Add autofix for lint on UNSAFE_CAST

Summary:
Offer an autofix when we know the best input type for the user to use.

Also reword the lint message to be a little more concise.

Reviewed By: vsiles

Differential Revision: D39913756

fbshipit-source-id: fbf093f665e27b721c2bdca7d6fc4a53e676caad

hphp/hack/src/lints/lints_errors.ml

@@ -392,18 +392,22 @@ let duplicate_property pos ~class_name ~prop_name ~class_names =
 
 let loose_unsafe_cast_lower_bound p ty_str_opt =
   let msg =
-    "HH\\FIXME\\UNSAFE_CAST input type annotation is too loose, please use a more specific type."
+    "The input type to `HH\\FIXME\\UNSAFE_CAST` should be as specific as possible."
   in
-  let msg =
+  let (msg, autofix) =
     match ty_str_opt with
     | Some ty_str ->
-      msg
-      ^ " The typechecker infers "
-      ^ Markdown_lite.md_codify ty_str
-      ^ " as the most specific type."
-    | None -> msg
+      let path = Pos.filename (Pos.to_absolute p) in
+      let lines = Errors.read_lines path in
+      let src = String.concat ~sep:"\n" lines in
+      let original = Pos.get_text_from_pos ~content:src p in
+      let (start_offset, end_offset) = Pos.info_raw p in
+      let width = end_offset - start_offset in
+      ( msg ^ " Consider using " ^ Markdown_lite.md_codify ty_str ^ " instead.",
+        Some (original, ty_str, start_offset, width) )
+    | None -> (msg, None)
   in
-  Lints.add Codes.loose_unsafe_cast_lower_bound Lint_error p msg
+  Lints.add ~autofix Codes.loose_unsafe_cast_lower_bound Lint_error p msg
 
 let loose_unsafe_cast_upper_bound p =
   Lints.add

hphp/hack/test/json/test_quickfix_unsafe_cast.php

@@ -0,0 +1,9 @@
+<?hh
+
+function nullable_str(): ?string {
+  return null;
+}
+
+function demo(): void {
+  $_ = HH\FIXME\UNSAFE_CAST<mixed, string>(nullable_str());
+}

hphp/hack/test/json/test_quickfix_unsafe_cast.php.exp

@@ -0,0 +1,19 @@
+{
+  "errors":[
+    {
+      "descr":"The input type to `HH\\FIXME\\UNSAFE_CAST` should be as specific as possible. Consider using `?string` instead.",
+      "severity":"error",
+      "path":"test_quickfix_unsafe_cast.php",
+      "line":8,
+      "start":29,
+      "end":33,
+      "code":5636,
+      "bypass_changed_lines":false,
+      "original":"mixed",
+      "replacement":"?string",
+      "start_offset":111,
+      "width":5
+    }
+  ],
+  "version":" Wed Dec 31 16:00:00 1969"
+}

hphp/hack/test/lint/unsafe_cast_lower_bound2.php.exp

@@ -1,2 +1,2 @@
 File "unsafe_cast_lower_bound2.php", line 4, characters 25-29:
-HH\FIXME\UNSAFE_CAST input type annotation is too loose, please use a more specific type. The typechecker infers `string` as the most specific type. (Lint[5636])
+The input type to `HH\FIXME\UNSAFE_CAST` should be as specific as possible. Consider using `string` instead. (Lint[5636])

hphp/hack/test/lint/unsafe_cast_lower_bound3.php.exp

@@ -1,2 +1,2 @@
 File "unsafe_cast_lower_bound3.php", line 4, characters 25-29:
-HH\FIXME\UNSAFE_CAST input type annotation is too loose, please use a more specific type. (Lint[5636])
+The input type to `HH\FIXME\UNSAFE_CAST` should be as specific as possible. (Lint[5636])

hphp/hack/test/lint/unsafe_cast_lower_bound4.php.exp

@@ -1,4 +1,4 @@
 File "unsafe_cast_lower_bound4.php", line 4, characters 25-29:
-HH\FIXME\UNSAFE_CAST input type annotation is too loose, please use a more specific type. The typechecker infers `arraykey` as the most specific type. (Lint[5636])
+The input type to `HH\FIXME\UNSAFE_CAST` should be as specific as possible. Consider using `arraykey` instead. (Lint[5636])
 File "unsafe_cast_lower_bound4.php", line 8, characters 25-29:
-HH\FIXME\UNSAFE_CAST input type annotation is too loose, please use a more specific type. The typechecker infers `num` as the most specific type. (Lint[5636])
+The input type to `HH\FIXME\UNSAFE_CAST` should be as specific as possible. Consider using `num` instead. (Lint[5636])

hphp/hack/test/lint/unsafe_cast_lower_bound5.php.exp

@@ -1,2 +1,2 @@
 File "unsafe_cast_lower_bound5.php", line 4, characters 25-29:
-HH\FIXME\UNSAFE_CAST input type annotation is too loose, please use a more specific type. The typechecker infers `?int` as the most specific type. (Lint[5636])
+The input type to `HH\FIXME\UNSAFE_CAST` should be as specific as possible. Consider using `?int` instead. (Lint[5636])