Band-Aid: fix one of the left recursion bugs (fixes #1047)

[Tartasprint]

This is a band-aid. Comments in the code indicate that left recursion validation has probably other edge cases/bugs.

This fixes #1047 and successfully detects the left-recursion. Here is the output message when running cargo build:

error: proc-macro derive panicked
 --> src\main.rs:4:10
  |
4 | #[derive(Parser)]
  |          ^^^^^^
  |
  = help: message: grammar error

           --> 1:19
            |
          1 | expr = { !(EOI) ~ expr ~ "infix" ~ expr }
            |                   ^--^
            |
            = rule expr is left-recursive (expr -> expr); pest::pratt_parser might be useful in this case

Summary by CodeRabbit

• New Features

  • Enhanced validation for parser rules to better detect potential left recursion issues, improving parsing reliability.

• Bug Fixes

  • Refined logic in the validation process to catch more cases of infinite repetitions or non-terminating behavior in grammar rules.

[coderabbitai]

Walkthrough

The changes involve an update to the validate_left_recursion function in the meta/src/validator.rs file. The update refines the logic for detecting left recursion in parser rules by adding a check for non-progressing expressions alongside the existing non-failing check. This modification is intended to improve the detection of potential left recursion issues, which can lead to infinite parsing loops.

Changes

File	Change Summary
meta/src/validator.rs	Modified the logic in validate_left_recursion to include a check for non-progressing expressions in addition to non-failing expressions.

Assessment against linked issues

Objective	Addressed	Explanation
Improve detection of left recursion to prevent stack overflow (#1047)	✅

Suggested reviewers

• tomtau

Poem

In fields of code where bunnies play,
We fixed the rules, come what may.
No more loops that spin and spin,
With logic sharp, let parsing begin! 🐇✨
Hopping forward, we’ve made it right,
A grammar’s dance, a joyful sight!

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📥 Commits

Files that changed from the base of the PR and between c93aae8 and 3ae5eba.

📒 Files selected for processing (1)

• meta/src/validator.rs (2 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• meta/src/validator.rs

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[tomtau]

LGTM; could you add a simple test case that captures that original issue?

MSRV failing seems unrelated to this PR (it's just that it failed to fetch cargo-msrv's binary for some reason)

[tomtau]

Thanks 🙏