commit 8fd0904a58d1256a531a89ecdb774ba970049185
author Vinh Tran <vinhdaitran@google.com> Thu Feb 15 23:04:34 2024 -0500
committer GitHub <noreply@github.com> Thu Feb 15 23:04:34 2024 -0500
tree db985bd9ccc382cab5a7291f9f839ec2dc11950d
parent 5c715ec50602e2ba6ca2ebfdd870662a6e6d1eda

Fix linkopts set in cc deps of bindgen (#2422)

### Problem

As of now (before this PR), we seem to mix `link_flags` file into using
for two purposes.

For
```
cc_library(
    name = "simple",
    srcs = ["simple.cc"],
    hdrs = ["simple.h"],
    linkopts = ["-framework"],
)
rust_bindgen_library(
    name = "simple_bindgen",
    cc_lib = ":simple",
    header = "simple.h",
)
rust_binary(
    name = "simple_example",
    srcs = ["main.rs"],
    deps = [":simple_bindgen"],
)
```
`rust_bindgen_library` produces a `link_flags` file with

```
-lstatic=simple
-C
link-args=-framework
```

`-lstatic=simple` is consumed by `rustc` whereas `-framework` is
expected to be consumed by an actual linker (either invoked by `rustc`
or `cc_common.link`)

The flags are then passed to `rustc` command to compile
`libsimple_bindgen.rlib`. It does not yield any error because
`-lstatic=simple` is correctly used whereas `-framework` is no-op (there
is no linker being invoked for producing `rlib`). However, the rustc
***doesn't*** pass `-framework` to the linker that link the
`rust_binary` (which is where the cc linkopts matters).

Another problem is that this approach is not compatible with
`experimental_use_cc_common_link` option which let `cc_common.link`
instead of `rustc` invoke the linker. See #2413

### Solution

We're referring "link" as at least two things (which I think what causes
problem here):

1.
https://doc.rust-lang.org/rustc/command-line-arguments.html#-l-link-the-generated-crate-to-a-native-library
2. https://doc.rust-lang.org/rustc/codegen-options/index.html#linker
https://doc.rust-lang.org/rustc/codegen-options/index.html#link-args

As proposed in
https://github.com/bazelbuild/rules_rust/pull/2415#issuecomment-1890109092,
this PR splits `<rust_library_bindgen>__bindgen.link_flags` produced by
`rust_bindgen` rule into two files:

1. `rustc_flags`
```
-lstatic=simple
```
2. `linker_flags`
```
-framework
```

We "sort-of" (but not perfectly) had it correct before
https://github.com/bazelbuild/rules_rust/pull/2361 when we link the
binary directly with the cc_library (aka both kinds of flags).

But since we want to support the Cargo style of linking
```
cc_lib -> bindgen -> lib_a  -> bin
```
instead of
```
cc_lib -> bindgen -> lib_a  -> bin
    \___________________________^
```
we can pass `-lstatic=simple` to the `rustc` command that builds
`simple_bindgen` (rust_library) and propagate `linkopts` to
`simple_example` (rust_binary) so that the linker can use it.

```
cc_library -> rust_bindgen ->    rust_library        -> rust_binary
                                                              
                                    -lstatic=simple
                                                            -Clink-args="-framework"
``` 

This is long and sounds like a proposal. I'm open for suggestion
@UebelAndre @illicitonion @krasimirgg

Fixes #2413