Support Cargo build-dir

[taiki-e]

Fixes #432

There is no support for nextest-archive because nextest itself doesn't seem to support it yet.

+ cargo nextest archive --archive-file a.tar.zst
warning: /home/runner/work/cargo-llvm-cov/cargo-llvm-cov/tests/fixtures/crates/build_dir/Cargo.toml: no edition set: defaulting to the 2015 edition while the latest is 2024
   Compiling build_dir v0.0.0 (/home/runner/work/cargo-llvm-cov/cargo-llvm-cov/tests/fixtures/crates/build_dir)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.11s
   Archiving 1 binary and 1 standard library to a.tar.zst
The application panicked (crashed).
Message:  binary paths must be within target directory: StripPrefixError(())
Location: /home/runner/work/nextest/nextest/nextest-runner/src/reuse_build/archiver.rs:344

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ BACKTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                                ⋮ 8 frames hidden ⋮                               
   9: nextest_runner::reuse_build::archiver::archive_to_file::{{closure}}::hfe6386f37325607b
      at <unknown source file>:<unknown line>
  10: cargo_nextest::dispatch::BaseApp::exec_archive::hb0c18033879d0a05
      at <unknown source file>:<unknown line>
  11: cargo_nextest::dispatch::AppOpts::exec::h0a45ae9b7294b285
      at <unknown source file>:<unknown line>
  12: cargo_nextest::dispatch::CargoNextestApp::exec::h5d63f524fe109718
      at <unknown source file>:<unknown line>
  13: cargo_nextest::main::h75d34d2809558cd2
      at <unknown source file>:<unknown line>
  14: std::sys::backtrace::__rust_begin_short_backtrace::h5f4ef41318e24363
      at <unknown source file>:<unknown line>
  15: main<unknown>
      at <unknown source file>:<unknown line>
  16: __libc_start_main<unknown>
      at <unknown source file>:<unknown line>
  17: _start<unknown>
      at /home/runner/.local/lib/python3.10/site-packages/ziglang/lib/libc/glibc/sysdeps/x86_64/start-2.33.S:120

[taiki-e]

Ah, the scope of --target-dir's effect depends on whether build-dir is specified in the configuration, so this is blocked on taiki-e/cargo-config2#29.

However, when using environment variables, it will probably not be a blocker.

[taiki-e]

This is likely inefficient as it triggers a rebuild, but unfortunately, build-dir lacks a flag like --target-dir and only has transparent environment variables, so it may be difficult to make it behave the same as target-dir here.

[ranger-ross]

Just want to point out that the build-dir contents are considered internal to Cargo.

We are planning to restructure the build-dir layout in rust-lang/cargo#15947 which may break this logic in the future

[ranger-ross]

Though given the nature of this tool we probably have no choice but to depend on the Cargo internals to get the correct files :/

[taiki-e]

Though given the nature of this tool we probably have no choice but to depend on the Cargo internals to get the correct files :/

Yeah. This isn't only happening with Cargo; it can occur with other testing tools as well. (e.g., #299 (comment))

If changes occur, we'll address them. As for future changes, we can only hope that "upstream developers are smart enough and only change internal structures when it's actually reasonable to do so".