fix: fall back to /tmp for buffers without a backing file (#60)

* fix: fall back to /tmp for buffers without a backing file

Problem: markdown and gfm presets fail when the buffer has no file on
disk (e.g. unnamed buffer with `ft=markdown`, or a named buffer whose
path doesn't exist yet) because `build_context` passes a nonexistent
path to pandoc and `compile` guards reject empty buffer names.

Solution: `build_context` now detects missing files and redirects
`ctx.file` to `/tmp/{bufnr}-{name}`. `compile` writes buffer contents
to that temp path via `vim.fn.writefile` instead of `:silent! update`.

* test: update unnamed buffer tests for tmpfile fallback

Problem: the unnamed buffer guard tests expected a "no file name"
warning that no longer exists after the tmpfile fallback change.

Solution: update assertions to expect the downstream messages that
unnamed buffers now reach ("no provider configured", "no output file").

lua/preview/compiler.lua

@@ -215,8 +215,14 @@ function M.compile(bufnr, name, provider, ctx, opts)
     return
   end
 
-  if vim.bo[bufnr].modified then
-    vim.cmd('silent! update')
+  local buf_file = vim.api.nvim_buf_get_name(bufnr)
+  if buf_file ~= '' and buf_file == ctx.file then
+    if vim.bo[bufnr].modified then
+      vim.cmd('silent! update')
+    end
+  else
+    local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+    vim.fn.writefile(lines, ctx.file)
   end
 
   local s = get_state(bufnr)

lua/preview/init.lua

@@ -139,27 +139,49 @@ function M.resolve_provider(bufnr)
   return ft
 end
 
+local ft_ext = {
+  markdown = 'md',
+  typst = 'typ',
+  tex = 'tex',
+  asciidoc = 'adoc',
+  plantuml = 'puml',
+  mermaid = 'mmd',
+  quarto = 'qmd',
+}
+
 ---@param bufnr? integer
 ---@return preview.Context
 function M.build_context(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
   local file = vim.api.nvim_buf_get_name(bufnr)
-  local root = vim.fs.root(bufnr, { '.git' }) or vim.fn.fnamemodify(file, ':h')
+  local ft = vim.bo[bufnr].filetype
+  local root
+
+  if file ~= '' and vim.uv.fs_stat(file) then
+    root = vim.fs.root(bufnr, { '.git' }) or vim.fn.fnamemodify(file, ':h')
+  else
+    local basename
+    if file ~= '' then
+      basename = string.format('%d-%s', bufnr, vim.fn.fnamemodify(file, ':t'))
+    else
+      local ext = ft_ext[ft] or ft
+      basename = string.format('preview-%d.%s', bufnr, ext)
+    end
+    file = '/tmp/' .. basename
+    root = '/tmp'
+  end
+
   return {
     bufnr = bufnr,
     file = file,
     root = root,
-    ft = vim.bo[bufnr].filetype,
+    ft = ft,
   }
 end
 
 ---@param bufnr? integer
 function M.compile(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
-  if vim.api.nvim_buf_get_name(bufnr) == '' then
-    vim.notify('[preview.nvim]: buffer has no file name', vim.log.levels.WARN)
-    return
-  end
   local name = M.resolve_provider(bufnr)
   if not name then
     vim.notify('[preview.nvim]: no provider configured for this filetype', vim.log.levels.WARN)
@@ -179,10 +201,6 @@ end
 ---@param bufnr? integer
 function M.clean(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
-  if vim.api.nvim_buf_get_name(bufnr) == '' then
-    vim.notify('[preview.nvim]: buffer has no file name', vim.log.levels.WARN)
-    return
-  end
   local name = M.resolve_provider(bufnr)
   if not name then
     vim.notify('[preview.nvim]: no provider configured for this filetype', vim.log.levels.WARN)
@@ -196,10 +214,6 @@ end
 ---@param bufnr? integer
 function M.toggle(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
-  if vim.api.nvim_buf_get_name(bufnr) == '' then
-    vim.notify('[preview.nvim]: buffer has no file name', vim.log.levels.WARN)
-    return
-  end
   local name = M.resolve_provider(bufnr)
   if not name then
     vim.notify('[preview.nvim]: no provider configured for this filetype', vim.log.levels.WARN)
@@ -212,10 +226,6 @@ end
 ---@param bufnr? integer
 function M.open(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
-  if vim.api.nvim_buf_get_name(bufnr) == '' then
-    vim.notify('[preview.nvim]: buffer has no file name', vim.log.levels.WARN)
-    return
-  end
   local name = M.resolve_provider(bufnr)
   local open_config = name and config.providers[name] and config.providers[name].open
   if not compiler.open(bufnr, open_config) then

spec/init_spec.lua

@@ -126,43 +126,43 @@ describe('preview', function()
       return msg
     end
 
-    it('compile warns on unnamed buffer', function()
-      local bufnr = helpers.create_buffer({}, 'typst')
+    it('compile falls through to provider check on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'lua')
       local msg = capture_notify(function()
         preview.compile(bufnr)
       end)
       assert.is_not_nil(msg)
-      assert.is_truthy(msg:find('no file name'))
+      assert.is_truthy(msg:find('no provider configured'))
       helpers.delete_buffer(bufnr)
     end)
 
-    it('toggle warns on unnamed buffer', function()
-      local bufnr = helpers.create_buffer({}, 'typst')
+    it('toggle falls through to provider check on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'lua')
       local msg = capture_notify(function()
         preview.toggle(bufnr)
       end)
       assert.is_not_nil(msg)
-      assert.is_truthy(msg:find('no file name'))
+      assert.is_truthy(msg:find('no provider configured'))
       helpers.delete_buffer(bufnr)
     end)
 
-    it('clean warns on unnamed buffer', function()
-      local bufnr = helpers.create_buffer({}, 'typst')
+    it('clean falls through to provider check on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'lua')
       local msg = capture_notify(function()
         preview.clean(bufnr)
       end)
       assert.is_not_nil(msg)
-      assert.is_truthy(msg:find('no file name'))
+      assert.is_truthy(msg:find('no provider configured'))
       helpers.delete_buffer(bufnr)
     end)
 
-    it('open warns on unnamed buffer', function()
-      local bufnr = helpers.create_buffer({}, 'typst')
+    it('open warns no output on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'lua')
       local msg = capture_notify(function()
         preview.open(bufnr)
       end)
       assert.is_not_nil(msg)
-      assert.is_truthy(msg:find('no file name'))
+      assert.is_truthy(msg:find('no output file'))
       helpers.delete_buffer(bufnr)
     end)
   end)