Rollup of 8 pull requests

[frewsxcv]

• Successful merges: Change how the 0 flag works in format! #40241, Rename TryFrom's associated type and implement str::parse using TryFrom. #40281, add test for nested macro def (#31946) #40523, macros: improve the TokenStream quoter #40532, Implement std::error::Error for !. #40566, Fix invalid debug display for associated consts #40587, Update gcc used for dist-x86-linux builds #40617, Replace hardcoded forward slash with path::MAIN_SEPARATOR #40620
• Failed merges:

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

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[frewsxcv]

@bors r+ p=10

[bors]

📌 Commit f39793a has been approved by frewsxcv

[bors]

⌛ Testing commit f39793a with merge cf61542...

[bors]

💔 Test failed - status-appveyor