Refine 'rewrite_comes_from' functions

Rather than a callable|str with dispatch, make this into a single
codepath: always a callable. The `str` case is converted into a
fixed-output function to fit this shape.

Also, preserve the space separator in string splitting, so that it is
clear that we are reconstructing data with the same shape (same
separator).

Co-authored-by: 🇺🇦 Sviatoslav Sydorenko (Святослав Сидоренко) <578543+webknjaz@users.noreply.github.com>

Author: sirosen

piptools/_compat/pip_compat.py

@@ -87,12 +87,12 @@ def parse_requirements(
     comes_from_stdin: bool = False,
 ) -> Iterator[InstallRequirement]:
     # the `comes_from` data will be rewritten in different ways in different conditions
-    # each rewrite rule is expressible as a str->str function or as a static replacement
-    rewrite_comes_from: Callable[[str], str] | str
+    # each rewrite rule is expressible as a str->str function
+    rewrite_comes_from: Callable[[str], str]
 
     if comes_from_stdin:
         # if data is coming from stdin, then `comes_from="-r -"`
-        rewrite_comes_from = "-r -"
+        rewrite_comes_from = _rewrite_comes_from_to_hardcoded_stdin_value
     elif pathlib.Path(filename).is_absolute():
         # if the input path is absolute, just normalize paths to posix-style
         rewrite_comes_from = _normalize_comes_from_location
@@ -113,14 +113,16 @@ def parse_requirements(
             install_req.link = file_link
         install_req = copy_install_requirement(install_req)
 
-        if isinstance(rewrite_comes_from, str):
-            install_req.comes_from = rewrite_comes_from
-        else:
-            install_req.comes_from = rewrite_comes_from(install_req.comes_from)
+        install_req.comes_from = rewrite_comes_from(install_req.comes_from)
 
         yield install_req
 
 
+def _rewrite_comes_from_to_hardcoded_stdin_value(_: str, /) -> str:
+    """Produce the hardcoded ``comes_from`` value for stdin."""
+    return "-r -"
+
+
 def _relativize_comes_from_location(original_comes_from: str, /) -> str:
     """
     Convert a ``comes_from`` path to a relative posix path.
@@ -136,7 +138,7 @@ def _relativize_comes_from_location(original_comes_from: str, /) -> str:
         return original_comes_from
 
     # split on the space
-    prefix, _, suffix = original_comes_from.partition(" ")
+    prefix, space_sep, suffix = original_comes_from.partition(" ")
 
     file_path = pathlib.Path(suffix)
 
@@ -146,7 +148,7 @@ def _relativize_comes_from_location(original_comes_from: str, /) -> str:
 
     # make it relative to the current working dir
     suffix = file_path.relative_to(pathlib.Path.cwd()).as_posix()
-    return f"{prefix} {suffix}"
+    return f"{prefix}{space_sep}{suffix}"
 
 
 def _normalize_comes_from_location(original_comes_from: str, /) -> str:
@@ -165,11 +167,11 @@ def _normalize_comes_from_location(original_comes_from: str, /) -> str:
         return original_comes_from
 
     # split on the space
-    prefix, _, suffix = original_comes_from.partition(" ")
+    prefix, space_sep, suffix = original_comes_from.partition(" ")
 
     # convert to a posix-style path
     suffix = pathlib.Path(suffix).as_posix()
-    return f"{prefix} {suffix}"
+    return f"{prefix}{space_sep}{suffix}"
 
 
 def create_wheel_cache(cache_dir: str, format_control: str | None = None) -> WheelCache: