feat(lsp): support willSave & willSaveWaitUntil capability (#21315)

`willSaveWaitUntil` allows servers to respond with text edits before
saving a document. That is used by some language servers to format a
document or apply quick fixes like removing unused imports.

runtime/lua/vim/lsp.lua

@@ -1611,9 +1611,37 @@ function lsp.buf_attach_client(bufnr, client_id)
     all_buffer_active_clients[bufnr] = buffer_client_ids
 
     local uri = vim.uri_from_bufnr(bufnr)
-    local augroup = ('lsp_c_%d_b_%d_did_save'):format(client_id, bufnr)
+    local augroup = ('lsp_c_%d_b_%d_save'):format(client_id, bufnr)
+    local group = api.nvim_create_augroup(augroup, { clear = true })
+    api.nvim_create_autocmd('BufWritePre', {
+      group = group,
+      buffer = bufnr,
+      desc = 'vim.lsp: textDocument/willSave',
+      callback = function(ctx)
+        for_each_buffer_client(ctx.buf, function(client)
+          local params = {
+            textDocument = {
+              uri = uri,
+            },
+            reason = protocol.TextDocumentSaveReason.Manual,
+          }
+          if vim.tbl_get(client.server_capabilities, 'textDocumentSync', 'willSave') then
+            client.notify('textDocument/willSave', params)
+          end
+          if vim.tbl_get(client.server_capabilities, 'textDocumentSync', 'willSaveWaitUntil') then
+            local result, err =
+              client.request_sync('textDocument/willSaveWaitUntil', params, 1000, ctx.buf)
+            if result and result.result then
+              util.apply_text_edits(result.result, ctx.buf, client.offset_encoding)
+            elseif err then
+              log.error(vim.inspect(err))
+            end
+          end
+        end)
+      end,
+    })
     api.nvim_create_autocmd('BufWritePost', {
-      group = api.nvim_create_augroup(augroup, { clear = true }),
+      group = group,
       buffer = bufnr,
       desc = 'vim.lsp: textDocument/didSave handler',
       callback = function(ctx)

runtime/lua/vim/lsp/protocol.lua

@@ -151,6 +151,7 @@ local constants = {
   },
 
   -- Represents reasons why a text document is saved.
+  ---@enum lsp.TextDocumentSaveReason
   TextDocumentSaveReason = {
     -- Manually triggered, e.g. by the user pressing save, by starting debugging,
     -- or by an API call.
@@ -631,11 +632,8 @@ function protocol.make_client_capabilities()
       synchronization = {
         dynamicRegistration = false,
 
-        -- TODO(ashkan) Send textDocument/willSave before saving (BufWritePre)
-        willSave = false,
-
-        -- TODO(ashkan) Implement textDocument/willSaveWaitUntil
-        willSaveWaitUntil = false,
+        willSave = true,
+        willSaveWaitUntil = true,
 
         -- Send textDocument/didSave after saving (BufWritePost)
         didSave = true,
@@ -870,8 +868,8 @@ function protocol._resolve_capabilities_compat(server_capabilities)
       text_document_sync_properties = {
         text_document_open_close = if_nil(textDocumentSync.openClose, false),
         text_document_did_change = if_nil(textDocumentSync.change, TextDocumentSyncKind.None),
-        text_document_will_save = if_nil(textDocumentSync.willSave, false),
-        text_document_will_save_wait_until = if_nil(textDocumentSync.willSaveWaitUntil, false),
+        text_document_will_save = if_nil(textDocumentSync.willSave, true),
+        text_document_will_save_wait_until = if_nil(textDocumentSync.willSaveWaitUntil, true),
         text_document_save = if_nil(textDocumentSync.save, false),
         text_document_save_include_text = if_nil(
           type(textDocumentSync.save) == 'table' and textDocumentSync.save.includeText,