Make function template async when fn returns a promise

CHANGELOG.md

@@ -18,6 +18,7 @@
 - Add completion to top level decorators. https://github.com/rescript-lang/rescript-vscode/pull/799
 - Add code action for wrapping patterns where option is expected with `Some`. https://github.com/rescript-lang/rescript-vscode/pull/806
 - Better completion from identifiers with inferred types. https://github.com/rescript-lang/rescript-vscode/pull/808
+- Make suggested template functions async when the target function returns a promise. https://github.com/rescript-lang/rescript-vscode/pull/816
 
 #### :nail_care: Polish
 

analysis/src/CompletionBackEnd.ml

@@ -1382,8 +1382,8 @@ let rec completeTypedValue ~full ~prefix ~completionContext ~mode
           ~env ();
       ]
     else []
-  | Tfunction {env; typ; args; uncurried} when prefix = "" && mode = Expression
-    ->
+  | Tfunction {env; typ; args; uncurried; returnType}
+    when prefix = "" && mode = Expression ->
     let shouldPrintAsUncurried = uncurried && !Config.uncurried <> Uncurried in
     let mkFnArgs ~asSnippet =
       match args with
@@ -1419,11 +1419,18 @@ let rec completeTypedValue ~full ~prefix ~completionContext ~mode
         in
         "(" ^ if shouldPrintAsUncurried then ". " else "" ^ argsText ^ ")"
     in
+    let isAsync =
+      match TypeUtils.extractType ~env ~package:full.package returnType with
+      | Some (Tpromise _) -> true
+      | _ -> false
+    in
+    let asyncPrefix = if isAsync then "async " else "" in
     [
       Completion.createWithSnippet
-        ~name:(mkFnArgs ~asSnippet:false ^ " => {}")
+        ~name:(asyncPrefix ^ mkFnArgs ~asSnippet:false ^ " => {}")
         ~insertText:
-          (mkFnArgs ~asSnippet:!Cfg.supportsSnippets
+          (asyncPrefix
+          ^ mkFnArgs ~asSnippet:!Cfg.supportsSnippets
           ^ " => "
           ^ if !Cfg.supportsSnippets then "{$0}" else "{}")
         ~sortText:"A" ~kind:(Value typ) ~env ();

analysis/src/SharedTypes.ml

@@ -343,6 +343,7 @@ and completionType =
       args: typedFnArg list;
       typ: Types.type_expr;
       uncurried: bool;
+      returnType: Types.type_expr;
     }
 
 module Env = struct

analysis/src/TypeUtils.ml

@@ -125,8 +125,8 @@ let rec extractType ~env ~package (t : Types.type_expr) =
   | Tconstr (Pident {name = "function$"}, [t; _], _) -> (
     (* Uncurried functions. *)
     match extractFunctionType t ~env ~package with
-    | args, _tRet when args <> [] ->
-      Some (Tfunction {env; args; typ = t; uncurried = true})
+    | args, tRet when args <> [] ->
+      Some (Tfunction {env; args; typ = t; uncurried = true; returnType = tRet})
     | _args, _tRet -> None)
   | Tconstr (path, typeArgs, _) -> (
     match References.digConstructor ~env ~package path with
@@ -168,8 +168,9 @@ let rec extractType ~env ~package (t : Types.type_expr) =
     Some (Tpolyvariant {env; constructors; typeExpr = t})
   | Tarrow _ -> (
     match extractFunctionType t ~env ~package with
-    | args, _tRet when args <> [] ->
-      Some (Tfunction {env; args; typ = t; uncurried = false})
+    | args, tRet when args <> [] ->
+      Some
+        (Tfunction {env; args; typ = t; uncurried = false; returnType = tRet})
     | _args, _tRet -> None)
   | _ -> None
 

analysis/tests/src/CompletionExpressions.res

@@ -250,3 +250,10 @@ external commitLocalUpdate: (~updater: RecordSourceSelectorProxy.t => unit) => u
 
 // commitLocalUpdate(~updater=)
 //                            ^com
+
+let fnTakingAsyncCallback = (cb: unit => promise<unit>) => {
+  let _ = cb
+}
+
+// fnTakingAsyncCallback()
+//                       ^com

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

@@ -1074,3 +1074,23 @@ Path commitLocalUpdate
     "insertTextFormat": 2
   }]
 
+Complete src/CompletionExpressions.res 257:25
+posCursor:[257:25] posNoWhite:[257:24] Found expr:[257:3->257:26]
+Pexp_apply ...[257:3->257:24] (...[257:25->257:26])
+Completable: Cexpression CArgument Value[fnTakingAsyncCallback]($0)
+Package opens Pervasives.JsxModules.place holder
+Resolved opens 1 pervasives
+ContextPath CArgument Value[fnTakingAsyncCallback]($0)
+ContextPath Value[fnTakingAsyncCallback]
+Path fnTakingAsyncCallback
+[{
+    "label": "async () => {}",
+    "kind": 12,
+    "tags": [],
+    "detail": "unit => promise<unit>",
+    "documentation": null,
+    "sortText": "A",
+    "insertText": "async () => {$0}",
+    "insertTextFormat": 2
+  }]
+