fix(filetype): match on <afile> rather than <abuf> (#16943)

Filetype detection runs on BufRead and BufNewFile autocommands, both of
which can fire without an underlying buffer, so it's incorrect to use
<abuf> to determine the file path. Instead, match on <afile> and assume
that the buffer we're operating on is the current buffer. This is the
same assumption that filetype.vim makes, so it should be safe.

runtime/filetype.lua

@@ -9,7 +9,7 @@ end
 
 vim.cmd [[
 augroup filetypedetect
-au BufRead,BufNewFile * call v:lua.vim.filetype.match(str2nr(expand('<abuf>')))
+au BufRead,BufNewFile * call v:lua.vim.filetype.match(expand('<afile>'))
 
 " These *must* be sourced after the autocommand above is created
 runtime! ftdetect/*.vim

runtime/lua/vim/filetype.lua

@@ -1449,15 +1449,20 @@ local function dispatch(ft, path, bufnr, ...)
 end
 
 ---@private
-function M.match(bufnr)
-  local path = api.nvim_buf_get_name(bufnr)
+function M.match(name, bufnr)
+  -- When fired from the main filetypedetect autocommand the {bufnr} argument is omitted, so we use
+  -- the current buffer. The {bufnr} argument is provided to allow extensibility in case callers
+  -- wish to perform filetype detection on buffers other than the current one.
+  bufnr = bufnr or api.nvim_get_current_buf()
+
   -- First check for the simple case where the full path exists as a key
+  local path = vim.fn.fnamemodify(name, ":p")
   if dispatch(filename[path], path, bufnr) then
     return
   end
 
   -- Next check against just the file name
-  local tail = vim.fn.fnamemodify(path, ":t")
+  local tail = vim.fn.fnamemodify(name, ":t")
   if dispatch(filename[tail], path, bufnr) then
     return
   end
@@ -1477,7 +1482,7 @@ function M.match(bufnr)
   end
 
   -- Finally, check file extension
-  local ext = vim.fn.fnamemodify(path, ":e")
+  local ext = vim.fn.fnamemodify(name, ":e")
   if dispatch(extension[ext], path, bufnr) then
     return
   end

test/functional/lua/filetype_spec.lua

@@ -24,8 +24,7 @@ describe('vim.filetype', function()
           rs = 'radicalscript',
         },
       })
-      vim.api.nvim_buf_set_name(0, 'main.rs')
-      vim.filetype.match(0)
+      vim.filetype.match('main.rs')
       return vim.bo.filetype
     ]])
   end)
@@ -40,8 +39,7 @@ describe('vim.filetype', function()
           ['main.rs'] = 'somethingelse',
         },
       })
-      vim.api.nvim_buf_set_name(0, 'main.rs')
-      vim.filetype.match(0)
+      vim.filetype.match('main.rs')
       return vim.bo.filetype
     ]])
   end)
@@ -53,8 +51,7 @@ describe('vim.filetype', function()
           ['s_O_m_e_F_i_l_e'] = 'nim',
         },
       })
-      vim.api.nvim_buf_set_name(0, 's_O_m_e_F_i_l_e')
-      vim.filetype.match(0)
+      vim.filetype.match('s_O_m_e_F_i_l_e')
       return vim.bo.filetype
     ]])
 
@@ -66,8 +63,7 @@ describe('vim.filetype', function()
           [root .. '/.config/fun/config'] = 'dosini',
         },
       })
-      vim.api.nvim_buf_set_name(0, root .. '/.config/fun/config')
-      vim.filetype.match(0)
+      vim.filetype.match(root .. '/.config/fun/config')
       return vim.bo.filetype
     ]], root))
   end)
@@ -80,8 +76,7 @@ describe('vim.filetype', function()
           [root .. '/blog/.*%.txt'] = 'markdown',
         }
       })
-      vim.api.nvim_buf_set_name(0, root .. '/blog/why_neovim_is_awesome.txt')
-      vim.filetype.match(0)
+      vim.filetype.match(root .. '/blog/why_neovim_is_awesome.txt')
       return vim.bo.filetype
     ]], root))
   end)
@@ -97,8 +92,7 @@ describe('vim.filetype', function()
           end,
         }
       })
-      vim.api.nvim_buf_set_name(0, 'relevant_to_me')
-      vim.filetype.match(0)
+      vim.filetype.match('relevant_to_me')
       return vim.bo.filetype
     ]])
   end)