Many repeated expressions cause stack overflow in parser

[endofunky]

Repeated quotes cause stack overflow in lexer

Good day, while fuzzing the compiler with honggfuzz for a competition I came across he following stack overflow in the lexer caused by large amounts of repeated quotation marks.

A (truncated) example:

fn f() {
  """"""""""""""""""""""""""" // <lots more of these...>
}

Full input file:

input.txt

Steps to reproduce:

1. Create a new project
2. Replace the src/<filename>.gleam with the example file attached
3. Run gleam build:

% gleam build
  Compiling fff

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
zsh: abort (core dumped)  gleam build

Stack trace from gdb:

 ► f 0   0x555555b7e04f _ZN10gleam_core5parse5lexer14Lexer$LT$T$GT$10inner_next17hb729e48a11fd24a8E.llvm.1354553687974732476+143
   f 1   0x555555c0ef6d gleam_core::parse::Parser<T>::next_tok+205
   f 2   0x555555bfa2aa gleam_core::parse::Parser<T>::parse_expression_unit+1226
   f 3   0x555555bf98d8 gleam_core::parse::Parser<T>::parse_expression+120
   f 4   0x555555bfe979 gleam_core::parse::Parser<T>::parse_expression_seq+265
   f 5   0x555555bfecfd gleam_core::parse::Parser<T>::parse_expression_seq+1165
   f 6   0x555555bfecfd gleam_core::parse::Parser<T>::parse_expression_seq+1165
   f 7   0x555555bfecfd gleam_core::parse::Parser<T>::parse_expression_seq+1165

I'm not sure why gdb didn't demangle the current frame, but that's gleam_core::parse::lexer::Lexer<T>::inner_next.

I have not investigated further.

[lpil]

Thank you. Let's fix this to return a lex error rather than crashing

[lpil]

After a bit of exploration I don't think this is the lexer that is the problem, it is that the parser's parse_expression_seq recursively calls itself, causing any block or function with a very large number of statements to crash the parser.

A test to replicate

// https://github.com/gleam-lang/gleam/issues/1707
#[test]
fn many_quotes_does_not_crash_the_lexer() {
    assert!(crate::parse::parse_module(&format!(
        "pub fn main() {{
  {}
}}",
        "Nil ".repeat(1024)
    ))
    .is_err());
}

This is fiddly to resolve currently with how we represent Try in the AST. This may get removed in future with #1709 at which point this code can be simplified