### What does this PR try to resolve?
This is a horrible hack,
which lets the rustc invocation for learning target info always emit
warnings as warnings.
But at least it unblocks people who pass `-Awarnings` via RUSTFLAGS.
A long-term solution is a better interface
between build systems and the compiler.
See the discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Improving.20communication.20interface.20between.20cargo.2FrustcFixesrust-lang/cargo#8010
### How should we test and review this PR?
Ensure `CFG_DISABLE_CROSS_TESTS` is not set,
and run `cargo t --test testsuite
always_emit_warnings_as_warnings_when_learning_target_info`
This also additionally adds `wasm32-unknown-unknown` target to Cargo's
CI.
### Additional information
Instead of allowing any type in metadata, we were specifying fields like
`"string": "<any string>"`.
Found this when looking into #15030
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I found a bug in the manifest parser and figured this would help make it
more obvious.
Since I was already changing the order, I figure I'm make things a
little more logical (user-facing first, implementtion details later)
### What does this PR try to resolve?
Instead of saying no package found when there are hidden `Summary`s,
we'll instead say why the summary was hidden in the cases of
- Yanked packages
- Schema mismatch
- Offline packages?
The schema mismatch covers part of #10623. Whats remaining is when we
can't parse the `Summary` but can parse a subset (name, version, schema
version, optionally rust-version). That will be handled in a follow up.
### How should we test and review this PR?
This has a couple of risky areas
- Moving the filtering of `IndexSummary` variants from the Index to the
Registry Source. On inspection, there were other code paths but they
seemed innocuous to not filter, like asking for a hash of a summary
- Switching `PackageRegistry` to preserve the `IndexSummary` variant for
regular sources and overrides
- I did not switch patches to preserve `IndexSummary` as that was more
invasive and the benefits seemed more minor (normally people patch over
registry dependencies and not to them)
### Additional information
As we abstract aware the env variables, users can't do a good job of
reporting these, so we'll do it ourselves.
We could make the traits public and take and explicit `env` parameter.
I figured we can wait until there is a motivating use case.
`build_env` does have a fancier `Env` impl where you pass it HOST and
TARGET and it automatically looks up values for those with a fallback
scheme.
