Mix format on map keys nil/true/false mixed with non-atom keys

[hypno2000]

Environment

• Elixir & Erlang/OTP versions (elixir --version):
  Erlang/OTP 22 [erts-10.6.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]
  Elixir 1.10.0 (compiled with Erlang/OTP 22)

• Operating system:
  MacOS Catalina, 10.15.2

Current behavior

mix format will transform

%{
  :nil => "foo",
  :false => "foo",
  :true => "foo",
  "bar" => "foo"
}

to:

%{
  nil => "foo",
  false => "foo",
  true => "foo",
  "bar" => "foo"
}

and when running mix format again it will be formatted to:

%{
  nil: "foo",
  false: "foo",
  true: "foo",
  "bar" => "foo"
}

The code above will not compile anymore: syntax error before: "bar"

Expected behavior

1. Mix format should output final format on first run
2. The output of mix format should compile

[dhedlund]

A git bisect with a failing test suggests the issue was introduced by the "Add literal encoder option to Code.string_to_quoted" (f38895c) commit.

[josevalim]

Thanks @dhedlund for the bisect. It was very helpful! I will push a fix soon!