Enable propagating host rustflags to build scripts
### What does this PR try to resolve?
This PR primarily fixes#10206, but in doing so it also slightly modifies the interface for the unstable `target-applies-to-host` feature (#9453), and adds the unstable `target-applies-to-host-kind` flag to mirror `target-applies-to-host` for build scripts and other host artifacts.
The commit messages have more in-depth discussion.
### How should we test and review this PR?
The test case from #10206 now works rather than producing an error. It has also been added a regression test case. A few additional test cases have also been added to handle the expected behavior around rustflags for build scripts with and without `target-applies-to-host-kind` enabled.
### Additional information
1. This changes the interface for `target-applies-to-host` so that it does not need to be specified twice to be used. And it can still be set through configuration files using the `[unstable]` table. However, we may(?) want to pick a stable format for in-file configuration of this setting unless we intend for it to only ever be a command-line flag.
2. It may be that `target-applies-to-host-kind` is never behavior we want users to turn on, and that it should therefore simply be removed and hard-coded as being `false`.
3. It's not entirely clear how `target-applies-to-host-kind` should interact with `-Zmultitarget`. If, for example, `requested_kinds = [HostTarget, SomeOtherTarget]` and `kind.is_host()`, should `RUSTFLAGS` take effect or not? For the time being I've just hard-coded the behavior for single targets, and the answer would be "no".
This patch implements more complete logic for applying rustflags to
build scripts and other host artifacts. In the default configuration, it
only makes build scripts (and plugins and whatnot) pick up on
`rustflags` from `[host]`, which fixes#10206 but otherwise preserves
existing behavior in all its inconsistent glory. The same is the case if
`target-applies-to-host` is explicitly set to `false`.
When `target-applies-to-host` is explicitly set to `true`, rustflags
will start to be applied in the same way that `linker` and `runner` are
today -- namely they'll be applied from `[target.<host triple>]` and
from `RUSTFLAGS`/`build.rustflags` if `--target <host triple>` is given.
.. with mutliple calls of:
fastmod --accept-all '\.cargo\("([^"]+)"\)\.arg\("([^"]+)"\)' '.cargo("${1} ${2}")' tests/testsuite/
until no changes are left.
* Collapse the nested cargotest::support module into the cargotest
module (merge the mod.rs's)
* Rename the cargotest module to support
* Nest the top-level hamcrest module into support
By rewriting the tests, with rerast (https://github.com/google/rerast),
to use the newly introduced "at" method.
First I added the following temporary function to cargotest::support:
pub fn project_foo() -> ProjectBuilder {
project("foo")
}
Then I defined the following rewrite.rs:
use cargotest::support::{ project, project_foo };
fn rule1(a: &'static str) {
replace!(project("foo") => project_foo());
replace!(project(a) => project_foo().at(a));
}
Then I ran rerast:
cargo +nightly rerast --rules_file=rewrite.rs --force --targets tests --file tests/testsuite/main.rs
Finally I searched and replaced the references to project_foo with
argument-less project (a little awkardly on macOS with a git clean).
find tests -type f -exec sed -i -e 's/project_foo/project/g' {} +
git clean -d tests
