Merge #3710

3710: Inlay hints for method chaining pattern r=matklad a=M-J-Hooper

This PR adds inlay hints on method call chains:
![image](https://user-images.githubusercontent.com/13765376/77472008-8dc2a880-6e13-11ea-9c18-2c2e2b809799.png)

It is not only explicit `MethodCall`s where this can be helpful. The heuristic used here is that whenever any expression is followed by a new line and then a dot, it resembles a call chain and type information can be  #useful.

Changes:
- A new `InlayKind` for chaining.
- New option for disabling this type of hints.
- Tree traversal rules for identifying the chaining hints.
- VSCode decorators in the extension layer (and associated types).

Notes:
- IntelliJ has additional rules and configuration on this topic. Eg. minimum length of chain to start displaying hints and only displaying distinct types in the chain.
- I am checking for chaining on every `ast::Expr` in the tree; Are there performance concerns there?

This is my first contribution (to RA and to Rust in general) so would appreciate any feedback.
The only issue I can find the references this feature is #2741.

Co-authored-by: Matt Hooper <[email protected]>

1 files changed, 3 insertions, 1 deletions

diff --git a/editors/code/src/rust-analyzer-api.ts b/editors/code/src/rust-analyzer-api.ts
index 9846f7343..400ac3714 100644
--- a/editors/code/src/rust-analyzer-api.ts
+++ b/editors/code/src/rust-analyzer-api.ts
@@ -86,12 +86,13 @@ export interface Runnable {
 }
 export const runnables = request<RunnablesParams, Vec<Runnable>>("runnables");
 
-export type InlayHint = InlayHint.TypeHint | InlayHint.ParamHint;
+export type InlayHint = InlayHint.TypeHint | InlayHint.ParamHint | InlayHint.ChainingHint;
 
 export namespace InlayHint {
     export const enum Kind {
         TypeHint = "TypeHint",
         ParamHint = "ParameterHint",
+        ChainingHint = "ChainingHint",
     }
     interface Common {
         range: lc.Range;
@@ -99,6 +100,7 @@ export namespace InlayHint {
     }
     export type TypeHint = Common & { kind: Kind.TypeHint };
     export type ParamHint = Common & { kind: Kind.ParamHint };
+    export type ChainingHint = Common & { kind: Kind.ChainingHint };
 }
 export interface InlayHintsParams {
     textDocument: lc.TextDocumentIdentifier;