This flag is intended to pair with `-Zbuild-std` as necessary to
configure the features that libstd is built with. This is highly
unlikely to ever be stabilized in any form (unlike `-Zbuild-std` which
we'd like to stabilize at some point), but can be useful for
experimenting with the standard library. For example today it can be
used to test changes to binary size by disabling backtraces.
My intention is that we won't need a `--no-default-features` equivalent
for libstd, where after rust-lang/rust#74377 is merged we can
unconditionally specify default features are disabled but the default
set of features lists `default`. That way if users want to override the
list *and* include the default feature, they can just be sure to include
`default`.
* Minimize the sysroot crates in play
* Don't use build scripts to inject args
* Use `RUSTC_WRAPPER` to dynamically switch `--sysroot` depending on
whether we're building sysroot crates or not.
* Minimize dependency graph in sysroot, only have each crate depend on a
dummy crates.io crate for testing and otherwise don't depend on
anything to load the desired sysroot crate directly.
This commit is aimed directly at rust-lang/wg-cargo-std-aware#33 and in
general making the `-Zbuild-std` tests more robust. The main change here
is that a new source tree is checked in, `tests/testsuite/mock-std`,
which mirrors rust-lang/rust's own tree for libstd. This mock tree is as
empty as it can be, ideally duplicating almost nothing but for not
requiring duplication of Cargo metadata about patches and such.
The end result here looks like:
* All `-Zbuild-std` tests are now run in parallel
* All tests run much more quickly since they're compiling tiny crates
instead of actually compiling libstd/libcore
* No tests require network access
* We verify that crates have access to the "custom" libraries
that we build
Coverage of tests is not currently expanded, but it's hoped that we
could add that shortly afterwards. Coverage has actually gone down
slightly since the custom target test was commented out temporarily and
the full integration test of running `-Zbuild-std` isn't run on CI any
more.
Closesrust-lang/wg-cargo-std-aware#33
