feat(lsp): on_accept can return item to customize behavior #37092

Problem:
`on_accept` is a bit cumbersome to customize.

Solution:
* Before: users had to override the entire `on_accept` logic for their changes to be applied.
* Now: users can modify the item and return it to apply the modified changes, or return `nil` to fully customize how the changes are applied.

runtime/doc/lsp.txt

@@ -2327,10 +2327,13 @@ get({opts})                                  *vim.lsp.inline_completion.get()*
       • {opts}  (`table?`) A table with the following fields:
                 • {bufnr}? (`integer`, default: 0) Buffer handle, or 0 for
                   current.
-                • {on_accept}? (`fun(item: vim.lsp.inline_completion.Item)`)
-                  Accept handler, called with the accepted item. If not
-                  provided, the default handler is used, which applies changes
-                  to the buffer based on the completion item.
+                • {on_accept}?
+                  (`fun(item: vim.lsp.inline_completion.Item): vim.lsp.inline_completion.Item?`)
+                  A callback triggered when a completion item is accepted. You
+                  can use it to modify the completion item that is about to be
+                  accepted and return it to apply the changes, or return `nil`
+                  to prevent the changes from being applied to the buffer so
+                  you can implement custom behavior.
 
     Return: ~
         (`boolean`) `true` if a completion was applied, else `false`.

runtime/lua/vim/lsp/inline_completion.lua

@@ -438,10 +438,12 @@ end
 --- (default: 0)
 ---@field bufnr? integer
 ---
---- Accept handler, called with the accepted item.
---- If not provided, the default handler is used,
---- which applies changes to the buffer based on the completion item.
----@field on_accept? fun(item: vim.lsp.inline_completion.Item)
+--- A callback triggered when a completion item is accepted.
+--- You can use it to modify the completion item that is about to be accepted
+--- and return it to apply the changes,
+--- or return `nil` to prevent the changes from being applied to the buffer
+--- so you can implement custom behavior.
+---@field on_accept? fun(item: vim.lsp.inline_completion.Item): vim.lsp.inline_completion.Item?
 
 --- Accept the currently displayed completion candidate to the buffer.
 ---
@@ -474,8 +476,13 @@ function M.get(opts)
         return
       end
 
+      -- Note that we do not intend for `on_accept`
+      -- to take effect when there is no current item.
       if on_accept then
-        on_accept(item)
+        item = on_accept(item)
+        if item then
+          completor:accept(item)
+        end
       else
         completor:accept(item)
       end