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

CHANGES.md

@@ -1,6 +1,8 @@
 unreleased
 ==========
+
   + merlin library
+    - Implement new refactor-extract-region command for extracting region to a fresh let binding (#1948)
     - Fix `merlin_reader` for OpenBSD (#1956)
 
 merlin 5.5

doc/dev/PROTOCOL.md

@@ -485,6 +485,25 @@ The result is returned as a list of:
 }
 ```
 
+### `refactor-extract-region -start <position> -end <position> -extract-name <name>`
+
+```
+    -start <position>  Where extracted region start
+    -end <position>  Where extracted region end
+    -extract-name <name>  Name used for the generated let binding
+```
+
+Returns the string `Nothing to do` (if extractor is not ables to select an expression to extract in the given position interval) or the following object:
+
+```javascript
+{
+  'start': position, // the start of the region to be substituted
+  'end': position, // the end of the region to be substituted
+  'content' string, // the content of the substitution
+  'selection_range': location // the location where to position the cursor for easy renaming of the generated let binding 
+}
+```
+
 ### `syntax-document -position <position>`
 
 	-position <position>  The position of the keyword to be documented

src/analysis/parsetree_utils.ml

@@ -1,5 +1,21 @@
+open Std
+
 open Parsetree
 
 type nonrec constant_desc = constant_desc
 
 let constant_desc c = c.pconst_desc
+
+let filter_merlin_attr =
+  let default = Ast_mapper.default_mapper in
+  let keep attr =
+    let { Location.txt; _ }, _ = Ast_helper.Attr.as_tuple attr in
+    not (Std.String.is_prefixed ~by:"merlin." txt)
+  in
+  let attributes mapper attrs =
+    default.Ast_mapper.attributes mapper (List.filter ~f:keep attrs)
+  in
+  { default with Ast_mapper.attributes }
+
+let expr_remove_merlin_attributes expr =
+  filter_merlin_attr.Ast_mapper.expr filter_merlin_attr expr

src/analysis/parsetree_utils.mli

@@ -6,3 +6,6 @@ open Parsetree
 type nonrec constant_desc = constant_desc
 
 val constant_desc : constant -> constant_desc
+
+(** Filter parsetree attributes which are prefixed by ["merlin."] in given expression. *)
+val expr_remove_merlin_attributes : expression -> expression