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

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 15bb455d12ca42b48b659e7c7c831ba621e95ec4, 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

Start by reproducing the issue in the linked cargo-guppy fixture at fixtures/workspace/inside-outside/workspace with the provided cargo +nightly build command. Compare the host and target rustc invocations and verify that proc-macro features from the initial set are excluded from target resolution while remaining available for the host.