v2 resolver: proc macros specified in initials participate in feature resolution for both target and host

[sunshowers]

Problem

I'm writing some tests with cargo-guppy + the v2 resolver, and it appears that currently, proc macros specified in the initial set participate in feature resolution for the host, not just the target platform.

Steps

Test workspace is here.

git clone https://github.com/facebookincubator/cargo-guppy && cd cargo-guppy
git checkout d5781f069ea57b2d0ec80fddb410978eb1b0f2d5
cd fixtures/workspace/inside-outside/workspace
cargo +nightly build -p internal-macro -p internal -Zfeatures=all --verbose

Note these two lines from the output:

     Running `rustc --crate-name external --edition=2018 /Users/rain1/local/cargo-guppy/fixtures/workspace/inside-outside/external/src/lib.rs
--error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -Cembed-bitcode=no -C debuginfo=2 --cfg 'feature="internal-build-feature"' --cfg 'feature="macro-build-feature"' --cfg 'feature="macro-normal-feature"' [...]
--error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -Cembed-bitcode=no -C debuginfo=2 --cfg 'feature="macro-normal-feature"' [...]

Note that external is built twice:

• The first line is for the host, which looks correct: normal + build + macro-normal
• The second line is for the target, in which case macro-normal-feature is included. This shouldn't be because proc macros are built on the host only, not the target.

Notes

Output of cargo version:

cargo 1.45.0-nightly (9fcb8c1d2 2020-05-25)

Also reproduces with 15bb455, which is the pinned version here.

This can also be reproduced with the cargo-compare tool checked into the repo. Check out my test branch https://github.com/sunshowers/cargo-guppy/tree/cargo-compare-fix, then run:

PROPTEST_MULTIPLIER=64 PROPTEST_VERBOSE=1 cargo test --release -p cargo-compare -- inside_outside --nocapture

cc @ehuss

[ehuss]

There is a comment here discussing this issue.

I'm trying to think if it would be possible to special-case this, but there are many edge cases that are difficult to deal with. I'm not sure if it is feasible to do it any other way.

[sunshowers]

Sounds good, thanks! Just wanted to make sure it was on the radar. If you decide not to fix it, feel free to close this issue and I can refer to it in guppy.

[leighmcculloch]

Another example that reproduces this is detailed in #10827.