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, 2 insertions, 0 deletions

diff --git a/docs/user/features.md b/docs/user/features.md
index 45360c633..56d2969fd 100644
--- a/docs/user/features.md
+++ b/docs/user/features.md
@@ -185,6 +185,7 @@ These contain extended information on the hovered language item.
 Two types of inlay hints are displayed currently:
 
 * type hints, displaying the minimal information on the type of the expression (if the information is available)
+* method chaining hints, type information for multi-line method chains
 * parameter name hints, displaying the names of the parameters in the corresponding methods
 
 #### VS Code
@@ -192,6 +193,7 @@ Two types of inlay hints are displayed currently:
 In VS Code, the following settings can be used to configure the inlay hints:
 
 * `rust-analyzer.inlayHints.typeHints` - enable hints for inferred types.
+* `rust-analyzer.inlayHints.chainingHints` - enable hints for inferred types on method chains.
 * `rust-analyzer.inlayHints.parameterHints` - enable hints for function parameters.
 * `rust-analyzer.inlayHints.maxLength` — shortens the hints if their length exceeds the value specified. If no value is specified (`null`), no shortening is applied.