feat: use scratch buffer for file previews (#467)

* Initial implementation of scratch based preview

* Fix call to buf is valid in loop

* Fixing call to be made only from the main event loop

* Improve handling of large files from @pkazmier

* Better error handling and simplifying the code

* Default to old behavior

* Add documentation

* Fix readfile

* Fix the configuration

* refactor: single config enum and load real buffer on BufEnter

* doc: regenerate documentation

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

lua/oil/util.lua

@@ -897,4 +897,59 @@ M.get_icon_provider = function()
   end
 end
 
+---Read a buffer into a scratch buffer and apply syntactic highlighting when possible
+---@param path string The path to the file to read
+---@param preview_method oil.PreviewMethod
+---@return nil|integer
+M.read_file_to_scratch_buffer = function(path, preview_method)
+  local bufnr = vim.api.nvim_create_buf(false, true)
+  if bufnr == 0 then
+    return
+  end
+
+  vim.bo[bufnr].bufhidden = "wipe"
+  vim.bo[bufnr].buftype = "nofile"
+
+  local max_lines = preview_method == "fast_scratch" and vim.o.lines or nil
+  local has_lines, read_res = pcall(vim.fn.readfile, path, "", max_lines)
+  local lines = has_lines and vim.split(table.concat(read_res, "\n"), "\n") or {}
+
+  local ok = pcall(vim.api.nvim_buf_set_lines, bufnr, 0, -1, false, lines)
+  if not ok then
+    return
+  end
+  local ft = vim.filetype.match({ filename = path, buf = bufnr })
+  if ft and ft ~= "" then
+    local lang = vim.treesitter.language.get_lang(ft)
+    if not pcall(vim.treesitter.start, bufnr, lang) then
+      vim.bo[bufnr].syntax = ft
+    else
+    end
+  end
+
+  -- Replace the scratch buffer with a real buffer if we enter it
+  vim.api.nvim_create_autocmd("BufEnter", {
+    desc = "oil.nvim replace scratch buffer with real buffer",
+    buffer = bufnr,
+    callback = function()
+      local winid = vim.api.nvim_get_current_win()
+      -- Have to schedule this so all the FileType, etc autocmds will fire
+      vim.schedule(function()
+        if vim.api.nvim_get_current_win() == winid then
+          vim.cmd.edit({ args = { path } })
+
+          -- If we're still in a preview window, make sure this buffer still gets treated as a
+          -- preview
+          if vim.wo.previewwindow then
+            vim.bo.bufhidden = "wipe"
+            vim.b.oil_preview_buffer = true
+          end
+        end
+      end)
+    end,
+  })
+
+  return bufnr
+end
+
 return M