Respect pre-release preferences from input files (astral-sh#5736)

## Summary

Right now, if you have a `requirements.txt` with a pre-release, but the
`requirements.in` does not have a pre-release marker for that dependency
we drop the pre-release. (In the selector, we end up returning
`AllowPrerelease::IfNecessary`, the default.)

I played with a few ways of solving this... The first was to remove that
guard altogether. But if we do that,
`universal_transitive_disjoint_prerelease_requirement` fails (we use
`1.17.0rc1` in both forks, when it should only apply to one of the two).

The second was to do that, but also avoid pushing pre-releases as
preferences when we solve a fork. But then
`universal_disjoint_prereleases` fails, because we return a different
pre-release in each fork.

Finally, I settled on allowing existing pre-releases in forks if they
have no markers on them, i.e., they are "global" preferences. I believe
this is true IFF the preference came from an existing lockfile.

Closes astral-sh#5729.

crates/uv-resolver/src/candidate_selector.rs

@@ -206,7 +206,7 @@ impl CandidateSelector {
         exclusions: &Exclusions,
         resolver_markers: &ResolverMarkers,
     ) -> Option<Candidate<'a>> {
-        for (_marker, version) in preferences {
+        for (marker, version) in preferences {
             // Respect the version range for this requirement.
             if !range.contains(version) {
                 continue;
@@ -240,13 +240,20 @@ impl CandidateSelector {
             }
 
             // Respect the pre-release strategy for this fork.
-            if version.any_prerelease()
-                && self
+            if version.any_prerelease() {
+                let allow = match self
                     .prerelease_strategy
                     .allows(package_name, resolver_markers)
-                    != AllowPrerelease::Yes
-            {
-                continue;
+                {
+                    AllowPrerelease::Yes => true,
+                    AllowPrerelease::No => false,
+                    // If the pre-release is "global" (i.e., provided via a lockfile, rather than
+                    // a fork), accept it unless pre-releases are completely banned.
+                    AllowPrerelease::IfNecessary => marker.is_none(),
+                };
+                if !allow {
+                    continue;
+                }
             }
 
             // Check for a remote distribution that matches the preferred version

crates/uv-resolver/src/requires_python.rs

@@ -494,7 +494,7 @@ mod tests {
         ];
         for (i, v1) in versions.iter().enumerate() {
             for v2 in &versions[i + 1..] {
-                assert_eq!(v1.cmp(v2), Ordering::Less, "less: {v1:?}\ngreater: {v2:?}",);
+                assert_eq!(v1.cmp(v2), Ordering::Less, "less: {v1:?}\ngreater: {v2:?}");
             }
         }
     }

crates/uv/tests/pip_compile.rs

@@ -7522,6 +7522,46 @@ fn universal_nested_disjoint_local_requirement() -> Result<()> {
     Ok(())
 }
 
+/// Respect an existing pre-release preference, even if preferences aren't enabled.
+#[test]
+fn existing_prerelease_preference() -> Result<()> {
+    static EXCLUDE_NEWER: &str = "2024-07-17T00:00:00Z";
+
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        cffi
+    "})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str(indoc::indoc! {r"
+        cffi==1.17.0rc1
+        pyparser==2.22
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .env("UV_EXCLUDE_NEWER", EXCLUDE_NEWER)
+            .arg("requirements.in")
+            .arg("-o")
+            .arg("requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -o requirements.txt
+    cffi==1.17.0rc1
+        # via -r requirements.in
+    pycparser==2.22
+        # via cffi
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Requested distinct pre-release strategies with disjoint markers.
 #[test]
 fn universal_disjoint_prereleases() -> Result<()> {
@@ -7530,8 +7570,8 @@ fn universal_disjoint_prereleases() -> Result<()> {
     let context = TestContext::new("3.12");
     let requirements_in = context.temp_dir.child("requirements.in");
     requirements_in.write_str(indoc::indoc! {r"
-        cffi ; os_name == 'linux'
-        cffi >= 1.17.0rc1 ; os_name != 'linux'
+        cffi >= 1.16.0rc1 ; os_name != 'linux'
+        cffi >= 1.16.0rc1, <1.16.0rc2 ; os_name == 'linux'
     "})?;
 
     uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
@@ -7543,15 +7583,137 @@ fn universal_disjoint_prereleases() -> Result<()> {
     ----- stdout -----
     # This file was autogenerated by uv via the following command:
     #    uv pip compile --cache-dir [CACHE_DIR] requirements.in --universal
-    cffi==1.16.0 ; os_name == 'linux'
+    cffi==1.16.0rc1
         # via -r requirements.in
-    cffi==1.17.0rc1 ; os_name != 'linux'
+    pycparser==2.22
+        # via cffi
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
+/// Requested distinct pre-release strategies with disjoint markers.
+#[test]
+fn universal_disjoint_prereleases_preference() -> Result<()> {
+    static EXCLUDE_NEWER: &str = "2024-07-17T00:00:00Z";
+
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        cffi ; os_name != 'linux'
+        cffi > 1.16.0 ; os_name == 'linux'
+    "})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str(indoc::indoc! {r"
+        cffi==1.17.0rc1
+        pyparser==2.22
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .env("UV_EXCLUDE_NEWER", EXCLUDE_NEWER)
+            .arg("requirements.in")
+            .arg("-o")
+            .arg("requirements.txt")
+            .arg("--universal"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -o requirements.txt --universal
+    cffi==1.17.0rc1
         # via -r requirements.in
     pycparser==2.22
         # via cffi
 
     ----- stderr -----
-    Resolved 3 packages in [TIME]
+    Resolved 2 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
+/// Requested distinct pre-release strategies with disjoint markers.
+///
+/// TODO(charlie): This should resolve to two different `cffi` versions, one for each fork.
+#[test]
+fn universal_disjoint_prereleases_preference_marker() -> Result<()> {
+    static EXCLUDE_NEWER: &str = "2024-07-17T00:00:00Z";
+
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        cffi ; os_name != 'linux'
+        cffi >= 1.16.0rc1 ; os_name == 'linux'
+    "})?;
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str(indoc::indoc! {r"
+        cffi==1.16.0 ; os_name != 'linux'
+        cffi==1.16.0rc1 ; os_name == 'linux'
+        pyparser==2.22
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .env("UV_EXCLUDE_NEWER", EXCLUDE_NEWER)
+            .arg("requirements.in")
+            .arg("-o")
+            .arg("requirements.txt")
+            .arg("--universal"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -o requirements.txt --universal
+    cffi==1.16.0
+        # via -r requirements.in
+    pycparser==2.22
+        # via cffi
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
+/// Resolve to a single version as `--prerelease=allow` is provided, even though the first branch
+/// doesn't include a pre-release marker.
+#[test]
+fn universal_disjoint_prereleases_allow() -> Result<()> {
+    static EXCLUDE_NEWER: &str = "2024-07-17T00:00:00Z";
+
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        cffi >= 1.15.0, < 1.17.0 ; os_name == 'linux'
+        cffi >= 1.15.0, <= 1.16.0rc2 ; os_name != 'linux'
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .env("UV_EXCLUDE_NEWER", EXCLUDE_NEWER)
+            .arg("requirements.in")
+            .arg("--universal")
+            .arg("--prerelease")
+            .arg("allow"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in --universal --prerelease allow
+    cffi==1.16.0rc2
+        # via -r requirements.in
+    pycparser==2.22
+        # via cffi
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
     "###
     );