add LinkHash.{from_archive_info(),__post_init__()}

these two objects are intended to be fungible, but play different roles, since LinkHash actually
parses its input and ArchiveInfo does not. ArchiveInfo's JSON (de)?serialization does not employ
hash name or value validation, while LinkHash does not offer a JSON serialization.

Author: cosmicexplorer

src/pip/_internal/models/link.py

@@ -37,7 +37,12 @@
 class LinkHash:
     """Links to content may have embedded hash values. This class parses those.
 
-    `name` must be any member of `_SUPPORTED_HASHES`."""
+    `name` must be any member of `_SUPPORTED_HASHES`.
+
+    This class can be converted to and from `ArchiveInfo`. While ArchiveInfo intends to
+    be JSON-serializable to conform to PEP 610, this class contains the logic for
+    parsing a hash name and value for correctness, and then checking whether that hash
+    conforms to a schema with `.is_hash_allowed()`."""
 
     name: str
     value: str
@@ -52,6 +57,9 @@ class LinkHash:
         )
     )
 
+    def __post_init__(self) -> None:
+        assert self._hash_re.match(f"{self.name}={self.value}")
+
     @classmethod
     @functools.lru_cache(maxsize=None)
     def split_hash_name_and_value(cls, url: str) -> Optional["LinkHash"]:
@@ -66,6 +74,13 @@ def to_archive_info(self) -> ArchiveInfo:
         """Convert to ArchiveInfo to form a DirectUrl instance (see PEP 610)."""
         return ArchiveInfo(hash=f"{self.name}={self.value}")
 
+    @classmethod
+    def from_archive_info(cls, info: ArchiveInfo) -> Optional["LinkHash"]:
+        """Parse an ArchiveInfo hash into a LinkHash instance."""
+        if info.hash is None:
+            return None
+        return cls.split_hash_name_and_value(info.hash)
+
     def is_hash_allowed(self, hashes: Optional[Hashes]) -> bool:
         """
         Return True if the current hash is allowed by `hashes`.

tests/unit/test_collector.py

@@ -1048,3 +1048,36 @@ def expand_path(path: str) -> str:
 )
 def test_link_hash_parsing(url: str, result: Optional[LinkHash]) -> None:
     assert LinkHash.split_hash_name_and_value(url) == result
+
+
+@pytest.mark.parametrize(
+    "archive_info, link_hash",
+    [
+        (
+            ArchiveInfo(hash=None),
+            None,
+        ),
+        (
+            ArchiveInfo(hash="sha256=aabe42af"),
+            LinkHash(name="sha256", value="aabe42af"),
+        ),
+        # Test invalid hash strings, which ArchiveInfo doesn't validate.
+        (
+            # Invalid hash name.
+            ArchiveInfo(hash="sha500=aabe42af"),
+            None,
+        ),
+        (
+            # Invalid hash value.
+            ArchiveInfo(hash="sha256=g42afbe"),
+            None,
+        ),
+    ],
+)
+def test_link_hash_archive_info_fungibility(
+    archive_info: ArchiveInfo,
+    link_hash: Optional[LinkHash],
+) -> None:
+    assert LinkHash.from_archive_info(archive_info) == link_hash
+    if link_hash is not None:
+        assert link_hash.to_archive_info() == archive_info