Disallow shorthand pipe after matches (#11433)

Closes #11418.

4 files changed, 61 insertions, 14 deletions

diff --git a/lib/iex/lib/iex.ex b/lib/iex/lib/iex.ex
index 47771c2b6..f49131809 100644
--- a/lib/iex/lib/iex.ex
+++ b/lib/iex/lib/iex.ex
@@ -136,6 +136,16 @@ defmodule IEx do
       iex(1)> |> List.flatten()
       ** (RuntimeError) v(-1) is out of bounds
 
+  If the previous expression was a match operation, the pipe operator will also
+  fail, to prevent an unsolicited break of the match:
+
+      iex(1)> x = 42
+      iex(2)> |> IO.puts()
+      ** (SyntaxError) iex:2:1: pipe shorthand is not allowed immediately after a match expression in IEx. To make it work, surround the whole pipeline with parentheses ('|>')
+          |
+        2 | |> IO.puts()
+          | ^
+
   Note however the above does not work for `+/2` and `-/2`, as they
   are ambiguous with the unary `+/1` and `-/1`:
 
diff --git a/lib/iex/lib/iex/evaluator.ex b/lib/iex/lib/iex/evaluator.ex
index 9c8d36ae8..cf4dcc0f8 100644
--- a/lib/iex/lib/iex/evaluator.ex
+++ b/lib/iex/lib/iex/evaluator.ex
@@ -62,13 +62,15 @@ defmodule IEx.Evaluator do
                [:three_op, :concat_op, :mult_op]
 
   @doc false
-  def parse(input, opts, buffer)
+  def parse(input, opts, parser_state)
 
-  def parse(@break_trigger, _opts, "") do
-    {:incomplete, ""}
+  def parse(input, opts, ""), do: parse(input, opts, {"", :other})
+
+  def parse(@break_trigger, _opts, {"", _} = parser_state) do
+    {:incomplete, parser_state}
   end
 
-  def parse(@break_trigger, opts, _buffer) do
+  def parse(@break_trigger, opts, _parser_state) do
     :elixir_errors.parse_error(
       [line: opts[:line]],
       opts[:file],
@@ -78,7 +80,7 @@ defmodule IEx.Evaluator do
     )
   end
 
-  def parse(input, opts, buffer) do
+  def parse(input, opts, {buffer, last_op}) do
     input = buffer <> input
     file = Keyword.get(opts, :file, "nofile")
     line = Keyword.get(opts, :line, 1)
@@ -86,16 +88,24 @@ defmodule IEx.Evaluator do
     charlist = String.to_charlist(input)
 
     result =
-      with {:ok, tokens} <- :elixir.string_to_tokens(charlist, line, column, file, opts) do
-        :elixir.tokens_to_quoted(adjust_operator(tokens, line, column, file, opts), file, opts)
+      with {:ok, tokens} <- :elixir.string_to_tokens(charlist, line, column, file, opts),
+           {:ok, adjusted_tokens} <- adjust_operator(tokens, line, column, file, opts, last_op),
+           {:ok, forms} <- :elixir.tokens_to_quoted(adjusted_tokens, file, opts) do
+        last_op =
+          case forms do
+            {:=, _, [_, _]} -> :match
+            _ -> :other
+          end
+
+        {:ok, forms, last_op}
       end
 
     case result do
-      {:ok, forms} ->
-        {:ok, forms, ""}
+      {:ok, forms, last_op} ->
+        {:ok, forms, {"", last_op}}
 
       {:error, {_, _, ""}} ->
-        {:incomplete, input}
+        {:incomplete, {input, last_op}}
 
       {:error, {location, error, token}} ->
         :elixir_errors.parse_error(
@@ -108,13 +118,21 @@ defmodule IEx.Evaluator do
     end
   end
 
-  defp adjust_operator([{op_type, _, _} | _] = tokens, line, column, file, opts)
+  defp adjust_operator([{op_type, _, token} | _] = _tokens, line, column, _file, _opts, :match)
+       when op_type in @op_tokens,
+       do:
+         {:error,
+          {[line: line, column: column],
+           "pipe shorthand is not allowed immediately after a match expression in IEx. To make it work, surround the whole pipeline with parentheses ",
+           "'#{token}'"}}
+
+  defp adjust_operator([{op_type, _, _} | _] = tokens, line, column, file, opts, _last_op)
        when op_type in @op_tokens do
     {:ok, prefix} = :elixir.string_to_tokens('v(-1)', line, column, file, opts)
-    prefix ++ tokens
+    {:ok, prefix ++ tokens}
   end
 
-  defp adjust_operator(tokens, _line, _column, _file, _opts), do: tokens
+  defp adjust_operator(tokens, _line, _column, _file, _opts, _last_op), do: {:ok, tokens}
 
   @doc """
   Gets a value out of the binding, using the provided
diff --git a/lib/iex/lib/iex/server.ex b/lib/iex/lib/iex/server.ex
index 5e4fe2c1a..b44ac8444 100644
--- a/lib/iex/lib/iex/server.ex
+++ b/lib/iex/lib/iex/server.ex
@@ -9,7 +9,15 @@ defmodule IEx.State do
             evaluator_options: [],
             previous_state: nil
 
-  @type t :: %__MODULE__{}
+  @type t :: %{
+          __struct__: __MODULE__,
+          parser_state: binary(),
+          counter: pos_integer(),
+          prefix: binary(),
+          on_eof: :stop_evaluator | :halt,
+          evaluator_options: keyword(),
+          previous_state: nil | t()
+        }
 end
 
 defmodule IEx.Server do
diff --git a/lib/iex/test/iex/helpers_test.exs b/lib/iex/test/iex/helpers_test.exs
index ac8f52dab..e7af366ae 100644
--- a/lib/iex/test/iex/helpers_test.exs
+++ b/lib/iex/test/iex/helpers_test.exs
@@ -1127,6 +1127,17 @@ defmodule IEx.HelpersTest do
       assert capture_iex("[42]\n++ [24]\n|> IO.inspect(label: \"foo\")") =~ "foo: [42, 24]"
       assert capture_iex("|> IO.puts()") =~ "(RuntimeError) v(-1) is out of bounds"
     end
+
+    test "raises if previous expression was a match" do
+      assert capture_iex("x = 42\n|> IO.puts()") =~
+               "surround the whole pipeline with parentheses '|>'"
+
+      assert capture_iex("%{x: x} = %{x: 42}\n|> IO.puts()") =~
+               "surround the whole pipeline with parentheses '|>'"
+
+      assert capture_iex("%{x: x} = map = %{x: 42}\n|> IO.puts()") =~
+               "surround the whole pipeline with parentheses '|>'"
+    end
   end
 
   describe "c" do