Implement basic support for Associated Methods and Constants

This is done in `infer_path_expr`. When `Resolver::resolve_path` returns
`PartiallyResolved`, we use the returned `Resolution` together with the given
`segment_index` to check if we can find something matching the segment at
segment_index in the impls for that particular type.

crates/ra_ide_api/src/completion/complete_path.rs

@@ -10,7 +10,7 @@ pub(super) fn complete_path(acc: &mut Completions, ctx: &CompletionContext) {
         Some(path) => path.clone(),
         _ => return,
     };
-    let def = match ctx.resolver.resolve_path(ctx.db, &path).take_types() {
+    let def = match ctx.resolver.resolve_path(ctx.db, &path).into_per_ns().take_types() {
         Some(Resolution::Def(def)) => def,
         _ => return,
     };

crates/ra_ide_api/src/goto_definition.rs

@@ -79,7 +79,7 @@ pub(crate) fn reference_definition(
     if let Some(path) =
         name_ref.syntax().ancestors().find_map(ast::Path::cast).and_then(hir::Path::from_ast)
     {
-        let resolved = resolver.resolve_path(db, &path);
+        let resolved = resolver.resolve_path(db, &path).into_per_ns();
         match resolved.clone().take_types().or_else(|| resolved.take_values()) {
             Some(Resolution::Def(def)) => return Exact(NavigationTarget::from_def(db, def)),
             Some(Resolution::LocalBinding(pat)) => {

crates/ra_ide_api/src/hover.rs

@@ -223,4 +223,24 @@ mod tests {
         assert_eq!("usize", &type_name);
     }
 
+    #[test]
+    fn test_hover_infer_associated_method_result() {
+        let (analysis, position) = single_file_with_position(
+            "
+            struct Thing { x: u32 };
+
+            impl Thing {
+                fn new() -> Thing {
+                    Thing { x: 0 }
+                }
+            }
+
+            fn main() {
+                let foo_<|>test = Thing::new();
+            }
+            ",
+        );
+        let hover = analysis.hover(position).unwrap().unwrap();
+        assert_eq!(hover.info, "Thing");
+    }
 }