Add an UnusableDependencies incompatibility kind (#4)

examples/unsat_root_message_no_version.rs

@@ -99,6 +99,13 @@ impl ReportFormatter<Package, Range<SemanticVersion>> for CustomReportFormatter
                     format!("{package} {package_set} depends on {dependency} {dependency_set}")
                 }
             }
+            External::UnusableDependencies(package, set, ..) => {
+                if set == &Range::full() {
+                    format!("dependencies of {package} are unusable")
+                } else {
+                    format!("dependencies of {package} at version {set} are unusable")
+                }
+            }
         }
     }
 }

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>>>,
 

src/internal/incompatibility.rs

@@ -34,14 +34,14 @@ use crate::version_set::VersionSet;
 #[derive(Debug, Clone)]
 pub struct Incompatibility<P: Package, VS: VersionSet> {
     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>,
 }
 
 /// Type alias of unique identifiers for incompatibilities.
 pub type IncompId<P, VS> = Id<Incompatibility<P, VS>>;
 
-/// 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> {
     /// Initial incompatibility aiming at picking the root package for the first decision.
@@ -50,6 +50,8 @@ pub enum Kind<P: Package, VS: VersionSet> {
     NoVersions(P, VS),
     /// Dependencies of the package are unavailable for versions in that range.
     UnavailableDependencies(P, VS),
+    /// Dependencies of the package are unusable for versions in that range.
+    UnusableDependencies(P, VS, Option<String>),
     /// Incompatibility coming from the dependencies of a given package.
     FromDependencyOf(P, VS, P, VS),
     /// Derived from two causes. Stores cause ids.
@@ -109,6 +111,17 @@ impl<P: Package, VS: VersionSet> Incompatibility<P, VS> {
         }
     }
 
+    /// 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: Option<String>) -> Self {
+        let set = VS::singleton(version);
+        Self {
+            package_terms: SmallMap::One([(package.clone(), Term::Positive(set.clone()))]),
+            kind: Kind::UnusableDependencies(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;
@@ -125,8 +138,7 @@ impl<P: Package, VS: VersionSet> Incompatibility<P, VS> {
         }
     }
 
-    /// 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)),
@@ -262,6 +274,9 @@ impl<P: Package, VS: VersionSet> Incompatibility<P, VS> {
             Kind::UnavailableDependencies(package, set) => {
                 DerivationTree::External(External::UnavailableDependencies(package, set))
             }
+            Kind::UnusableDependencies(package, set, reason) => {
+                DerivationTree::External(External::UnusableDependencies(package, set, reason))
+            }
             Kind::FromDependencyOf(package, set, dep_package, dep_set) => DerivationTree::External(
                 External::FromDependencyOf(package, set, dep_package, dep_set),
             ),

src/report.rs

@@ -49,6 +49,8 @@ pub enum External<P: Package, VS: VersionSet> {
     NoVersions(P, VS),
     /// Dependencies of the package are unavailable for versions in that set.
     UnavailableDependencies(P, VS),
+    /// Dependencies of the package are unusable for versions in that set.
+    UnusableDependencies(P, VS, Option<String>),
     /// Incompatibility coming from the dependencies of a given package.
     FromDependencyOf(P, VS, P, VS),
 }
@@ -124,6 +126,13 @@ impl<P: Package, VS: VersionSet> DerivationTree<P, VS> {
             DerivationTree::External(External::UnavailableDependencies(_, r)) => Some(
                 DerivationTree::External(External::UnavailableDependencies(package, set.union(&r))),
             ),
+            DerivationTree::External(External::UnusableDependencies(_, r, reason)) => {
+                Some(DerivationTree::External(External::UnusableDependencies(
+                    package,
+                    set.union(&r),
+                    reason,
+                )))
+            }
             DerivationTree::External(External::FromDependencyOf(p1, r1, p2, r2)) => {
                 if p1 == package {
                     Some(DerivationTree::External(External::FromDependencyOf(
@@ -169,6 +178,29 @@ impl<P: Package, VS: VersionSet> fmt::Display for External<P, VS> {
                     )
                 }
             }
+            Self::UnusableDependencies(package, set, reason) => {
+                if let Some(reason) = reason {
+                    if set == &VS::full() {
+                        write!(f, "dependencies of {} are unusable: {reason}", package)
+                    } else {
+                        write!(
+                            f,
+                            "dependencies of {} at version {} are unusable: {reason}",
+                            package, set
+                        )
+                    }
+                } else {
+                    if set == &VS::full() {
+                        write!(f, "dependencies of {} are unusable", package)
+                    } else {
+                        write!(
+                            f,
+                            "dependencies of {} at version {} are unusable",
+                            package, set
+                        )
+                    }
+                }
+            }
             Self::FromDependencyOf(p, set_p, dep, set_dep) => {
                 if set_p == &VS::full() && set_dep == &VS::full() {
                     write!(f, "{} depends on {}", p, dep)