From 4091cce1f1f57d4424a20ceeb8be82c7b670b63b Mon Sep 17 00:00:00 2001
From: Zanie Blue <contact@zanie.dev>
Date: Mon, 16 Dec 2024 15:31:35 -0600
Subject: [PATCH] Fix redundant enumeration of all package versions in some
 resolver errors (#9885)

Closes #4075

There are many more redundant enumerations I want to look into as well.
---
 crates/uv-resolver/src/error.rs   | 57 +++++++++++++++++++++++++++++++
 crates/uv/tests/it/pip_compile.rs | 11 +-----
 2 files changed, 58 insertions(+), 10 deletions(-)

diff --git a/crates/uv-resolver/src/error.rs b/crates/uv-resolver/src/error.rs
index 01b3688fa..45d0184d2 100644
--- a/crates/uv-resolver/src/error.rs
+++ b/crates/uv-resolver/src/error.rs
@@ -333,6 +333,7 @@ impl std::fmt::Display for NoSolutionError {
         }
 
         collapse_unavailable_versions(&mut tree);
+        collapse_redundant_no_versions(&mut tree);
         collapse_redundant_depends_on_no_versions(&mut tree);
 
         if should_display_tree {
@@ -427,6 +428,62 @@ fn display_tree_inner(
     }
 }
 
+fn collapse_redundant_no_versions(
+    tree: &mut DerivationTree<PubGrubPackage, Range<Version>, UnavailableReason>,
+) {
+    match tree {
+        DerivationTree::External(_) => {}
+        DerivationTree::Derived(derived) => {
+            match (
+                Arc::make_mut(&mut derived.cause1),
+                Arc::make_mut(&mut derived.cause2),
+            ) {
+                // If we have a node for a package with no versions...
+                (
+                    DerivationTree::External(External::NoVersions(package, versions)),
+                    ref mut other,
+                )
+                | (
+                    ref mut other,
+                    DerivationTree::External(External::NoVersions(package, versions)),
+                ) => {
+                    // First, always recursively visit the other side of the tree
+                    collapse_redundant_no_versions(other);
+
+                    let DerivationTree::Derived(derived) = other else {
+                        return;
+                    };
+
+                    // If the range in the conclusion (terms) matches the range of no versions,
+                    // then we'll drop this node
+                    let Some(Term::Positive(term)) = derived.terms.get(package) else {
+                        return;
+                    };
+                    let versions = versions.complement();
+
+                    // If we're disqualifying a single version, this is important to retain, e.g,
+                    // for `only foo==1.0.0 is available`
+                    if versions.as_singleton().is_some() {
+                        return;
+                    }
+
+                    if *term != versions {
+                        return;
+                    }
+
+                    // Replace this node with the other tree
+                    *tree = other.clone();
+                }
+                // If not, just recurse
+                _ => {
+                    collapse_redundant_no_versions(Arc::make_mut(&mut derived.cause1));
+                    collapse_redundant_no_versions(Arc::make_mut(&mut derived.cause2));
+                }
+            }
+        }
+    }
+}
+
 /// Given a [`DerivationTree`], collapse any `NoVersion` incompatibilities for workspace members
 /// to avoid saying things like "only <workspace-member>==0.1.0 is available".
 fn collapse_no_versions_of_workspace_members(
diff --git a/crates/uv/tests/it/pip_compile.rs b/crates/uv/tests/it/pip_compile.rs
index 9d570446e..7df2cc33e 100644
--- a/crates/uv/tests/it/pip_compile.rs
+++ b/crates/uv/tests/it/pip_compile.rs
@@ -13126,16 +13126,7 @@ fn compile_enumerate_no_versions() -> Result<()> {
     ----- stderr -----
       × No solution found when resolving dependencies:
       ╰─▶ Because the current Python version (3.10.[X]) does not satisfy Python>=3.11,<4.0 and all versions of rooster-blue depend on Python>=3.11,<4.0, we can conclude that all versions of rooster-blue cannot be used.
-          And because only the following versions of rooster-blue are available:
-              rooster-blue==0.0.1
-              rooster-blue==0.0.2
-              rooster-blue==0.0.3
-              rooster-blue==0.0.4
-              rooster-blue==0.0.5
-              rooster-blue==0.0.6
-              rooster-blue==0.0.7
-              rooster-blue==0.0.8
-          and you require rooster-blue, we can conclude that your requirements are unsatisfiable.
+          And because you require rooster-blue, we can conclude that your requirements are unsatisfiable.
     "###);
 
     Ok(())