feat: allow modifying expand-position in pre_expand-callback (#1219).

DOC.md

@@ -3400,6 +3400,8 @@ The node and `event_args` can be accessed through `require("luasnip").session`:
   `event_args`:
   * `expand_pos`: `{<row>, <column>}`, position at which the snippet will be
   	expanded. `<row>` and `<column>` are both 0-indexed.
+  * `expand_pos_mark_id`: `number`, the id of the extmark luasnip uses to track
+    `expand_pos`. This may be moved around freely.
   `event_res`:
   * `env_override`: `map string->(string[]|string)`, override or extend the
     snippet's environment (`snip.env`).

lua/luasnip/nodes/snippet.lua

@@ -632,6 +632,12 @@ end
 function Snippet:trigger_expand(current_node, pos_id, env, indent_nodes)
 	local pos = vim.api.nvim_buf_get_extmark_by_id(0, session.ns_id, pos_id, {})
 
+	local pre_expand_res = self:event(events.pre_expand, { expand_pos = pos, expand_pos_mark_id = pos_id })
+		or {}
+
+	-- update pos, event-callback might have moved the extmark.
+	pos = vim.api.nvim_buf_get_extmark_by_id(0, session.ns_id, pos_id, {})
+
 	-- find tree-node the snippet should be inserted at (could be before another node).
 	local _, sibling_snippets, own_indx, parent_node =
 		node_util.snippettree_find_undamaged_node(pos, {
@@ -697,11 +703,6 @@ function Snippet:trigger_expand(current_node, pos_id, env, indent_nodes)
 		end
 	end
 
-	local pre_expand_res = self:event(events.pre_expand, { expand_pos = pos })
-		or {}
-	-- update pos, event-callback might have moved the extmark.
-	pos = vim.api.nvim_buf_get_extmark_by_id(0, session.ns_id, pos_id, {})
-
 	Environ:override(env, pre_expand_res.env_override or {})
 
 	if indent_nodes then

tests/integration/snippet_basics_spec.lua

@@ -1626,4 +1626,24 @@ describe("snippets_basic", function()
 			  {2:-- INSERT --}                                      |]],
 		})
 	end)
+
+	it("expand-position may be moved in pre_expand.", function()
+		feed("i.")
+
+		exec_lua[[
+			snip = s("foo", {
+				t"asdf"
+			}, {callbacks = {[-1] = { [events.pre_expand] = function(node, event_args)
+				vim.api.nvim_buf_set_extmark(0, ls.session.ns_id, 0,0, {id = event_args.expand_pos_mark_id})
+			end}} } )
+
+			ls.snip_expand(snip)
+		]]
+
+		screen:expect({
+			grid = [[
+				asdf^.                                             |
+				{0:~                                                 }|
+				{2:-- INSERT --}                                      |]]})
+	end)
 end)