fix(api): not using TRY_WRAP, generic error messages #31595

Problem:
- API functions using `try_start` directly instead of `TRY_WRAP`, do not
  surface the underlying error message, and instead show generic things
  like "Failed to set buffer".
- Error handling code is duplicated in the API impl, instead of
  delegating to the vim buffer/window handling logic.

Solution:
- Use `TRY_WRAP`.

src/nvim/api/private/helpers.c

@@ -101,9 +101,9 @@ bool try_leave(const TryState *const tstate, Error *const err)
   return ret;
 }
 
-/// Start block that may cause vimscript exceptions
+/// Starts a block that may cause Vimscript exceptions; must be mirrored by `try_end()` call.
 ///
-/// Each try_start() call should be mirrored by try_end() call.
+/// Note: use `TRY_WRAP` instead (except in `FUNC_API_FAST` functions such as nvim_get_runtime_file).
 ///
 /// To be used as a replacement of `:try … catch … endtry` in C code, in cases
 /// when error flag could not already be set. If there may be pending error
@@ -114,8 +114,9 @@ void try_start(void)
   trylevel++;
 }
 
-/// End try block, set the error message if any and return true if an error
-/// occurred.
+/// Ends a `try_start` block; sets error message if any and returns true if an error occurred.
+///
+/// Note: use `TRY_WRAP` instead (except in `FUNC_API_FAST` functions such as nvim_get_runtime_file).
 ///
 /// @param err Pointer to the stack-allocated error object
 /// @return true if an error occurred

src/nvim/api/vim.c

@@ -596,6 +596,7 @@ ArrayOf(String) nvim_get_runtime_file(String name, Boolean all, Arena *arena, Er
   int flags = DIP_DIRFILE | (all ? DIP_ALL : 0);
   TryState tstate;
 
+  // XXX: intentionally not using `TRY_WRAP`, to avoid `did_emsg=false` in `try_end`.
   try_enter(&tstate);
   do_in_runtimepath((name.size ? name.data : ""), flags, find_runtime_cb, &cookie);
   vim_ignored = try_leave(&tstate, err);
@@ -888,23 +889,13 @@ void nvim_set_current_buf(Buffer buffer, Error *err)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
 
-  if (!buf || curwin->w_buffer == buf) {
+  if (!buf) {
     return;
   }
 
-  if (curwin->w_p_wfb) {
-    api_set_error(err, kErrorTypeException, "%s", e_winfixbuf_cannot_go_to_buffer);
-    return;
-  }
-
-  try_start();
-  int result = do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, buf->b_fnum, 0);
-  if (!try_end(err) && result == FAIL) {
-    api_set_error(err,
-                  kErrorTypeException,
-                  "Failed to switch to buffer %d",
-                  buffer);
-  }
+  TRY_WRAP(err, {
+    do_buffer(DOBUF_GOTO, DOBUF_FIRST, FORWARD, buf->b_fnum, 0);
+  });
 }
 
 /// Gets the current list of window handles.

src/nvim/api/window.c

@@ -59,12 +59,7 @@ void nvim_win_set_buf(Window window, Buffer buffer, Error *err)
 {
   win_T *win = find_window_by_handle(window, err);
   buf_T *buf = find_buffer_by_handle(buffer, err);
-  if (!win || !buf || win->w_buffer == buf) {
-    return;
-  }
-
-  if (win->w_p_wfb) {
-    api_set_error(err, kErrorTypeException, "%s", e_winfixbuf_cannot_go_to_buffer);
+  if (!win || !buf) {
     return;
   }