From 6042699fc3643ab8f3ee081f8c85f969f70fcff9 Mon Sep 17 00:00:00 2001
From: Zanie Blue <contact@zanie.dev>
Date: Thu, 16 Nov 2023 13:56:02 -0600
Subject: [PATCH] Add an `UnusableDependencies` incompatibility kind (#4)

---
 src/internal/core.rs            |  2 +-
 src/internal/incompatibility.rs | 26 +++++++++++++++++---------
 src/report.rs                   | 17 +++++++----------
 3 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/src/internal/core.rs b/src/internal/core.rs
index d3281d24..6c3b5d55 100644
--- a/src/internal/core.rs
+++ b/src/internal/core.rs
@@ -25,7 +25,7 @@ pub struct State<DP: DependencyProvider> {
     root_package: DP::P,
     root_version: DP::V,
 
-    /// All incompatibilities indexed by package.
+    /// The set of incompatibilities for each package.
     #[allow(clippy::type_complexity)]
     incompatibilities: Map<DP::P, Vec<IncompDpId<DP>>>,
 
diff --git a/src/internal/incompatibility.rs b/src/internal/incompatibility.rs
index fb45ca48..da906c70 100644
--- a/src/internal/incompatibility.rs
+++ b/src/internal/incompatibility.rs
@@ -34,14 +34,14 @@ use crate::version_set::VersionSet;
 #[derive(Debug, Clone)]
 pub struct Incompatibility<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> {
     package_terms: SmallMap<P, Term<VS>>,
-    /// The reason for the incompatibility.
+    /// The reason why this version or combination of versions can't be selected.
     pub kind: Kind<P, VS, M>,
 }
 
 /// Type alias of unique identifiers for incompatibilities.
 pub type IncompId<P, VS, M> = Id<Incompatibility<P, VS, M>>;
 
-/// The reason for the incompatibility.
+/// The reason why a version or combination of versions can't be selected.
 #[derive(Debug, Clone)]
 pub enum Kind<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> {
     /// Initial incompatibility aiming at picking the root package for the first decision.
@@ -138,6 +138,17 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> Incompatibilit
         }
     }
 
+    /// Create an incompatibility to remember
+    /// that a package version is not selectable
+    /// because its dependencies are not usable.
+    pub fn unusable_dependencies(package: P, version: VS::V, reason: M) -> Self {
+        let set = VS::singleton(version);
+        Self {
+            package_terms: SmallMap::One([(package.clone(), Term::Positive(set.clone()))]),
+            kind: Kind::Custom(package, set, reason),
+        }
+    }
+
     /// Build an incompatibility from a given dependency.
     pub fn from_dependency(package: P, versions: VS, dep: (&P, &VS)) -> Self {
         let (p2, set2) = dep;
@@ -154,8 +165,7 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> Incompatibilit
         }
     }
 
-    /// Return the two packages where this incompatibility when the incompatibility was created
-    /// through a dependency edge between the two.
+    /// The two packages causing the incompatibility, if it was derived from dependencies.
     pub fn as_dependency(&self) -> Option<(&P, &P)> {
         match &self.kind {
             Kind::FromDependencyOf(p1, _, p2, _) => Some((p1, p2)),
@@ -296,11 +306,9 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> Incompatibilit
                     dep_set.clone(),
                 ))
             }
-            Kind::Custom(package, set, metadata) => DerivationTree::External(External::Custom(
-                package.clone(),
-                set.clone(),
-                metadata.clone(),
-            )),
+            Kind::Custom(package, set, reason) => {
+                DerivationTree::External(External::Custom(package, set, reason))
+            }
         }
     }
 }
diff --git a/src/report.rs b/src/report.rs
index 6a309165..35880dce 100644
--- a/src/report.rs
+++ b/src/report.rs
@@ -121,6 +121,9 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> DerivationTree
             //
             // Cannot be merged because the reason may not match
             DerivationTree::External(External::NoVersions(_, _)) => None,
+            DerivationTree::External(External::Custom(_, r, reason)) => Some(
+                DerivationTree::External(External::Custom(package, set.union(&r), reason)),
+            ),
             DerivationTree::External(External::FromDependencyOf(p1, r1, p2, r2)) => {
                 if p1 == package {
                     Some(DerivationTree::External(External::FromDependencyOf(
@@ -138,8 +141,6 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> DerivationTree
                     )))
                 }
             }
-            // Cannot be merged because the reason may not match
-            DerivationTree::External(External::Custom(_, _, _)) => None,
         }
     }
 }
@@ -159,18 +160,14 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> fmt::Display
                     write!(f, "there is no version of {} in {}", package, set)
                 }
             }
-            Self::Custom(package, set, metadata) => {
+            Self::Custom(package, set, reason) => {
                 if set == &VS::full() {
-                    write!(
-                        f,
-                        "dependencies of {} are unavailable {}",
-                        package, metadata
-                    )
+                    write!(f, "dependencies of {} are unusable: {reason}", package)
                 } else {
                     write!(
                         f,
-                        "dependencies of {} at version {} are unavailable {}",
-                        package, set, metadata
+                        "dependencies of {} at version {} are unusable: {reason}",
+                        package, set
                     )
                 }
             }