internal error when translating Ast to Hir

[conradludgate]

What version of regex are you using?

regex-syntax 0.7.4

Describe the bug at a high level.

There are Asts that can be created that cause internal errors to be panicked when being translated into Hir.

These Ast's are nonsense, but can be generated using the Arbitrary integration.

What are the steps to reproduce the behavior?

use regex_syntax::{ast::{Span, Position, Ast, Alternation, Concat}, hir::translate::Translator};

let span = Span::splat(Position::new(0, 0, 0));
let ast = Ast::Alternation(Alternation {
    span,
    asts: vec![Ast::Concat(Concat { span, asts: vec![] })],
});

let mut t = Translator::new();
let hir_result = t.translate("", &ast);

What is the actual behavior?

panicked at 'internal error: entered unreachable code: expected expr or concat, got alt branch marker', /Users/conrad/.cargo/registry/src/index.crates.io-6f17d22bba15001f/regex-syntax-0.7.4/src/hir/translate.rs:769:17

What is the expected behavior?

Return an error case from translate