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ndk-build: Invoke clang directly instead of through wrapper scripts #306

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merged 1 commit into from
Jul 4, 2022

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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.

@MarijnS95 MarijnS95 force-pushed the invoke-clang-directly branch from d2927f7 to 0f61129 Compare June 25, 2022 11:08
@MarijnS95 MarijnS95 force-pushed the invoke-clang-directly branch 2 times, most recently from e11fab6 to cb3a4cb Compare July 4, 2022 18:51
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
@MarijnS95 MarijnS95 force-pushed the invoke-clang-directly branch from cb3a4cb to bc6eb22 Compare July 4, 2022 21:16
@MarijnS95 MarijnS95 merged commit 63f99d8 into rust-mobile:master Jul 4, 2022
@MarijnS95 MarijnS95 deleted the invoke-clang-directly branch July 4, 2022 21:39
rib added a commit to rib/cargo-ndk that referenced this pull request 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
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2 participants