Display dep requirement info for cyclic dependencies

src/cargo/core/resolver/mod.rs

@@ -47,7 +47,7 @@
 //! that we're implementing something that probably shouldn't be allocating all
 //! over the place.
 
-use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::mem;
 use std::rc::Rc;
 use std::time::{Duration, Instant};
@@ -1007,13 +1007,15 @@ fn check_cycles(resolve: &Resolve) -> CargoResult<()> {
     // dev-dependency since that doesn't count for cycles.
     let mut graph = BTreeMap::new();
     for id in resolve.iter() {
-        let set = graph.entry(id).or_insert_with(BTreeSet::new);
-        for (dep, listings) in resolve.deps_not_replaced(id) {
-            let is_transitive = listings.iter().any(|d| d.is_transitive());
-
-            if is_transitive {
-                set.insert(dep);
-                set.extend(resolve.replacement(dep));
+        let map = graph.entry(id).or_insert_with(BTreeMap::new);
+        for (dep_id, listings) in resolve.deps_not_replaced(id) {
+            let transitive_dep = listings.iter().find(|d| d.is_transitive());
+
+            if let Some(transitive_dep) = transitive_dep.cloned() {
+                map.insert(dep_id, transitive_dep.clone());
+                resolve
+                    .replacement(dep_id)
+                    .map(|p| map.insert(p, transitive_dep));
             }
         }
     }
@@ -1033,23 +1035,29 @@ fn check_cycles(resolve: &Resolve) -> CargoResult<()> {
     return Ok(());
 
     fn visit(
-        graph: &BTreeMap<PackageId, BTreeSet<PackageId>>,
+        graph: &BTreeMap<PackageId, BTreeMap<PackageId, Dependency>>,
         id: PackageId,
         visited: &mut HashSet<PackageId>,
         path: &mut Vec<PackageId>,
         checked: &mut HashSet<PackageId>,
     ) -> CargoResult<()> {
         path.push(id);
         if !visited.insert(id) {
+            let iter = path.iter().rev().skip(1).scan(id, |child, parent| {
+                let dep = graph.get(parent).and_then(|adjacent| adjacent.get(child));
+                *child = *parent;
+                Some((parent, dep))
+            });
+            let iter = std::iter::once((&id, None)).chain(iter);
             anyhow::bail!(
                 "cyclic package dependency: package `{}` depends on itself. Cycle:\n{}",
                 id,
-                errors::describe_path(&path.iter().rev().collect::<Vec<_>>()),
+                errors::describe_path(iter),
             );
         }
 
         if checked.insert(id) {
-            for dep in graph[&id].iter() {
+            for dep in graph[&id].keys() {
                 visit(graph, *dep, visited, path, checked)?;
             }
         }