backend/ninja: stop adding random attributes to BuildTargets

Instead of storing the generated header cache in the Target, store it in
the NinjaBackend, as it really is just data for the NinjaBackend to use.

mesonbuild/backend/ninjabackend.py

@@ -503,6 +503,7 @@ class NinjaBackend(backends.Backend):
         self.rust_crates: T.Dict[str, RustCrate] = {}
         self.implicit_meson_outs: T.List[str] = []
         self._uses_dyndeps = False
+        self._generated_header_cache: T.Dict[str, T.List[FileOrString]] = {}
         # nvcc chokes on thin archives:
         #   nvlink fatal   : Could not open input file 'libfoo.a.p'
         #   nvlink fatal   : elfLink internal error
@@ -738,8 +739,9 @@ class NinjaBackend(backends.Backend):
     # Get all generated headers. Any source file might need them so
     # we need to add an order dependency to them.
     def get_generated_headers(self, target: build.BuildTarget) -> T.List[FileOrString]:
-        if hasattr(target, 'cached_generated_headers'):
-            return target.cached_generated_headers
+        tid = target.get_id()
+        if tid in self._generated_header_cache:
+            return self._generated_header_cache[tid]
         header_deps: T.List[FileOrString] = []
         # XXX: Why don't we add deps to CustomTarget headers here?
         for genlist in target.get_generated_sources():
@@ -757,7 +759,7 @@ class NinjaBackend(backends.Backend):
                 header_deps += self.get_generated_headers(dep)
         if isinstance(target, build.CompileTarget):
             header_deps.extend(target.get_generated_headers())
-        target.cached_generated_headers = header_deps
+        self._generated_header_cache[tid] = header_deps
         return header_deps
 
     def get_target_generated_sources(self, target: build.BuildTarget) -> T.MutableMapping[str, File]:

mesonbuild/build.py

@@ -721,9 +721,6 @@ class BuildTarget(Target):
         ('cpp', 'cuda'),
     ])
 
-    # This is used by the backend to cache complex computation.
-    cached_generated_headers: T.Optional[T.List[FileOrString]]
-
     def __init__(
             self,
             name: str,