Preserve fragments when applying verbatim redirects (#4038)

## Summary

`echo
"git+https://github.com/pypa/sample-namespace-packages.git#subdirectory=pkg_resources/pkg_a"
| cargo run pip compile -` now resolves to a compliant URL.

Closes https://github.com/astral-sh/uv/issues/4037.

crates/uv-resolver/src/redirect.rs

@@ -44,6 +44,9 @@ fn apply_redirect(url: &VerbatimUrl, redirect: Url) -> VerbatimUrl {
     // The redirect should be the "same" URL, but with a specific commit hash added after the `@`.
     // We take advantage of this to preserve as much of the verbatim representation as possible.
     if let Some(given) = url.given() {
+        let (given, fragment) = given
+            .split_once('#')
+            .map_or((given, None), |(prefix, suffix)| (prefix, Some(suffix)));
         if let Some(precise_suffix) = redirect
             .raw()
             .path()
@@ -56,13 +59,25 @@ fn apply_redirect(url: &VerbatimUrl, redirect: Url) -> VerbatimUrl {
                     // And the portion after the `@` is stable between the parsed and given representations...
                     if given_suffix == parsed_suffix {
                         // Preserve everything that precedes the `@` in the precise representation.
-                        return redirect.with_given(format!("{given_prefix}@{precise_suffix}"));
+                        let given = format!("{given_prefix}@{precise_suffix}");
+                        let given = if let Some(fragment) = fragment {
+                            format!("{given}#{fragment}")
+                        } else {
+                            given
+                        };
+                        return redirect.with_given(given);
                     }
                 }
             } else {
                 // If there was no `@` in the original representation, we can just append the
                 // precise suffix to the given representation.
-                return redirect.with_given(format!("{given}@{precise_suffix}"));
+                let given = format!("{given}@{precise_suffix}");
+                let given = if let Some(fragment) = fragment {
+                    format!("{given}#{fragment}")
+                } else {
+                    given
+                };
+                return redirect.with_given(given);
             }
         }
     }
@@ -118,6 +133,18 @@ mod tests {
         )?;
         assert_eq!(apply_redirect(&verbatim, redirect), expected);
 
+        // We should preserve subdirectory fragments.
+        let verbatim = VerbatimUrl::parse_url("https://github.com/flask.git#subdirectory=src")?
+            .with_given("git+https://github.com/flask.git#subdirectory=src");
+        let redirect =
+            Url::parse("https://github.com/flask.git@b90a4f1f4a370e92054b9cc9db0efcb864f87ebe#subdirectory=src")?;
+
+        let expected = VerbatimUrl::parse_url(
+            "https://github.com/flask.git@b90a4f1f4a370e92054b9cc9db0efcb864f87ebe#subdirectory=src",
+        )?.with_given("git+https://github.com/flask.git@b90a4f1f4a370e92054b9cc9db0efcb864f87ebe#subdirectory=src");
+
+        assert_eq!(apply_redirect(&verbatim, redirect), expected);
+
         Ok(())
     }
 }

crates/uv/tests/pip_compile.rs

@@ -1565,7 +1565,7 @@ fn compile_git_subdirectory_static_metadata() -> Result<()> {
     ----- stdout -----
     # This file was autogenerated by uv via the following command:
     #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in
-    uv-public-pypackage @ git+https://github.com/astral-test/uv-workspace-pypackage#subdirectory=uv-public-pypackage@b8c4e192456d736c27f2c84c61175c896dba8373
+    uv-public-pypackage @ git+https://github.com/astral-test/uv-workspace-pypackage@b8c4e192456d736c27f2c84c61175c896dba8373#subdirectory=uv-public-pypackage
         # via -r requirements.in
 
     ----- stderr -----