Merge #866

866: Implement basic support for Associated Methods r=flodiebold a=vipentti

This is my attempt at learning to understand how the type inference works by adding basic support for associated methods. Currently it does not resolve associated types or constants. 

The basic idea is that `Resolver::resolve_path` returns a new `PathResult` type, which has two variants, `FullyResolved` and `PartiallyResolved`, fully resolved matches the previous behavior, where as `PartiallyResolved` contains the `PerNs<Resolution` in addition to a `segment_index` which contains the index of the segment which we failed to resolve. This index can then be used to continue inference in `infer_path_expr` using the `Type` we managed to resolve.

This changes some of the previous apis, so looking for feedback and suggestions.

This should enable fixing #832

Co-authored-by: Ville Penttinen <villem.penttinen@gmail.com>

crates/ra_ide_api/src/hover.rs

@@ -240,4 +240,24 @@ 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");
+    }
 }