trailing_fragment option not working as expected in case stubs

[lukaszsamson]

Elixir and Erlang/OTP versions

Erlang/OTP 26 [erts-14.2.5.4] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.18.1 (compiled with Erlang/OTP 26)

Operating system

any

Current behavior

This code (where | represents the cursor)

defmodule MyModule do
  def my_fun(a, b) do
    case a do
      ^b -> b
      %{b: ^b} = |a -> b
    end
  end
end

The cursor AST element lands on the wrong side of the second -> operator and the first one is transformed into a block as if the code was

defmodule MyModule do
  def my_fun(a, b) do
    case a do
      ^b ->
        b
        %{b: ^b} = __cursor__()
    end
  end
end

Code.Fragment.container_cursor_to_quoted("defmodule MyModule do\n def my_fun(a, b) do\n  case a do\n   ^b -> b\n   %{b: ^b} = ", trailing_fragment: "a ->b\n  end\n end\nend")
{:ok,
 {:defmodule, [line: 1],
  [
    {:__aliases__, [line: 1], [:MyModule]},
    [
      do: {:def, [line: 2],
       [
         {:my_fun, [line: 2], [{:a, [line: 2], nil}, {:b, [line: 2], nil}]},
         [
           do: {:case, [line: 3],
            [
              {:a, [line: 3], nil},
              [
                do: [
                  {:->, [line: 4],
                   [
                     [{:^, [line: 4], [{:b, [line: 4], nil}]}],
                     {:__block__, [],
                      [
                        {:b, [line: 4], nil},
                        {:=, [line: 5],
                         [
                           {:%{}, [line: 5],
                            [b: {:^, [line: 5], [{:b, [line: 5], nil}]}]},
                           {:__cursor__, [line: 5], []}
                         ]}
                      ]}
                   ]}
                ]
              ]
            ]}
         ]
       ]}
    ]
  ]}}

trailing_fragment option does not seem to have any effect

Code.Fragment.container_cursor_to_quoted("defmodule MyModule do\n def my_fun(a, b) do\n  case a do\n   ^b -> b\n   %{b: ^b} = ")
{:ok,
 {:defmodule, [line: 1],
  [
    {:__aliases__, [line: 1], [:MyModule]},
    [
      do: {:def, [line: 2],
       [
         {:my_fun, [line: 2], [{:a, [line: 2], nil}, {:b, [line: 2], nil}]},
         [
           do: {:case, [line: 3],
            [
              {:a, [line: 3], nil},
              [
                do: [
                  {:->, [line: 4],
                   [
                     [{:^, [line: 4], [{:b, [line: 4], nil}]}],
                     {:__block__, [],
                      [
                        {:b, [line: 4], nil},
                        {:=, [line: 5],
                         [
                           {:%{}, [line: 5],
                            [b: {:^, [line: 5], [{:b, [line: 5], nil}]}]},
                           {:__cursor__, [line: 5], []}
                         ]}
                      ]}
                   ]}
                ]
              ]
            ]}
         ]
       ]}
    ]
  ]}}

Expected behavior

I'm aware that this case in ambiguous but given the trailing fragment container_cursor_to_quoted should be able to resolve it in favour of correct AST

[josevalim]

@lukaszsamson let me know if this is all good and I will do a new Elixir v1.18 release.

[lukaszsamson]

@josevalim I tried out 1.18 branch with ba945e9 cherry picked and it looks good