unexpected "impossible" exception in lexer

[ceedubs]

Describe and demonstrate the bug

I see this error in my terminal, though seems seem to continue to run fine. I think that it may be coming from an LSP thread similar to #4737.

 Uh oh, an unexpected exception brought the process down! That should never happen. Please file a bug report.

Here's a stringy rendering of the exception:

  impossible
  CallStack (from HasCallStack):
    error, called at src/Unison/Syntax/Lexer.hs:241:22 in unison-syntax-0.0.0-3wBkjZ3Vjv91L4a1yNKHwZ:Unison.Syntax.Lexer

I'm not exactly sure what caused this. But I think that it was something like this:

• add a type to the project and a function that matches on one of its constructors
• Bring the type and the function into your scratch file
• Add an argument to the constructor, and modify the function to use it. Save the file but don't update.
• Delete or comment out the updated type, but leave the function depending on the modified constructor in the scratch file.

I'm using LSP; not sure how relevant that is

Environment (please complete the following information):

• ucm --version a74d4e8
• OS/Architecture: x86 NixOS
• Editor: Neovim with lspconfig

[sellout]

Here’s the code at that commit:

unison/unison-syntax/src/Unison/Syntax/Lexer.hs

Lines 229 to 241 in a74d4e8

	pops :: Pos -> P [Token Lexeme]
	pops p = do
	env <- S.get
	let l = layout env
	if top l == column p && topContainsVirtualSemis l
	then pure [Token (Semi True) p p]
	else
	if column p > top l || topHasClosePair l
	then pure []
	else
	if column p < top l
	then S.put (env {layout = pop l}) >> ((Token Close p p :) <$> pops p)
	else error "impossible"

A first step might just be returning a parser failure rather than error (and maybe changing the description).