Let mypyc optimise os.path.join

See python#17948

There's one call site which has varargs that I leave as os.path.join, it
doesn't show up on my profile. I do see the `endswith` on the profile,
we could try `path[-1] == '/'` instead

In my work environment, this is about a 10% speedup:
```
λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_6eddd3ab1/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python'
Benchmark 1: /tmp/mypy_primer/timer_mypy_6eddd3ab1/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python
  Time (mean ± σ):     30.842 s ±  0.119 s    [User: 26.383 s, System: 4.396 s]
  Range (min … max):   30.706 s … 30.927 s    3 runs
```
Compared to:
```
λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_88ae62b4a/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python'
Benchmark 1: /tmp/mypy_primer/timer_mypy_88ae62b4a/venv/bin/mypy  -c "import torch" --no-incremental --python-executable /opt/oai/bin/python
  Time (mean ± σ):     34.161 s ±  0.163 s    [User: 29.818 s, System: 4.289 s]
  Range (min … max):   34.013 s … 34.336 s    3 runs
```

In the toy "long" environment mentioned in the issue, this is about a 7%
speedup:
```
λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_6eddd3ab1/venv/bin/mypy  -c "import torch" --no-incremental --python-executable long/bin/python'
Benchmark 1: /tmp/mypy_primer/timer_mypy_6eddd3ab1/venv/bin/mypy  -c "import torch" --no-incremental --python-executable long/bin/python
  Time (mean ± σ):     23.177 s ±  0.317 s    [User: 20.265 s, System: 2.873 s]
  Range (min … max):   22.815 s … 23.407 s    3 runs
```
Compared to:
```
λ hyperfine -w 1 -M 3 '/tmp/mypy_primer/timer_mypy_88ae62b4a/venv/bin/mypy -c "import torch" --python-executable=long/bin/python --no-incremental'
Benchmark 1: /tmp/mypy_primer/timer_mypy_88ae62b4a/venv/bin/mypy -c "import torch" --python-executable=long/bin/python --no-incremental
  Time (mean ± σ):     24.838 s ±  0.237 s    [User: 22.038 s, System: 2.750 s]
  Range (min … max):   24.598 s … 25.073 s    3 runs
```

mypy/modulefinder.py

@@ -22,6 +22,7 @@
 from mypy.nodes import MypyFile
 from mypy.options import Options
 from mypy.stubinfo import approved_stub_package_exists
+from mypy.util import os_path_join
 
 
 # Paths to be searched in find_module().
@@ -205,7 +206,7 @@ def find_module_via_source_set(self, id: str) -> ModuleSearchResult | None:
             d = os.path.dirname(p)
             for _ in range(id.count(".")):
                 if not any(
-                    self.fscache.isfile(os.path.join(d, "__init__" + x)) for x in PYTHON_EXTENSIONS
+                    self.fscache.isfile(os_path_join(d, "__init__" + x)) for x in PYTHON_EXTENSIONS
                 ):
                     return None
                 d = os.path.dirname(d)
@@ -249,7 +250,7 @@ def find_lib_path_dirs(self, id: str, lib_path: tuple[str, ...]) -> PackageDirs:
         dirs = []
         for pathitem in self.get_toplevel_possibilities(lib_path, components[0]):
             # e.g., '/usr/lib/python3.4/foo/bar'
-            dir = os.path.normpath(os.path.join(pathitem, dir_chain))
+            dir = os.path.normpath(os_path_join(pathitem, dir_chain))
             if self.fscache.isdir(dir):
                 dirs.append((dir, True))
         return dirs
@@ -320,8 +321,8 @@ def _find_module_non_stub_helper(
         plausible_match = False
         dir_path = pkg_dir
         for index, component in enumerate(components):
-            dir_path = os.path.join(dir_path, component)
-            if self.fscache.isfile(os.path.join(dir_path, "py.typed")):
+            dir_path = os_path_join(dir_path, component)
+            if self.fscache.isfile(os_path_join(dir_path, "py.typed")):
                 return os.path.join(pkg_dir, *components[:-1]), index == 0
             elif not plausible_match and (
                 self.fscache.isdir(dir_path) or self.fscache.isfile(dir_path + ".py")
@@ -418,9 +419,9 @@ def _find_module(self, id: str, use_typeshed: bool) -> ModuleSearchResult:
         # Third-party stub/typed packages
         for pkg_dir in self.search_paths.package_path:
             stub_name = components[0] + "-stubs"
-            stub_dir = os.path.join(pkg_dir, stub_name)
+            stub_dir = os_path_join(pkg_dir, stub_name)
             if fscache.isdir(stub_dir):
-                stub_typed_file = os.path.join(stub_dir, "py.typed")
+                stub_typed_file = os_path_join(stub_dir, "py.typed")
                 stub_components = [stub_name] + components[1:]
                 path = os.path.join(pkg_dir, *stub_components[:-1])
                 if fscache.isdir(path):
@@ -430,7 +431,7 @@ def _find_module(self, id: str, use_typeshed: bool) -> ModuleSearchResult:
                         # Partial here means that mypy should look at the runtime
                         # package if installed.
                         if fscache.read(stub_typed_file).decode().strip() == "partial":
-                            runtime_path = os.path.join(pkg_dir, dir_chain)
+                            runtime_path = os_path_join(pkg_dir, dir_chain)
                             third_party_inline_dirs.append((runtime_path, True))
                             # if the package is partial, we don't verify the module, as
                             # the partial stub package may not have a __init__.pyi
@@ -580,7 +581,7 @@ def find_modules_recursive(self, module: str) -> list[BuildSource]:
             # Skip certain names altogether
             if name in ("__pycache__", "site-packages", "node_modules") or name.startswith("."):
                 continue
-            subpath = os.path.join(package_path, name)
+            subpath = os_path_join(package_path, name)
 
             if self.options and matches_exclude(
                 subpath, self.options.exclude, self.fscache, self.options.verbosity >= 2
@@ -590,8 +591,8 @@ def find_modules_recursive(self, module: str) -> list[BuildSource]:
             if self.fscache.isdir(subpath):
                 # Only recurse into packages
                 if (self.options and self.options.namespace_packages) or (
-                    self.fscache.isfile(os.path.join(subpath, "__init__.py"))
-                    or self.fscache.isfile(os.path.join(subpath, "__init__.pyi"))
+                    self.fscache.isfile(os_path_join(subpath, "__init__.py"))
+                    or self.fscache.isfile(os_path_join(subpath, "__init__.pyi"))
                 ):
                     seen.add(name)
                     sources.extend(self.find_modules_recursive(module + "." + name))
@@ -636,7 +637,7 @@ def verify_module(fscache: FileSystemCache, id: str, path: str, prefix: str) ->
     for i in range(id.count(".")):
         path = os.path.dirname(path)
         if not any(
-            fscache.isfile_case(os.path.join(path, f"__init__{extension}"), prefix)
+            fscache.isfile_case(os_path_join(path, f"__init__{extension}"), prefix)
             for extension in PYTHON_EXTENSIONS
         ):
             return False
@@ -651,7 +652,7 @@ def highest_init_level(fscache: FileSystemCache, id: str, path: str, prefix: str
     for i in range(id.count(".")):
         path = os.path.dirname(path)
         if any(
-            fscache.isfile_case(os.path.join(path, f"__init__{extension}"), prefix)
+            fscache.isfile_case(os_path_join(path, f"__init__{extension}"), prefix)
             for extension in PYTHON_EXTENSIONS
         ):
             level = i + 1
@@ -842,11 +843,11 @@ def load_stdlib_py_versions(custom_typeshed_dir: str | None) -> StdlibVersions:
 
     None means there is no maximum version.
     """
-    typeshed_dir = custom_typeshed_dir or os.path.join(os.path.dirname(__file__), "typeshed")
-    stdlib_dir = os.path.join(typeshed_dir, "stdlib")
+    typeshed_dir = custom_typeshed_dir or os_path_join(os.path.dirname(__file__), "typeshed")
+    stdlib_dir = os_path_join(typeshed_dir, "stdlib")
     result = {}
 
-    versions_path = os.path.join(stdlib_dir, "VERSIONS")
+    versions_path = os_path_join(stdlib_dir, "VERSIONS")
     assert os.path.isfile(versions_path), (custom_typeshed_dir, versions_path, __file__)
     with open(versions_path) as f:
         for line in f:

mypy/util.py

@@ -417,6 +417,20 @@ def is_sub_path(path1: str, path2: str) -> bool:
     return pathlib.Path(path2) in pathlib.Path(path1).parents
 
 
+if sys.platform == "linux" or sys.platform == "darwin":
+    def os_path_join(path: str, b: str) -> str:
+        # Based off of os.path.join, but simplified to str-only, 2 args and mypyc can compile it.
+        if b.startswith("/") or not path:
+            return b
+        elif path.endswith("/"):
+            return path + b
+        else:
+            return path + "/" + b
+else:
+    def os_path_join(a: str, p: str) -> str:
+        return os.path.join(a, p)
+
+
 def hard_exit(status: int = 0) -> None:
     """Kill the current process without fully cleaning up.