build: Move build subdir logic into compute_build_subdir()

mesonbuild/build.py

@@ -458,7 +458,7 @@ def to_string_list(self, sourcedir: str, builddir: str) -> T.List[str]:
             be added if this is unset
         :returns: A list of strings (without compiler argument)
         """
-        bsubdir = f'build.{self.curdir}' if self.is_build_only_subproject else self.curdir
+        bsubdir = compute_build_subdir(self.curdir, self.is_build_only_subproject)
 
         strlist: T.List[str] = []
         for idir in self.incdirs:
@@ -672,9 +672,7 @@ def get_source_subdir(self) -> str:
         return self.subdir
 
     def get_output_subdir(self) -> str:
-        if self.build_only_subproject:
-            return f'build.{self.subdir}'
-        return self.get_source_subdir()
+        return compute_build_subdir(self.subdir, self.build_only_subproject)
 
     def get_typename(self) -> str:
         return self.typename
@@ -2005,9 +2003,7 @@ def get_source_subdir(self) -> str:
         return self.subdir
 
     def get_output_subdir(self) -> str:
-        if self.is_build_only_subproject:
-            return f'build.{self.subdir}'
-        return self.get_source_subdir()
+        return compute_build_subdir(self.subdir, self.is_build_only_subproject)
 
 
 class Executable(BuildTarget):
@@ -3186,6 +3182,11 @@ def get_sources_string_names(sources, backend):
             raise AssertionError(f'Unknown source type: {s!r}')
     return names
 
+def compute_build_subdir(subdir: str, build_only_subproject: bool) -> str:
+    if build_only_subproject:
+        return f'build.{subdir}'
+    return subdir
+
 def load(build_dir: str) -> Build:
     filename = os.path.join(build_dir, 'meson-private', 'build.dat')
     try:

mesonbuild/interpreter/interpreter.py

@@ -971,7 +971,7 @@ def _do_subproject_meson(self, subp_name: SubProject, subdir: str,
                 ast.accept(printer)
                 printer.post_process()
                 bsubdir = os.path.join(self.build.environment.get_build_dir(),
-                                       subdir if not new_build.environment.coredata.is_build_only else f'build.{subdir}')
+                                       build.compute_build_subdir(subdir, new_build.environment.coredata.is_build_only))
                 os.makedirs(bsubdir, exist_ok=True)
                 meson_filename = os.path.join(bsubdir, 'meson.build')
                 with open(meson_filename, "w", encoding='utf-8') as f: