Add --migrate flag to mix format (#13846)

Author: sabiwara

lib/elixir/src/elixir_bitstring.erl

@@ -403,10 +403,13 @@ format_error({undefined_bittype, Expr}) ->
   io_lib:format("unknown bitstring specifier: ~ts", ['Elixir.Macro':to_string(Expr)]);
 format_error({unknown_bittype, Name}) ->
   io_lib:format("bitstring specifier \"~ts\" does not exist and is being expanded to \"~ts()\","
-                " please use parentheses to remove the ambiguity", [Name, Name]);
+                " please use parentheses to remove the ambiguity.\n"
+                "You may run \"mix format --migrate\" to fix this warning automatically.", [Name, Name]);
 format_error({parens_bittype, Name}) ->
     io_lib:format("extra parentheses on a bitstring specifier \"~ts()\" have been deprecated. "
-    "Please remove the parentheses: \"~ts\"",
+    "Please remove the parentheses: \"~ts\".\n"
+    "You may run \"mix format --migrate\" to fix this warning automatically."
+    ,
     [Name, Name]);
 format_error({bittype_mismatch, Val1, Val2, Where}) ->
   io_lib:format("conflicting ~ts specification for bit field: \"~p\" and \"~p\"", [Where, Val1, Val2]);

lib/elixir/src/elixir_tokenizer.erl

@@ -267,7 +267,10 @@ tokenize([$" | T], Line, Column, Scope, Tokens) ->
 
 %% TODO: Remove me in Elixir v2.0
 tokenize([$' | T], Line, Column, Scope, Tokens) ->
-  NewScope = prepend_warning(Line, Column, "single-quoted strings represent charlists. Use ~c\"\" if you indeed want a charlist or use \"\" instead", Scope),
+  Message = "single-quoted strings represent charlists. "
+    "Use ~c\"\" if you indeed want a charlist or use \"\" instead.\n"
+    "You may run \"mix format --migrate\" to fix this warning automatically.",
+  NewScope = prepend_warning(Line, Column, Message, Scope),
   handle_strings(T, Line, Column + 1, $', NewScope, Tokens);
 
 % Operator atoms

lib/mix/lib/mix/tasks/format.ex

@@ -78,6 +78,14 @@ defmodule Mix.Tasks.Format do
       as `.heex`. Without passing this flag, it is assumed that the code being
       passed via stdin is valid Elixir code. Defaults to "stdin.exs".
 
+    * `--migrate` - enables the `:migrate` option, which should be able to
+      automatically fix some deprecation warnings but is changing the AST.
+      This should be safe in typical projects, but there is a non-zero risk
+      of breaking code for meta-programming heavy projects that relied on a
+      specific AST, or projects that are re-defining functions from the `Kernel`.
+      See the "Migration formatting" section in `Code.format_string!/2` for
+      more information.
+
   ## When to format code
 
   We recommend developers to format code directly in their editors, either
@@ -184,7 +192,8 @@ defmodule Mix.Tasks.Format do
     dot_formatter: :string,
     dry_run: :boolean,
     stdin_filename: :string,
-    force: :boolean
+    force: :boolean,
+    migrate: :boolean
   ]
 
   @manifest_timestamp "format_timestamp"
@@ -362,17 +371,21 @@ defmodule Mix.Tasks.Format do
   end
 
   defp eval_dot_formatter(cwd, opts) do
-    cond do
-      dot_formatter = opts[:dot_formatter] ->
-        {dot_formatter, eval_file_with_keyword_list(dot_formatter)}
+    {dot_formatter, format_opts} =
+      cond do
+        dot_formatter = opts[:dot_formatter] ->
+          {dot_formatter, eval_file_with_keyword_list(dot_formatter)}
 
-      File.regular?(Path.join(cwd, ".formatter.exs")) ->
-        dot_formatter = Path.join(cwd, ".formatter.exs")
-        {".formatter.exs", eval_file_with_keyword_list(dot_formatter)}
+        File.regular?(Path.join(cwd, ".formatter.exs")) ->
+          dot_formatter = Path.join(cwd, ".formatter.exs")
+          {".formatter.exs", eval_file_with_keyword_list(dot_formatter)}
 
-      true ->
-        {".formatter.exs", []}
-    end
+        true ->
+          {".formatter.exs", []}
+      end
+
+    # the --migrate flag overrides settings from the dot formatter
+    {dot_formatter, Keyword.take(opts, [:migrate]) ++ format_opts}
   end
 
   # This function reads exported configuration from the imported

lib/mix/test/mix/tasks/format_test.exs

@@ -534,6 +534,18 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  test "respects the --migrate flag", context do
+    in_tmp(context.test, fn ->
+      File.write!("a.ex", "unless foo, do: 'bar'\n")
+
+      Mix.Tasks.Format.run(["a.ex"])
+      assert File.read!("a.ex") == "unless foo, do: 'bar'\n"
+
+      Mix.Tasks.Format.run(["a.ex", "--migrate"])
+      assert File.read!("a.ex") == "if !foo, do: ~c\"bar\"\n"
+    end)
+  end
+
   test "uses inputs and configuration from --dot-formatter", context do
     in_tmp(context.test, fn ->
       File.write!("custom_formatter.exs", """