Handle xwin cache paths containing spaces

[Audacity88]

Summary

• Pass generated MSVC linker search paths through a target-scoped Cargo --config array so each path remains one rustc argument when the xwin cache directory contains spaces.
• Quote generated MSVC include paths and enable cc-rs shell parsing so C, C++, bindgen, and resource compiler consumers preserve the same paths.
• Add regression coverage for both environment-string contracts with a cache path containing spaces.

Problem

cargo xwin currently serializes generated -Lnative arguments through the space-separated target rustflags variable. A cache path containing a space is rejected by cargo-config2 before compilation with flag in rustflags must not contain its separator (' ').

Using a TOML array resolves that failure without restoring global encoded rustflags, which would leak Windows linker flags into cross-target artifact dependencies. The generated clang-cl include flags also need quoting. Without shell-escaped parsing, cc-rs splits an SDK include path at the space and clang-cl cannot find the Windows headers.

Approach

The spawned Cargo command receives the already-resolved target.<triple>.rustflags as a --config TOML array. This preserves cargo-xwin's existing flag-resolution behavior while keeping each complete SDK path intact and retaining the target isolation introduced for cross-target artifact dependencies. The option is inserted before any trailing -- arguments used by cargo test, cargo run, or cargo bench. The include-path helper emits quoted arguments for CL_FLAGS, target CFLAGS and CXXFLAGS, bindgen, and RCFLAGS; CC_SHELL_ESCAPED_FLAGS=1 tells cc-rs to parse those quoted arguments as shell-escaped flags.

Validation

• A downstream cargo xwin check --locked -p zeroclaw-providers --target x86_64-pc-windows-msvc --tests completed successfully with xwin cache and Cargo target paths containing spaces.
• git diff --check passes.
• cargo test --locked compiler::clang_cl::tests passes (3 passed, 0 failed).

[messense]

Thanks for the PR! The --config target.<triple>.rustflags=[…] approach is the right way around cargo-config2's separator error, and the unit tests / fmt / clippy all pass locally. A few issues before this can land, though:

Blocking

1. Quoted include paths break everyone on cc < 1.1.11, even without spaces.
CC_SHELL_ESCAPED_FLAGS was only added in cc-rs 1.1.11 (Aug 2024, rust-lang/cc-rs#1181). Older cc ignores the env var and whitespace-splits CFLAGS_*, so /imsvc "/home/u/.cache/cargo-xwin/crt/include" becomes a literal argument with the quote characters in it and clang-cl can't find the headers. Lockfiles pinned to cc 1.0.x are still very common, so this would regress the default (no-spaces) case for a lot of users. Could we only quote / set CC_SHELL_ESCAPED_FLAGS when the cache path actually contains whitespace? That keeps today's behaviour for everyone else, and we can document the cc-rs requirement for the spaces case.

2. cargo xwin env regression.
src/env.rs only prints cmd.get_envs(). With the rustflags now passed as a --config argument, the -Lnative=… paths disappear from cargo xwin env output entirely, so eval "$(cargo xwin env)" && cargo build --target x86_64-pc-windows-msvc no longer links. env needs to keep emitting something equivalent (e.g. fall back to CARGO_TARGET_<T>_RUSTFLAGS there), or the --config path should be limited to the spawned command.

Should address

3. CC_SHELL_ESCAPED_FLAGS=1 is global, not target-scoped.
It changes how cc-rs parses all *FLAGS for the whole cargo run, including host builds (build scripts, proc-macros) and the user's own CFLAGS/CXXFLAGS/CL_FLAGS, which are appended verbatim. Anything with backslashes (/IC:\foo on a Windows host) or quotes now gets reinterpreted. Another reason to only enable it when needed, plus a README note.

4. RCFLAGS quoting is unverified.
RCFLAGS is consumed by embed-resource, not cc-rs, and I don't think the cargo check in the validation section exercised any .rc files. Please confirm embed-resource handles quoted args before changing the format. src/compiler/clang.rs still emits unquoted RCFLAGS, so the two backends would also disagree.

5. Rebuilding the Command in insert_cargo_config.
This silently drops anything get_* can't read back (env_clear, stdio, pre_exec). Nothing sets those today so it works, but it's fragile. Simpler: add --config where the command is built (build_command in src/macros.rs, run.rs, test.rs, bench.rs), e.g. by having apply_command_env return the extra args.

Minor

• Please keep the note that RUSTFLAGS / CARGO_ENCODED_RUSTFLAGS are already folded into rustflags via cargo-config2; otherwise the new env_remove calls look like they drop user flags.
• The new tests cover the string formats well, but nothing exercises the real case end-to-end. A CI step with a cache path containing a space (XWIN_CACHE_DIR="… /xwin cache") would be the actual regression guard.

[Audacity88]

Thanks for the detailed review. I revised the branch locally to address each point:

• Cache paths are quoted and CC_SHELL_ESCAPED_FLAGS=1 is set only when the resolved path contains whitespace. Ordinary paths retain the previous unquoted flag format, so projects pinned to cc < 1.1.11 are unchanged. The README now documents the cc >= 1.1.11 requirement for spaced paths.
• Spawned Cargo commands use target-scoped --config target.<triple>.rustflags=[...], while cargo xwin env keeps emitting CARGO_TARGET_<TRIPLE>_RUSTFLAGS. The comment explaining that cargo-config2 has already resolved inherited rustflags is retained.
• The command-rebuilding helper is removed. Each builder now adds generated --config arguments before its trailing -- arguments, preserving the original Command and its process configuration.
• RCFLAGS is restored to its existing unquoted format because its quoting contract was not verified. The README explicitly excludes Windows resource compilation from the spaced-cache-path claim.
• CI now performs a real hello-tls build with an XWIN_CACHE_DIR containing a space on the stable Ubuntu clang-cl job.

Local validation on the revised tree:

• cargo fmt --all -- --check
• RUST_TEST_THREADS=1 cargo test --locked (7 passed)

The serial test setting avoids the existing process-wide RUSTFLAGS race between the is_static_crt_enabled unit tests. Hosted CI is green on ab43fb54cce0, including the stable Ubuntu clang-cl job with the new end-to-end spaced-cache-path build.