Add a new command to extract expression into a fresh let binding

tests/test-dirs/refactor-extract-region/extraction-issue.t/foo.ml

@@ -0,0 +1,20 @@
+let z = 100
+
+let complicated_function x y =
+  let a = 10 in
+  let b = 11 in
+  let c = 12 in
+  let module D = struct
+    let x = 13
+  end in
+  a + b + (c * x * y) + z + D.x
+
+let f () =
+  let module D = struct
+    let x = 42
+  end in
+  let module M = struct
+    let x = 1
+  end in
+  let a, b, c, x = (1, 2, 3, 4) in
+  a + b + c + D.x + x + M.(x + a)

tests/test-dirs/refactor-extract-region/extraction-issue.t/run.t

@@ -0,0 +1,77 @@
+FIXME: `x` is used instead of `d_x` in the extracted function body!
+Should be: (((a + b) + ((c * x) * y)) + z) + d_x
+
+  $ $MERLIN single refactoring-extract-region -start 10:2 -end 10:31 < foo.ml
+  {
+    "class": "return",
+    "value": {
+      "start": {
+        "line": 3,
+        "col": 0
+      },
+      "end": {
+        "line": 10,
+        "col": 31
+      },
+      "content": "let fun_name1 x y a b c d_x = (((a + b) + ((c * x) * y)) + z) + x
+  let complicated_function x y =
+    let a = 10 in
+    let b = 11 in
+    let c = 12 in
+    let module D = struct
+      let x = 13
+    end in
+    (fun_name1 x y a b c D.x)",
+      "selection-range": {
+        "start": {
+          "line": 3,
+          "col": 4
+        },
+        "end": {
+          "line": 3,
+          "col": 13
+        }
+      }
+    },
+    "notifications": []
+  }
+
+FIXME: the extracted function body is wrong. 
+Should be: a + b + c + d_x + x + (m_x + a)
+
+  $ $MERLIN single refactoring-extract-region -start 20:2 -end 20:33 < foo.ml
+  {
+    "class": "return",
+    "value": {
+      "start": {
+        "line": 12,
+        "col": 0
+      },
+      "end": {
+        "line": 20,
+        "col": 33
+      },
+      "content": "let fun_name1 a b c x d_x m_x =
+    ((((a + b) + c) + x) + x) + (let open M in x + a)
+  let f () =
+    let module D = struct
+      let x = 42
+    end in
+    let module M = struct
+      let x = 1
+    end in
+    let a, b, c, x = (1, 2, 3, 4) in
+    (fun_name1 a b c x D.x M.x)",
+      "selection-range": {
+        "start": {
+          "line": 12,
+          "col": 4
+        },
+        "end": {
+          "line": 12,
+          "col": 13
+        }
+      }
+    },
+    "notifications": []
+  }

tests/test-dirs/refactor-extract-region/func-extraction.t/func.ml

@@ -156,24 +156,3 @@ module T = struct
     let printer = pp_print_list pp_print_int in
     printf "%a\n" printer a_list
 end
-
-let z = 100
-
-let complicated_function x y =
-  let a = 10 in
-  let b = 11 in
-  let c = 12 in
-  let module D = struct
-    let x = 13
-  end in
-  a + b + (c * x * y) + z + D.x
-
-let f () =
-  let module D = struct
-    let x = 42
-  end in
-  let module M = struct
-    let x = 1
-  end in
-  let a, b, c, x = (1, 2, 3, 4) in
-  a + b + c + D.x + x + M.(x + a)

tests/test-dirs/refactor-extract-region/func-extraction.t/run.t

@@ -867,40 +867,3 @@ TODO: This extraction shouldn't be allowed.
     },
     "notifications": []
   }
-
-  $ $MERLIN single refactoring-extract-region -start 169:2 -end 169:31 < func.ml
-  {
-    "class": "return",
-    "value": {
-      "start": {
-        "line": 162,
-        "col": 0
-      },
-      "end": {
-        "line": 169,
-        "col": 31
-      },
-      "content": "let fun_name2 x y a b c d_x = (((a + b) + ((c * x) * y)) + z) + d_x
-  let complicated_function x y =
-    let a = 10 in
-    let b = 11 in
-    let c = 12 in
-    let module D = struct
-      let x = 13
-    end in
-    (fun_name2 x y a b c D.x)",
-      "selection-range": {
-        "start": {
-          "line": 162,
-          "col": 4
-        },
-        "end": {
-          "line": 162,
-          "col": 13
-        }
-      }
-    },
-    "notifications": []
-  }
-
-  $ $MERLIN single refactoring-extract-region -start 179:2 -end 179:33 < func.ml