Fix special case in switch expr completion (#1002)

* fixes a special case in switch expr completion

* changelog

* add comments

Author: zth

CHANGELOG.md

@@ -12,6 +12,10 @@
 
 ## master
 
+#### :bug: Bug Fix
+
+- Fix edge case in switch expr completion. https://github.com/rescript-lang/rescript-vscode/pull/1002
+
 ## 1.52.0
 
 #### :bug: Bug Fix

analysis/src/CompletionFrontEnd.ml

@@ -584,9 +584,19 @@ let completionWithParser1 ~currentFile ~debug ~offset ~path ~posCursor
           | Some contextPath ->
             setResult (CexhaustiveSwitch {contextPath; exprLoc = exp.pexp_loc}))
       | Pexp_match (_expr, []) ->
+        (* switch x { } *)
         if Debug.verbose () && debugTypedCompletionExpr then
           print_endline "[typedCompletionExpr] No cases, rest";
         ()
+      | Pexp_match (expr, [{pc_lhs; pc_rhs}])
+        when locHasCursor expr.pexp_loc
+             && CompletionExpressions.isExprHole pc_rhs
+             && CompletionPatterns.isPatternHole pc_lhs ->
+        (* switch x { | } when we're in the switch expr itself. *)
+        if Debug.verbose () && debugTypedCompletionExpr then
+          print_endline
+            "[typedCompletionExpr] No cases (expr and pat holes), rest";
+        ()
       | Pexp_match
           ( exp,
             [

analysis/tests/src/CompletionExpressions.res

@@ -385,3 +385,6 @@ let {name} = hook({
   includeName: true,
   showMore: true,
 })
+
+// switch someTyp. { | }
+//                ^com

analysis/tests/src/expected/CompletionExpressions.res.txt

@@ -1459,3 +1459,22 @@ Path hook
     "documentation": {"kind": "markdown", "value": "```rescript\noperator?: [#\"and\" | #or]\n```\n\n```rescript\ntype config = {includeName: bool, operator: option<[#\"and\" | #or]>, showMore: bool}\n```"}
   }]
 
+Complete src/CompletionExpressions.res 388:18
+posCursor:[388:18] posNoWhite:[388:17] Found expr:[388:3->388:24]
+posCursor:[388:18] posNoWhite:[388:17] Found expr:[388:10->388:18]
+Pexp_field [388:10->388:17] _:[388:19->388:18]
+Completable: Cpath Value[someTyp].""
+Raw opens: 1 CompletionSupport.place holder
+Package opens Pervasives.JsxModules.place holder
+Resolved opens 2 pervasives CompletionSupport.res
+ContextPath Value[someTyp].""
+ContextPath Value[someTyp]
+Path someTyp
+[{
+    "label": "test",
+    "kind": 5,
+    "tags": [],
+    "detail": "bool",
+    "documentation": {"kind": "markdown", "value": "```rescript\ntest: bool\n```\n\n```rescript\ntype someTyp = {test: bool}\n```"}
+  }]
+