How can I detect whether an unlisted buffer contains a new file or an existing file?

Question

Upon opening a new file in an active buffer in the current window, the message line at the very bottom of the screen shows "%f" [New File].

How does Vim know that the buffer contains a new file, rather than an existing file? I want to detect this in order to test whether quickfix has correctly parsed a file name from an error message. Since getqflist() gives 'bufno' instead of 'filename', I can't use filereadable() for this.

I've also tried checking getbufvar() but until you actually try to jump to the error with :cc, any buffers created by quickfix after parsing an error message for a file name are unlisted and getbufvar() returns an empty dictionary. I want to determine whether the buffer will contain a new or existing file and intervene before jumping to the error and opening the file.

Answer 1

The list of built-in functions includes bufname, which can retrieve the filename displayed in the output of ls! from an unlisted buffer, before/without listing it:

bufname( {expr})      String  Name of the buffer {expr}

So, if you know the number of the buffer, you can use filereadable() as in the following example:

let qflist = getqflist()
for item in qflist
    let readable = filereadable(bufname(item.bufnr))
    echo bufname(item.bufnr) . " is " . (readable ? "" : "not ") . "a readable file."
endfor