commit e7f55168ace5920eb41488133f3c03ada7fcd998
author Vinh Tran <vinhdaitran@google.com> Fri Feb 23 10:58:54 2024 -0500
committer GitHub <noreply@github.com> Fri Feb 23 15:58:54 2024 +0000
tree 647270b2803eb0f47f16807becf33485fdce520d
parent 33fdddd03c9a958d82efcf9658f9dce13a9f06df

Implement support for dylib linkage (#2414)

This PR implements dylib linkage against the standard library behind a
feature flag
`--@rules_rust//rust/settings:experimental_use_dylib_linkage`.


The main part of this feature is
[here](https://github.com/bazelbuild/rules_rust/pull/2414/files#diff-2a806da393e47c07ffe67c78ace69eb488b4ac44b029a46d8237b8e2a05637beR258)
where we skip exporting static rust stdlibs and export only `libstd.so`
instead.

This feature is useful when the subset of libstd being statically linked
to downstream shared libraries and binaries is **larger** than the
entire dylib version of libstd. The following diagram is the high level
of what dylib linkage is trying to achieve.


![Untitled Diagram
drawio](https://github.com/bazelbuild/rules_rust/assets/13268391/d19f18f5-c2d1-4ddc-b170-773a6004f732)

Running the feature against `android_binary` yields a size reduction on
the shared library produced by `android_binary` because it doesn't
statically link the rust stdlibs anymore.

```
> bazel build //:android_app --config=android_x86_64 
> unzip -l bazel-bin/android_app.apk
Archive:  bazel-bin/android_app.apk
  Length      Date    Time    Name
---------  ---------- -----   ----
  1381968  2010-01-01 00:00   lib/x86_64/libandroid_app.so <--- static link with rust stdlibs
---------                     -------
  1390294                     9 files
```

```
> bazel build //:android_app --config=android_x86_64 --config=dylib_linkage
> unzip -l bazel-bin/android_app.apk
Archive:  bazel-bin/android_app.apk
  Length      Date    Time    Name
---------  ---------- -----   ----
     8080  2010-01-01 00:00   lib/x86_64/libandroid_app.so <--- reduced size because of dynamic linking
 13055776  2010-01-01 00:00   lib/x86_64/libstd-8d416d49cf02ecea.so
---------                     -------
 13072400                     10 files
 ```

Here, the benefit comes when there are enough shared libraries statically linking against the rust stdlibs. "Enough" here means that the total up size of those libraries being more than just the entire `libstd.so`.

TODO: I'm leaving this PR without unit tests until I get some feedback or suggestions on my approach.

---------

Co-authored-by: scentini <rosica@google.com>