Don't panic on crates depending on themselves

Fixes #3883.

crates/ra_db/src/input.rs

@@ -254,12 +254,12 @@ impl CrateGraph {
             return false;
         }
 
-        for dep in &self[from].dependencies {
+        if target == from {
-            let crate_id = dep.crate_id;
-            if crate_id == target {
             return true;
         }
 
+        for dep in &self[from].dependencies {
+            let crate_id = dep.crate_id;
             if self.dfs_find(target, crate_id, visited) {
                 return true;
             }
@@ -369,7 +369,7 @@ mod tests {
     use super::{CfgOptions, CrateGraph, CrateName, Dependency, Edition::Edition2018, Env, FileId};
 
     #[test]
-    fn it_should_panic_because_of_cycle_dependencies() {
+    fn detect_cyclic_dependency_indirect() {
         let mut graph = CrateGraph::default();
         let crate1 = graph.add_crate_root(
             FileId(1u32),
@@ -403,6 +403,31 @@ mod tests {
         assert!(graph.add_dep(crate3, CrateName::new("crate1").unwrap(), crate1).is_err());
     }
 
+    #[test]
+    fn detect_cyclic_dependency_direct() {
+        let mut graph = CrateGraph::default();
+        let crate1 = graph.add_crate_root(
+            FileId(1u32),
+            Edition2018,
+            None,
+            CfgOptions::default(),
+            Env::default(),
+            Default::default(),
+            Default::default(),
+        );
+        let crate2 = graph.add_crate_root(
+            FileId(2u32),
+            Edition2018,
+            None,
+            CfgOptions::default(),
+            Env::default(),
+            Default::default(),
+            Default::default(),
+        );
+        assert!(graph.add_dep(crate1, CrateName::new("crate2").unwrap(), crate2).is_ok());
+        assert!(graph.add_dep(crate2, CrateName::new("crate2").unwrap(), crate2).is_err());
+    }
+
     #[test]
     fn it_works() {
         let mut graph = CrateGraph::default();

crates/ra_hir_def/src/nameres/collector.rs

@@ -36,8 +36,8 @@ pub(super) fn collect_defs(db: &dyn DefDatabase, mut def_map: CrateDefMap) -> Cr
 
     // populate external prelude
     for dep in &crate_graph[def_map.krate].dependencies {
-        let dep_def_map = db.crate_def_map(dep.crate_id);
         log::debug!("crate dep {:?} -> {:?}", dep.name, dep.crate_id);
+        let dep_def_map = db.crate_def_map(dep.crate_id);
         def_map.extern_prelude.insert(
             dep.as_name(),
             ModuleId { krate: dep.crate_id, local_id: dep_def_map.root }.into(),