cargo: support lib.path in Cargo.toml

Cargo implements this configuration option to override the entry point
of the library.

To get test coverage, I've modified one of the two Cargo subprojects
in the test to use a non-default library entrypoint.  The other still
uses the default.

mesonbuild/cargo/interpreter.py

@@ -206,6 +206,7 @@ class Library(BuildTarget):
 
     doctest: bool = True
     doc: bool = True
+    path: str = os.path.join('src', 'lib.rs')
     proc_macro: bool = False
     crate_type: T.List[manifest.CRATE_TYPE] = dataclasses.field(default_factory=lambda: ['lib'])
     doc_scrape_examples: bool = True
@@ -606,7 +607,7 @@ def _create_lib(cargo: Manifest, build: builder.Builder, crate_type: manifest.CR
 
     posargs: T.List[mparser.BaseNode] = [
         build.string(fixup_meson_varname(cargo.package.name)),
-        build.string(os.path.join('src', 'lib.rs')),
+        build.string(cargo.lib.path),
     ]
 
     kwargs: T.Dict[str, mparser.BaseNode] = {
@@ -696,7 +697,7 @@ def interpret(subp_name: str, subdir: str, env: Environment) -> T.Tuple[mparser.
 
     # Libs are always auto-discovered and there's no other way to handle them,
     # which is unfortunate for reproducability
-    if os.path.exists(os.path.join(env.source_dir, cargo.subdir, cargo.path, 'src', 'lib.rs')):
+    if os.path.exists(os.path.join(env.source_dir, cargo.subdir, cargo.path, cargo.lib.path)):
         for crate_type in cargo.lib.crate_type:
             ast.extend(_create_lib(cargo, build, crate_type))
 

test cases/rust/22 cargo subproject/subprojects/foo-rs/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [lib]
 crate-type = ["cdylib"]
+path = "lib.rs"
 
 # This dependency does not exist, verify optional works.
 [dependencies.notfound]