fix(lsp): inlay hints are rendered in the correct order (#29707)

Problem:
When there are multiple inlay hints present at the same position, they
should be rendered in the order they are received in the response from
LSP as per the LSP spec. Currently, this is not respected.

Solution:
Gather all hints for a given position, and then set it in a single
extmark call instead of multiple set_extmark calls. This leads to fewer
extmark calls and correct inlay hints being rendered.

2 files changed, 107 insertions, 29 deletions

diff --git a/runtime/lua/vim/lsp/inlay_hint.lua b/runtime/lua/vim/lsp/inlay_hint.lua
index aa84294cc4..1e224d1bef 100644
--- a/runtime/lua/vim/lsp/inlay_hint.lua
+++ b/runtime/lua/vim/lsp/inlay_hint.lua
@@ -336,6 +336,8 @@ api.nvim_set_decoration_provider(namespace, {
     for lnum = topline, botline do
       if bufstate.applied[lnum] ~= bufstate.version then
         api.nvim_buf_clear_namespace(bufnr, namespace, lnum, lnum + 1)
+
+        local hint_virtual_texts = {} --- @type table<integer, [string, string?][]>
         for _, lnum_hints in pairs(client_hints) do
           local hints = lnum_hints[lnum] or {}
           for _, hint in pairs(hints) do
@@ -348,7 +350,7 @@ api.nvim_set_decoration_provider(namespace, {
                 text = text .. part.value
               end
             end
-            local vt = {} --- @type [string, string?][]
+            local vt = hint_virtual_texts[hint.position.character] or {}
             if hint.paddingLeft then
               vt[#vt + 1] = { ' ' }
             end
@@ -356,13 +358,18 @@ api.nvim_set_decoration_provider(namespace, {
             if hint.paddingRight then
               vt[#vt + 1] = { ' ' }
             end
-            api.nvim_buf_set_extmark(bufnr, namespace, lnum, hint.position.character, {
-              virt_text_pos = 'inline',
-              ephemeral = false,
-              virt_text = vt,
-            })
+            hint_virtual_texts[hint.position.character] = vt
           end
         end
+
+        for pos, vt in pairs(hint_virtual_texts) do
+          api.nvim_buf_set_extmark(bufnr, namespace, lnum, pos, {
+            virt_text_pos = 'inline',
+            ephemeral = false,
+            virt_text = vt,
+          })
+        end
+
         bufstate.applied[lnum] = bufstate.version
       end
     end
diff --git a/test/functional/plugin/lsp/inlay_hint_spec.lua b/test/functional/plugin/lsp/inlay_hint_spec.lua
index d3b5ae0e4e..00f79b9963 100644
--- a/test/functional/plugin/lsp/inlay_hint_spec.lua
+++ b/test/functional/plugin/lsp/inlay_hint_spec.lua
@@ -12,7 +12,8 @@ local api = n.api
 local clear_notrace = t_lsp.clear_notrace
 local create_server_definition = t_lsp.create_server_definition
 
-local text = dedent([[
+describe('vim.lsp.inlay_hint', function()
+  local text = dedent([[
 auto add(int a, int b) { return a + b; }
 
 int main() {
@@ -22,7 +23,7 @@ int main() {
 }
 }]])
 
-local response = [==[
+  local response = [==[
 [
 {"kind":1,"paddingLeft":false,"label":"-> int","position":{"character":22,"line":0},"paddingRight":false},
 {"kind":2,"paddingLeft":false,"label":"a:","position":{"character":15,"line":5},"paddingRight":true},
@@ -30,7 +31,7 @@ local response = [==[
 ]
 ]==]
 
-local grid_without_inlay_hints = [[
+  local grid_without_inlay_hints = [[
   auto add(int a, int b) { return a + b; }          |
                                                     |
   int main() {                                      |
@@ -42,7 +43,7 @@ local grid_without_inlay_hints = [[
                                                     |
 ]]
 
-local grid_with_inlay_hints = [[
+  local grid_with_inlay_hints = [[
   auto add(int a, int b){1:-> int} { return a + b; }    |
                                                     |
   int main() {                                      |
@@ -54,16 +55,16 @@ local grid_with_inlay_hints = [[
                                                     |
 ]]
 
---- @type test.functional.ui.screen
-local screen
-before_each(function()
-  clear_notrace()
-  screen = Screen.new(50, 9)
-  screen:attach()
+  --- @type test.functional.ui.screen
+  local screen
+  before_each(function()
+    clear_notrace()
+    screen = Screen.new(50, 9)
+    screen:attach()
 
-  exec_lua(create_server_definition)
-  exec_lua(
-    [[
+    exec_lua(create_server_definition)
+    exec_lua(
+      [[
     local response = ...
     server = _create_server({
       capabilities = {
@@ -81,19 +82,18 @@ before_each(function()
 
     client_id = vim.lsp.start({ name = 'dummy', cmd = server.cmd })
   ]],
-    response
-  )
+      response
+    )
 
-  insert(text)
-  exec_lua([[vim.lsp.inlay_hint.enable(true, { bufnr = bufnr })]])
-  screen:expect({ grid = grid_with_inlay_hints })
-end)
+    insert(text)
+    exec_lua([[vim.lsp.inlay_hint.enable(true, { bufnr = bufnr })]])
+    screen:expect({ grid = grid_with_inlay_hints })
+  end)
 
-after_each(function()
-  api.nvim_exec_autocmds('VimLeavePre', { modeline = false })
-end)
+  after_each(function()
+    api.nvim_exec_autocmds('VimLeavePre', { modeline = false })
+  end)
 
-describe('vim.lsp.inlay_hint', function()
   it('clears inlay hints when sole client detaches', function()
     exec_lua([[vim.lsp.stop_client(client_id)]])
     screen:expect({ grid = grid_without_inlay_hints, unchanged = true })
@@ -258,3 +258,74 @@ describe('vim.lsp.inlay_hint', function()
     end)
   end)
 end)
+
+describe('Inlay hints handler', function()
+  local text = dedent([[
+test text
+  ]])
+
+  local response = [==[
+  [
+      { "position": { "line": 0, "character": 0 }, "label": "0" },
+      { "position": { "line": 0, "character": 0 }, "label": "1" },
+      { "position": { "line": 0, "character": 0 }, "label": "2" },
+      { "position": { "line": 0, "character": 0 }, "label": "3" },
+      { "position": { "line": 0, "character": 0 }, "label": "4" }
+  ]
+  ]==]
+
+  local grid_without_inlay_hints = [[
+  test text                                         |
+   ^                                                 |
+                                                    |
+]]
+
+  local grid_with_inlay_hints = [[
+  {1:01234}test text                                    |
+   ^                                                 |
+                                                    |
+]]
+
+  --- @type test.functional.ui.screen
+  local screen
+  before_each(function()
+    clear_notrace()
+    screen = Screen.new(50, 3)
+    screen:attach()
+
+    exec_lua(create_server_definition)
+    exec_lua(
+      [[
+    local response = ...
+    server = _create_server({
+      capabilities = {
+        inlayHintProvider = true,
+      },
+      handlers = {
+        ['textDocument/inlayHint'] = function(_, _, callback)
+          callback(nil, vim.json.decode(response))
+        end,
+      }
+    })
+
+    bufnr = vim.api.nvim_get_current_buf()
+    vim.api.nvim_win_set_buf(0, bufnr)
+
+    client_id = vim.lsp.start({ name = 'dummy', cmd = server.cmd })
+  ]],
+      response
+    )
+    insert(text)
+  end)
+
+  it('renders hints with same position in received order', function()
+    exec_lua([[vim.lsp.inlay_hint.enable(true, { bufnr = bufnr })]])
+    screen:expect({ grid = grid_with_inlay_hints })
+    exec_lua([[vim.lsp.stop_client(client_id)]])
+    screen:expect({ grid = grid_without_inlay_hints, unchanged = true })
+  end)
+
+  after_each(function()
+    api.nvim_exec_autocmds('VimLeavePre', { modeline = false })
+  end)
+end)