Fix #1661 field-erasure for destruct punning field (#1734)

from xvw/1661-destructing-punned-record-field-breaks-syntax

CHANGES.md

@@ -3,6 +3,8 @@ merlin NEXT_VERSION
 
   + merlin binary
     - destruct: Removal of residual patterns (#1737, fixes #1560)
+    - Do not erase fields' names when destructing punned record fields (#1734, 
+      fixes #1661)
 
 merlin 4.14
 ===========

src/analysis/destruct.ml

@@ -481,6 +481,25 @@ let find_branch patterns sub =
   in
   aux [] patterns
 
+(* In the presence of record punning fields, the definition must be reconstructed
+   with the label. ie: [{a; b}] with destruction on [a] becomes *)
+(*  [{a = destruct_result; b}]. *)
+let find_field_name_for_punned_field patt = function
+  | Pattern {pat_desc = Tpat_record (fields, _); _} :: _ ->
+    List.find_opt ~f:(fun (_, _, opat) ->
+        let ppat_loc = patt.Typedtree.pat_loc
+        and opat_loc = opat.Typedtree.pat_loc in
+        Int.equal (Location_aux.compare ppat_loc opat_loc) 0
+      ) fields |> Option.map ~f:(fun (_, label, _) -> label)
+  | _ -> None
+
+let print_pretty ?punned_field config source subject =
+  let result = Mreader.print_pretty config source subject in
+  match punned_field with
+  | None -> result
+  | Some label ->
+    label.Types.lbl_name ^ " = " ^ result
+
 let rec node config source selected_node parents =
   let open Extend_protocol.Reader in
   let loc = Mbrowse.node_loc selected_node in
@@ -571,9 +590,9 @@ let rec node config source selected_node parents =
             | [ more_precise ] ->
               (* If only one pattern is generated, then we're only refining the
                 current pattern, not generating new branches. *)
+              let punned_field = find_field_name_for_punned_field patt parents in
               let ppat = filter_pat_attr (Untypeast.untype_pattern more_precise) in
-              let str = Mreader.print_pretty
-                  config source (Pretty_pattern ppat) in
+              let str = print_pretty ?punned_field config source (Pretty_pattern ppat) in
               patt.Typedtree.pat_loc, str
             | sub_patterns ->
               let rev_before, after, top_patt =
@@ -609,9 +628,9 @@ let rec node config source selected_node parents =
                 in
                 (* Format.eprintf "por %a \n%!" (Printtyped.pattern 0) p; *)
                 let ppat = filter_pat_attr (Untypeast.untype_pattern p) in
+
                 (* Format.eprintf "ppor %a \n%!" (Pprintast.pattern) ppat; *)
-                let str = Mreader.print_pretty
-                    config source (Pretty_pattern ppat) in
+                let str = Mreader.print_pretty config source (Pretty_pattern ppat) in
                 (* Format.eprintf "STR: %s \n %!" str; *)
                 top_patt.Typedtree.pat_loc, str
             end

tests/test-dirs/destruct/issue1661.t

@@ -0,0 +1,46 @@
+  $ $MERLIN single case-analysis -start 2:9 -end 2:9 \
+  > -filename main.ml <<EOF
+  > type t = {a: int * int; b: string}
+  > let f ({a; b} : t) = assert false
+  > EOF
+  {
+    "class": "return",
+    "value": [
+      {
+        "start": {
+          "line": 2,
+          "col": 8
+        },
+        "end": {
+          "line": 2,
+          "col": 9
+        }
+      },
+      "a = (_, _)"
+    ],
+    "notifications": []
+  }
+
+
+  $ $MERLIN single case-analysis -start 2:9 -end 2:9 \
+  > -filename main.ml <<EOF
+  > type t = {a: int option; b: string}
+  > let f ({a; b} : t) = assert false
+  > EOF
+  {
+    "class": "return",
+    "value": [
+      {
+        "start": {
+          "line": 2,
+          "col": 7
+        },
+        "end": {
+          "line": 2,
+          "col": 13
+        }
+      },
+      "({ a = None; b } : t) | ({ a = Some _; b } : t)"
+    ],
+    "notifications": []
+  }