4 files changed, 128 insertions, 55 deletions

diff --git a/crates/ra_hir_def/src/find_path.rs b/crates/ra_hir_def/src/find_path.rs
index 8cc2fb160..43b9b124a 100644
--- a/crates/ra_hir_def/src/find_path.rs
+++ b/crates/ra_hir_def/src/find_path.rs
@@ -7,10 +7,39 @@ use crate::{
     visibility::Visibility,
     CrateId, ModuleDefId, ModuleId,
 };
-use hir_expand::name::Name;
+use hir_expand::name::{known, Name};
+use test_utils::tested_by;
 
 const MAX_PATH_LEN: usize = 15;
 
+impl ModPath {
+    fn starts_with_std(&self) -> bool {
+        self.segments.first().filter(|&first_segment| first_segment == &known::std).is_some()
+    }
+
+    // When std library is present, paths starting with `std::`
+    // should be preferred over paths starting with `core::` and `alloc::`
+    fn should_start_with_std(&self) -> bool {
+        self.segments
+            .first()
+            .filter(|&first_segment| {
+                first_segment == &known::alloc || first_segment == &known::core
+            })
+            .is_some()
+    }
+
+    fn len(&self) -> usize {
+        self.segments.len()
+            + match self.kind {
+                PathKind::Plain => 0,
+                PathKind::Super(i) => i as usize,
+                PathKind::Crate => 1,
+                PathKind::Abs => 0,
+                PathKind::DollarCrate(_) => 1,
+            }
+    }
+}
+
 // FIXME: handle local items
 
 /// Find a path that can be used to refer to a certain item. This can depend on
@@ -112,23 +141,27 @@ fn find_path_inner(
             Some(path) => path,
         };
         path.segments.push(name);
-        if path_len(&path) < best_path_len {
-            best_path_len = path_len(&path);
-            best_path = Some(path);
-        }
+
+        let new_path =
+            if let Some(best_path) = best_path { select_best_path(best_path, path) } else { path };
+        best_path_len = new_path.len();
+        best_path = Some(new_path);
     }
     best_path
 }
 
-fn path_len(path: &ModPath) -> usize {
-    path.segments.len()
-        + match path.kind {
-            PathKind::Plain => 0,
-            PathKind::Super(i) => i as usize,
-            PathKind::Crate => 1,
-            PathKind::Abs => 0,
-            PathKind::DollarCrate(_) => 1,
-        }
+fn select_best_path(old_path: ModPath, new_path: ModPath) -> ModPath {
+    if old_path.starts_with_std() && new_path.should_start_with_std() {
+        tested_by!(prefer_std_paths);
+        old_path
+    } else if new_path.starts_with_std() && old_path.should_start_with_std() {
+        tested_by!(prefer_std_paths);
+        new_path
+    } else if new_path.len() < old_path.len() {
+        new_path
+    } else {
+        old_path
+    }
 }
 
 fn find_importable_locations(
@@ -201,6 +234,7 @@ mod tests {
     use hir_expand::hygiene::Hygiene;
     use ra_db::fixture::WithFixture;
     use ra_syntax::ast::AstNode;
+    use test_utils::covers;
 
     /// `code` needs to contain a cursor marker; checks that `find_path` for the
     /// item the `path` refers to returns that same path when called from the
@@ -452,4 +486,41 @@ mod tests {
         "#;
         check_found_path(code, "crate::foo::S");
     }
+
+    #[test]
+    fn prefer_std_paths_over_alloc() {
+        covers!(prefer_std_paths);
+        let code = r#"
+        //- /main.rs crate:main deps:alloc,std
+        <|>
+
+        //- /std.rs crate:std deps:alloc
+        pub mod sync {
+            pub use alloc::sync::Arc;
+        }
+
+        //- /zzz.rs crate:alloc
+        pub mod sync {
+            pub struct Arc;
+        }
+        "#;
+        check_found_path(code, "std::sync::Arc");
+    }
+
+    #[test]
+    fn prefer_shorter_paths_if_not_alloc() {
+        let code = r#"
+        //- /main.rs crate:main deps:megaalloc,std
+        <|>
+
+        //- /std.rs crate:std deps:megaalloc
+        pub mod sync {
+            pub use megaalloc::sync::Arc;
+        }
+
+        //- /zzz.rs crate:megaalloc
+        pub struct Arc;
+        "#;
+        check_found_path(code, "megaalloc::Arc");
+    }
 }
diff --git a/crates/ra_hir_def/src/marks.rs b/crates/ra_hir_def/src/marks.rs
index 457ba4abe..daa49d5f1 100644
--- a/crates/ra_hir_def/src/marks.rs
+++ b/crates/ra_hir_def/src/marks.rs
@@ -13,4 +13,5 @@ test_utils::marks!(
     macro_dollar_crate_self
     macro_dollar_crate_other
     infer_resolve_while_let
+    prefer_std_paths
 );
diff --git a/crates/ra_hir_def/src/nameres.rs b/crates/ra_hir_def/src/nameres.rs
index 27c12e46c..852304dd0 100644
--- a/crates/ra_hir_def/src/nameres.rs
+++ b/crates/ra_hir_def/src/nameres.rs
@@ -229,6 +229,46 @@ impl CrateDefMap {
             self.resolve_path_fp_with_macro(db, ResolveMode::Other, original_module, path, shadow);
         (res.resolved_def, res.segment_index)
     }
+
+    // FIXME: this can use some more human-readable format (ideally, an IR
+    // even), as this should be a great debugging aid.
+    pub fn dump(&self) -> String {
+        let mut buf = String::new();
+        go(&mut buf, self, "\ncrate", self.root);
+        return buf.trim().to_string();
+
+        fn go(buf: &mut String, map: &CrateDefMap, path: &str, module: LocalModuleId) {
+            *buf += path;
+            *buf += "\n";
+
+            let mut entries: Vec<_> = map.modules[module].scope.resolutions().collect();
+            entries.sort_by_key(|(name, _)| name.clone());
+
+            for (name, def) in entries {
+                *buf += &format!("{}:", name);
+
+                if def.types.is_some() {
+                    *buf += " t";
+                }
+                if def.values.is_some() {
+                    *buf += " v";
+                }
+                if def.macros.is_some() {
+                    *buf += " m";
+                }
+                if def.is_none() {
+                    *buf += " _";
+                }
+
+                *buf += "\n";
+            }
+
+            for (name, child) in map.modules[module].children.iter() {
+                let path = path.to_string() + &format!("::{}", name);
+                go(buf, map, &path, *child);
+            }
+        }
+    }
 }
 
 impl ModuleData {
diff --git a/crates/ra_hir_def/src/nameres/tests.rs b/crates/ra_hir_def/src/nameres/tests.rs
index 78bcdc850..82f0f835c 100644
--- a/crates/ra_hir_def/src/nameres/tests.rs
+++ b/crates/ra_hir_def/src/nameres/tests.rs
@@ -10,11 +10,10 @@ use insta::assert_snapshot;
 use ra_db::{fixture::WithFixture, SourceDatabase};
 use test_utils::covers;
 
-use crate::{db::DefDatabase, nameres::*, test_db::TestDB, LocalModuleId};
+use crate::{db::DefDatabase, nameres::*, test_db::TestDB};
 
 fn def_map(fixture: &str) -> String {
-    let dm = compute_crate_def_map(fixture);
-    render_crate_def_map(&dm)
+    compute_crate_def_map(fixture).dump()
 }
 
 fn compute_crate_def_map(fixture: &str) -> Arc<CrateDefMap> {
@@ -23,44 +22,6 @@ fn compute_crate_def_map(fixture: &str) -> Arc<CrateDefMap> {
     db.crate_def_map(krate)
 }
 
-fn render_crate_def_map(map: &CrateDefMap) -> String {
-    let mut buf = String::new();
-    go(&mut buf, map, "\ncrate", map.root);
-    return buf.trim().to_string();
-
-    fn go(buf: &mut String, map: &CrateDefMap, path: &str, module: LocalModuleId) {
-        *buf += path;
-        *buf += "\n";
-
-        let mut entries: Vec<_> = map.modules[module].scope.resolutions().collect();
-        entries.sort_by_key(|(name, _)| name.clone());
-
-        for (name, def) in entries {
-            *buf += &format!("{}:", name);
-
-            if def.types.is_some() {
-                *buf += " t";
-            }
-            if def.values.is_some() {
-                *buf += " v";
-            }
-            if def.macros.is_some() {
-                *buf += " m";
-            }
-            if def.is_none() {
-                *buf += " _";
-            }
-
-            *buf += "\n";
-        }
-
-        for (name, child) in map.modules[module].children.iter() {
-            let path = path.to_string() + &format!("::{}", name);
-            go(buf, map, &path, *child);
-        }
-    }
-}
-
 #[test]
 fn crate_def_map_smoke_test() {
     let map = def_map(