ndk-build: Invoke clang directly instead of through wrapper scripts
#306
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The NDK's wrapper scripts for `clang`/`clang++` currently only, and will only ever pass `--target` to the compiler. That is something we can do ourselves, especially now that we're already updating `RUSTFLAGS` anyway. [Upstream even clarified] that it is recommended to pass `--target` yourself instead of incurring extra overhead while going through the wrapper scripts (especially costly on Windows), further guaranteeing that we won't miss out on any flags possibly being added to the wrapper scripts in the future. [Upstream even clarified]: https://r.android.com/2134712
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This was referenced May 16, 2023
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May 18, 2023
This is an alternative fix for bbqsrc#92 that avoids hitting command line length limitations and avoids needing to use `cargo-ndk` as a spring board for running `clang`, which avoids needing temporary hard links or copying the `cargo-ndk` binary to the target/ directory. This takes a similar approach to `ndk-build` here: rust-mobile/ndk#306 but also tries to be more thorough with reading configured rustflags. Although this solution initially looked like it would be much simpler it soon became apparent that for it to work reliably it needs to be able to read the user's configured rustflags before adding the required `-Clink-arg=--target` rustflag (otherwise we would potentially discard rustflags needed by the project). Unfortunately it turns out to be practically impossible to reliably read configured rustflags from outside cargo! Rustflags are read by cargo from one of four, mutually-exclusive, places: 1. CARGO_ENCODED_RUSTFLAGS environment variable. 2. RUSTFLAGS environment variable. 3. All matching target.<triple>.rustflags and target.<cfg>.rustflags config entries joined together. 4. build.rustflags config value. (cargo also supports --config command line options that can set these) Although we can easily handle the first two environment variables, the second two involve: - parsing all of Cargo's config files - evaluating cfg options and handling a paradox whereby --cfg arguments specified as rustflags need to then also affect which target.<cfg>rustflags to use! (cargo does this internally with 2 iterations) This experimental patch handles the first two cases and by adding a dependency on the `cargo` crate will even attempt to handle the second two options, with these caveats: 1. --config options on the command line aren't handled 2. _Any_ use of target.<cfg>.rustflags are treated as an error because we have no way of knowing if these are needed but we can't handle them 3. It's always possible that future versions of Cargo could add new ways to configure rustflags that we might not handle
This was referenced May 18, 2023
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The NDK's wrapper scripts for
clang/clang++currently only, and will only ever pass--targetto the compiler. That is something we can do ourselves, especially now that we're already updatingRUSTFLAGSanyway.Upstream even clarified that it is recommended to pass
--targetyourself instead of incurring extra overhead while going through the wrapper scripts (especially costly on Windows), further guaranteeing that we won't miss out on any flags possibly being added to the wrapper scripts in the future.