[rust] Rebuild libclang dependents on clang rolls

Sometimes clang libraries can be added or removed, so we need to ensure
we rebuild libclang's dependencies whenever clang rolls. Previously we
(effectively) expanded *.a into a .rsp file which wouldn't necessarily
be updated in these circumstances, leading to stale information and
potential build failure.

This CL contains two significant changes:
* We rerun find_clanglibs.py whenever the clang revision changes;
* We cease to generate a .rsp file and instead generate a synthetic Rust
  static library.

The reason for this second change is that the names of the various
clang libraries were persisted in intermediate .rlib files as metadata.
We therefore needed to force rebuild of every dependent .rlib, not
just the final linker invocation, when the clang libraries were
updated. Altering 'inputs' for many such rlib targets would be fragile;
instead, have them all (transitively) depend on a static library.

Bug: 1296236
Change-Id: Iea406b2394bbd2231ca8f4cb1b367666277fff9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3516695
Reviewed-by: danakj <[email protected]>
Commit-Queue: Adrian Taylor <[email protected]>
Cr-Commit-Position: refs/heads/main@{#979978}
NOKEYCHECK=True
GitOrigin-RevId: f8b3a9a06116a0d2a9d87473eac635a2605577e3

Author: adetaylor

clang_sys/v1/crate/src/lib.rs

@@ -34,6 +34,17 @@
 #![allow(non_camel_case_types, non_snake_case, non_upper_case_globals)]
 #![cfg_attr(feature = "cargo-clippy", allow(clippy::unreadable_literal))]
 
+/// Chromium customization: in order to find the libraries shipped with our
+/// version of clang, we generate a synthetic Rust crate called clanglibs
+/// containing lots of #[link(...)] directives and nothing else. We need to
+/// ensure that clang_sys and all its dependencies duly absorb that crate
+/// so that the final linking step (performed by rustc) reads all these
+/// library names from crate metadata and passes them to the linker.
+/// If we don't have this 'extern crate' line, the clanglibs crate does not
+/// get used by clang_sys or its dependencies, and those extra libraries
+/// are discarded.
+extern crate clanglibs;
+
 extern crate glob;
 extern crate libc;
 #[cfg(feature = "runtime")]

clang_sys/v1/patches/0002-link-against-clanglibs.patch

@@ -0,0 +1,35 @@
+From f8cada38a88b48ca4ab68e690b1977abc23bb53b Mon Sep 17 00:00:00 2001
+From: Adrian Taylor <[email protected]>
+Date: Thu, 10 Mar 2022 10:59:47 -0800
+Subject: [PATCH 1/1] Link against clanglibs
+
+Change-Id: Icaedc6dc749850df73ded4a8b0e49beb7da2e4c9
+---
+ third_party/rust/clang_sys/v1/crate/src/lib.rs | 11 +++++++++++
+ 1 file changed, 11 insertions(+)
+
+diff --git a/third_party/rust/clang_sys/v1/crate/src/lib.rs b/third_party/rust/clang_sys/v1/crate/src/lib.rs
+index 5bc0d2a68bef2..fec6acf7bf795 100644
+--- a/third_party/rust/clang_sys/v1/crate/src/lib.rs
++++ b/third_party/rust/clang_sys/v1/crate/src/lib.rs
+@@ -34,6 +34,17 @@
+ #![allow(non_camel_case_types, non_snake_case, non_upper_case_globals)]
+ #![cfg_attr(feature = "cargo-clippy", allow(clippy::unreadable_literal))]
+ 
++/// Chromium customization: in order to find the libraries shipped with our
++/// version of clang, we generate a synthetic Rust crate called clanglibs
++/// containing lots of #[link(...)] directives and nothing else. We need to
++/// ensure that clang_sys and all its dependencies duly absorb that crate
++/// so that the final linking step (performed by rustc) reads all these
++/// library names from crate metadata and passes them to the linker.
++/// If we don't have this 'extern crate' line, the clanglibs crate does not
++/// get used by clang_sys or its dependencies, and those extra libraries
++/// are discarded.
++extern crate clanglibs;
++
+ extern crate glob;
+ extern crate libc;
+ #[cfg(feature = "runtime")]
+-- 
+2.35.1.723.g4982287a31-goog
+