From bc59f83991a6444ff2f2364b0e942e8a82943b6d Mon Sep 17 00:00:00 2001
From: Florian Diebold <flodiebold@gmail.com>
Date: Sun, 12 May 2019 14:58:37 +0200
Subject: Use traits from prelude for method resolution

---
 crates/ra_hir/src/resolve.rs              | 25 ++++++++++++-------------
 crates/ra_hir/src/ty/method_resolution.rs |  2 +-
 crates/ra_hir/src/ty/tests.rs             | 29 +++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/crates/ra_hir/src/resolve.rs b/crates/ra_hir/src/resolve.rs
index 707556ef8..2fb219908 100644
--- a/crates/ra_hir/src/resolve.rs
+++ b/crates/ra_hir/src/resolve.rs
@@ -3,7 +3,7 @@ use std::sync::Arc;
 
 use ra_syntax::ast;
 
-use rustc_hash::FxHashMap;
+use rustc_hash::{FxHashMap, FxHashSet};
 
 use crate::{
     ModuleDef, Trait,
@@ -193,19 +193,18 @@ impl Resolver {
         names
     }
 
-    pub(crate) fn traits_in_scope<'a>(&'a self) -> impl Iterator<Item = Trait> + 'a {
-        // FIXME prelude
-        self.scopes
-            .iter()
-            .rev()
-            .flat_map(|scope| {
-                match scope {
-                    Scope::ModuleScope(m) => Some(m.crate_def_map[m.module_id].scope.traits()),
-                    _ => None,
+    pub(crate) fn traits_in_scope(&self, db: &impl HirDatabase) -> FxHashSet<Trait> {
+        let mut traits = FxHashSet::default();
+        for scope in &self.scopes {
+            if let Scope::ModuleScope(m) = scope {
+                if let Some(prelude) = m.crate_def_map.prelude() {
+                    let prelude_def_map = db.crate_def_map(prelude.krate);
+                    traits.extend(prelude_def_map[prelude.module_id].scope.traits());
                 }
-                .into_iter()
-            })
-            .flatten()
+                traits.extend(m.crate_def_map[m.module_id].scope.traits());
+            }
+        }
+        traits
     }
 
     fn module(&self) -> Option<(&CrateDefMap, CrateModuleId)> {
diff --git a/crates/ra_hir/src/ty/method_resolution.rs b/crates/ra_hir/src/ty/method_resolution.rs
index e8cfa0b85..34817a5ec 100644
--- a/crates/ra_hir/src/ty/method_resolution.rs
+++ b/crates/ra_hir/src/ty/method_resolution.rs
@@ -185,7 +185,7 @@ fn iterate_trait_method_candidates<T>(
     mut callback: impl FnMut(&Ty, Function) -> Option<T>,
 ) -> Option<T> {
     let krate = resolver.krate()?;
-    'traits: for t in resolver.traits_in_scope() {
+    'traits: for t in resolver.traits_in_scope(db) {
         let data = t.trait_data(db);
         // we'll be lazy about checking whether the type implements the
         // trait, but if we find out it doesn't, we'll skip the rest of the
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index 510fa5333..978cc2587 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -2501,6 +2501,35 @@ fn test() { (&S).foo()<|>; }
     assert_eq!(t, "u128");
 }
 
+#[test]
+fn method_resolution_trait_from_prelude() {
+    let (mut db, pos) = MockDatabase::with_position(
+        r#"
+//- /main.rs
+struct S;
+impl Clone for S {}
+
+fn test() {
+    S.clone()<|>;
+}
+
+//- /lib.rs
+#[prelude_import] use foo::*;
+
+mod foo {
+    trait Clone {
+        fn clone(&self) -> Self;
+    }
+}
+"#,
+    );
+    db.set_crate_graph_from_fixture(crate_graph! {
+        "main": ("/main.rs", ["other_crate"]),
+        "other_crate": ("/lib.rs", []),
+    });
+    assert_eq!("S", type_at_pos(&db, pos));
+}
+
 #[test]
 fn method_resolution_where_clause_for_unknown_trait() {
     // The blanket impl shouldn't apply because we can't even resolve UnknownTrait
-- 
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