Merge #1652

1652: Improve type hints behavior r=matklad a=SomeoneToIgnore

This PR fixed the following type hints issues:

* Restructures the `InlayKind` enum contents based on the discussion here: https://github.com/rust-analyzer/rust-analyzer/pull/1606#issuecomment-515968055
* Races described in #1639 
* Caches the latest decorations received for each file to show them the next time the file is opened (instead of a new server request)

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>

crates/ra_lsp_server/src/main_loop/handlers.rs

@@ -885,14 +885,7 @@ pub fn handle_inlay_hints(
             label: api_type.label.to_string(),
             range: api_type.range.conv_with(&line_index),
             kind: match api_type.kind {
-                ra_ide_api::InlayKind::LetBindingType => InlayKind::LetBindingType,
-                ra_ide_api::InlayKind::ClosureParameterType => InlayKind::ClosureParameterType,
-                ra_ide_api::InlayKind::ForExpressionBindingType => {
-                    InlayKind::ForExpressionBindingType
-                }
-                ra_ide_api::InlayKind::IfExpressionType => InlayKind::IfExpressionType,
-                ra_ide_api::InlayKind::WhileLetExpressionType => InlayKind::WhileLetExpressionType,
-                ra_ide_api::InlayKind::MatchArmType => InlayKind::MatchArmType,
+                ra_ide_api::InlayKind::TypeHint => InlayKind::TypeHint,
             },
         })
         .collect())

crates/ra_lsp_server/src/req.rs

@@ -213,12 +213,7 @@ pub struct InlayHintsParams {
 
 #[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub enum InlayKind {
-    LetBindingType,
-    ClosureParameterType,
-    ForExpressionBindingType,
-    IfExpressionType,
-    WhileLetExpressionType,
-    MatchArmType,
+    TypeHint,
 }
 
 #[derive(Debug, Deserialize, Serialize)]