cargo: Call into meson subdir if it exists

This allows projects to manually add extra rust args and deps. This is
intended to replace build.rs logic.

docs/markdown/Wrap-dependency-system-manual.md

@@ -356,6 +356,14 @@ project(...,
 )
 ```
 
+In addition, if the file `meson/meson.build` exists, Meson will call `subdir('meson')`
+where the project can add manual logic that would usually be part of `build.rs`.
+Some naming conventions need to be respected:
+- The `extra_args` variable is pre-defined and can be used to add any Rust arguments.
+  This is typically used as `extra_args += ['--cfg', 'foo']`.
+- The `extra_deps` variable is pre-defined and can be used to add extra dependencies.
+  This is typically used as `extra_deps += dependency('foo')`.
+
 ## Using wrapped projects
 
 Wraps provide a convenient way of obtaining a project into your

mesonbuild/cargo/interpreter.py

@@ -366,6 +366,14 @@ def _options_varname(depname: str) -> str:
     return f'{fixup_meson_varname(depname)}_options'
 
 
+def _extra_args_varname() -> str:
+    return 'extra_args'
+
+
+def _extra_deps_varname() -> str:
+    return 'extra_deps'
+
+
 def _create_project(cargo: Manifest, build: builder.Builder) -> T.List[mparser.BaseNode]:
     """Create a function call
 
@@ -561,6 +569,25 @@ def _create_dependencies(cargo: Manifest, build: builder.Builder) -> T.List[mpar
     return ast
 
 
+def _create_meson_subdir(cargo: Manifest, build: builder.Builder) -> T.List[mparser.BaseNode]:
+    # Allow Cargo subprojects to add extra Rust args in meson/meson.build file.
+    # This is used to replace build.rs logic.
+
+    # extra_args = []
+    # extra_deps = []
+    # fs = import('fs')
+    # if fs.is_dir('meson')
+    #  subdir('meson')
+    # endif
+    return [
+        build.assign(build.array([]), _extra_args_varname()),
+        build.assign(build.array([]), _extra_deps_varname()),
+        build.assign(build.function('import', [build.string('fs')]), 'fs'),
+        build.if_(build.method('is_dir', build.identifier('fs'), [build.string('meson')]),
+                  build.block([build.function('subdir', [build.string('meson')])]))
+    ]
+
+
 def _create_lib(cargo: Manifest, build: builder.Builder, crate_type: manifest.CRATE_TYPE) -> T.List[mparser.BaseNode]:
     dependencies: T.List[mparser.BaseNode] = []
     dependency_map: T.Dict[mparser.BaseNode, mparser.BaseNode] = {}
@@ -570,7 +597,12 @@ def _create_lib(cargo: Manifest, build: builder.Builder, crate_type: manifest.CR
         if name != package_name:
             dependency_map[build.string(fixup_meson_varname(package_name))] = build.string(name)
 
-    rust_args: T.List[mparser.BaseNode] = [build.identifier('features_args')]
+    rust_args: T.List[mparser.BaseNode] = [
+        build.identifier('features_args'),
+        build.identifier(_extra_args_varname())
+    ]
+
+    dependencies.append(build.identifier(_extra_deps_varname()))
 
     posargs: T.List[mparser.BaseNode] = [
         build.string(fixup_meson_varname(cargo.package.name)),
@@ -660,6 +692,7 @@ def interpret(subp_name: str, subdir: str, env: Environment) -> T.Tuple[mparser.
     ast += [build.assign(build.function('import', [build.string('rust')]), 'rust')]
     ast += _create_features(cargo, build)
     ast += _create_dependencies(cargo, build)
+    ast += _create_meson_subdir(cargo, build)
 
     # Libs are always auto-discovered and there's no other way to handle them,
     # which is unfortunate for reproducability

test cases/rust/22 cargo subproject/subprojects/foo-rs/meson/meson.build

@@ -0,0 +1 @@
+extra_args += ['--cfg', 'feature="foo"']

test cases/rust/22 cargo subproject/subprojects/foo-rs/src/lib.rs

@@ -2,6 +2,7 @@ extern "C" {
     fn extra_func() -> i32;
 }
 
+#[cfg(feature = "foo")]
 #[no_mangle]
 pub extern "C" fn rust_func() -> i32 {
     let v: i32;