Speed up suppressed deps options processing (#20806)

`suppressed_deps_opts` was consuming 5% of total CPU in a small
incremental run targeting a very large codebase. Make it faster by
avoiding json dump, and overall using a more efficient encoding.

We could still make this faster by encoding the options as a single
integer, but maybe this is already good enough.

Author: JukkaL

mypy/build.py

@@ -905,8 +905,9 @@ def __init__(
         # location of a symbol used as a location for an error message.
         self.extra_trees: dict[str, MypyFile] = {}
         # Snapshot of import-related options per module. We record these even for
-        # suppressed imports, since they can affect errors in the callers.
-        self.import_options: dict[str, dict[str, object]] = {}
+        # suppressed imports, since they can affect errors in the callers. Bytes
+        # value is opaque but can be compared to detect changes in options.
+        self.import_options: dict[str, bytes] = {}
         # Cache for transitive dependency check (expensive).
         self.transitive_deps_cache: dict[tuple[int, int], bool] = {}
 
@@ -3065,13 +3066,15 @@ def update_fine_grained_deps(self, deps: dict[str, set[str]]) -> None:
             type_state.update_protocol_deps(deps)
 
     def suppressed_deps_opts(self) -> bytes:
-        return json_dumps(
-            {
-                dep: self.manager.import_options[dep]
-                for dep in self.suppressed
-                if self.priorities.get(dep) != PRI_INDIRECT
-            }
-        )
+        if not self.suppressed:
+            return b""
+        buf = WriteBuffer()
+        import_options = self.manager.import_options
+        for dep in sorted(self.suppressed):
+            if self.priorities.get(dep) != PRI_INDIRECT:
+                write_str_bare(buf, dep)
+                write_bytes_bare(buf, import_options[dep])
+        return buf.getvalue()
 
     def write_cache(self) -> tuple[CacheMeta, str] | None:
         assert self.tree is not None, "Internal error: method must be called on parsed file only"

mypy/options.py

@@ -8,6 +8,8 @@
 from re import Pattern
 from typing import Any, Final
 
+from librt.internal import WriteBuffer, write_bool, write_str
+
 from mypy import defaults
 from mypy.errorcodes import ErrorCode, error_codes
 from mypy.util import get_class_descriptors, replace_object_state
@@ -628,10 +630,13 @@ def select_options_affecting_cache(self) -> tuple[str, list[object]]:
             result.append(val)
         return self.platform, result
 
-    def dep_import_options(self) -> dict[str, object]:
-        # These are options that can affect dependent modules as well.
-        return {
-            "ignore_missing_imports": self.ignore_missing_imports,
-            "follow_imports": self.follow_imports,
-            "follow_imports_for_stubs": self.follow_imports_for_stubs,
-        }
+    def dep_import_options(self) -> bytes:
+        """Return opaque bytes with options that can affect dependent modules as well.
+
+        The value can be compared for equality to detect changed options.
+        """
+        buf = WriteBuffer()
+        write_bool(buf, self.ignore_missing_imports)
+        write_str(buf, self.follow_imports)
+        write_bool(buf, self.follow_imports_for_stubs)
+        return buf.getvalue()