rust: recursively pull proc-macro deps for rust targets

aee941559 ("rust: recursively pull proc-macro dependencies as well")
had to be reverted (in a66cb97e8) because it broke Mesa cross
compilation.  This happened because a C shared library was linked with
a Rust C-ABI static library, which caused it to inherit the proc macro
dependency the Rust static library was linked with.

The right way to handle this is for only Rust targets to inherit proc
macro dependencies from static libraries they link with.  A Rust
executable, library, or whatever will need the proc macros its Rust
dependencies use, as illustrated in the test case that I've
reintroduced here.

I've verified that Mesa still cross compiles correctly with this
change.  The same failure was also identified by the "rust/21
transitive dependencies" test case, but only when cross compiling, so
it wasn't caught by CI.

Co-authored-by: Xavier Claessens <xavier.claessens@collabora.com>

mesonbuild/build.py

@@ -1292,10 +1292,10 @@ class BuildTarget(Target):
             if t not in result:
                 result.add(t)
                 if isinstance(t, StaticLibrary):
-                    t.get_dependencies_recurse(result)
+                    t.get_dependencies_recurse(result, include_proc_macros = self.uses_rust())
         return result
 
-    def get_dependencies_recurse(self, result: OrderedSet[BuildTargetTypes], include_internals: bool = True) -> None:
+    def get_dependencies_recurse(self, result: OrderedSet[BuildTargetTypes], include_internals: bool = True, include_proc_macros: bool = False) -> None:
         # self is always a static library because we don't need to pull dependencies
         # of shared libraries. If self is installed (not internal) it already
         # include objects extracted from all its internal dependencies so we can
@@ -1304,14 +1304,14 @@ class BuildTarget(Target):
         for t in self.link_targets:
             if t in result:
                 continue
-            if t.rust_crate_type == 'proc-macro':
+            if not include_proc_macros and t.rust_crate_type == 'proc-macro':
                 continue
             if include_internals or not t.is_internal():
                 result.add(t)
             if isinstance(t, StaticLibrary):
-                t.get_dependencies_recurse(result, include_internals)
+                t.get_dependencies_recurse(result, include_internals, include_proc_macros)
         for t in self.link_whole_targets:
-            t.get_dependencies_recurse(result, include_internals)
+            t.get_dependencies_recurse(result, include_internals, include_proc_macros)
 
     def get_source_subdir(self):
         return self.subdir

test cases/rust/18 proc-macro/lib.rs

@@ -0,0 +1,8 @@
+extern crate proc_macro_examples;
+use proc_macro_examples::make_answer;
+
+make_answer!();
+
+pub fn func() -> u32 {
+    answer()
+}

test cases/rust/18 proc-macro/meson.build

@@ -31,3 +31,14 @@ main = executable(
 )
 
 test('main_test2', main)
+
+subdir('subdir')
+
+staticlib = static_library('staticlib', 'lib.rs',
+  link_with: pm_in_subdir,
+  rust_dependency_map : {'proc_macro_examples3' : 'proc_macro_examples'}
+)
+
+executable('transitive-proc-macro', 'transitive-proc-macro.rs',
+  link_with: staticlib,
+)

test cases/rust/18 proc-macro/subdir/meson.build

@@ -0,0 +1 @@
+pm_in_subdir = rust.proc_macro('proc_macro_examples3', '../proc.rs')

test cases/rust/18 proc-macro/transitive-proc-macro.rs

@@ -0,0 +1,7 @@
+extern crate staticlib;
+use staticlib::func;
+
+
+fn main() {
+    assert_eq!(42, func());
+}

test cases/rust/21 transitive dependencies/meson.build

@@ -21,6 +21,7 @@ foo = static_library('foo-rs', 'foo.rs',
   rust_abi: 'c',
   link_with: pm,
 )
+shared_library('shared', 'foo.c', link_with : foo)
 exe = executable('footest', 'foo.c',
   link_with: foo,
 )