cache for host is gone and cargo rebuild all from scratch.

Notes

Output of cargo version:
cargo 1.40.0 (bc8e4c8 2019-11-22)

This is happens only if rppal used as dependency, but any way why armv7-unknown-linux-gnueabihf and my host (x86_64-unknown-linux-gnu) can interfere to each other?

rustc 1.40.0 (73528e339 2019-12-16) / linux / amd64, stable installed via rustup.

[ehuss]

Hm, I'm unable to reproduce your example. Can you run with the CARGO_LOG=cargo::core::compiler::fingerprint=trace environment variable? It should print some debug information explaining why Cargo thinks it needs to be rebuilt.

[davemilter]

@ehuss

I find out culprit:

$ cat ~/.cargo/config 
[target.x86_64-unknown-linux-gnu]
rustflags = ["-Clink-arg=-fuse-ld=gold"]

If I remove ~/.cargo/config from my home directory caching starts work,
if I return it back then every target change cause rebuild from scratch.

Have no idea why change of linker for x86_64 breaks cargo caching though.

[ehuss]

Ah. The reason is that when using the --target flag, rustflags are not included for build scripts. However, without the --target flag, Cargo will include the rustflags for build scripts. This causes the libc build script to switch to being built with and without the flag based on whether or not the --target flag is passed. There are a variety of issues that cover this (#6375, and those linked there).

[ehuss]

I'm going to close this as it seems like the problem was figured out, and there are other issues tracking handling of RUSTFLAGS.