[DRAFT] Change Symbol::as_str() to &self -> &str.

[nnethercote]

This is based on an idea from @oli-obk. The code is incomplete for the moment, and I won't have time to work on it further, but somebody else might be interested in taking it over.

Pros:

• &str is nicer to work with than SymbolStr, e.g. avoids the need for some & and &* occurrences.
• It would be nice to eliminate SymbolStr.
• It avoids the lie that SymbolStr strings have a static lifetime, replacing it with a smaller lie that the strings have a lifetime tied to the lifetime of the input Symbol.

Cons:

• That lifetime is still a lie, and it's possible to cause crashes with this.
• The lie is less obvious in a vanilla &str than it is within a SymbolStr.
• We can't mark &str as !Send and !Sync.
• I haven't yet eliminated SymbolStr entirely. The to_stable_hash_key() is the hardest case.

I'm totally ambivalent about whether this should land, but it seemed worth putting up here for others to look at.

r? @ghost

[petrochenkov]

We can't mark &str as !Send and !Sync.

IIRC, this is why SymbolStr (or what it was called previously) was introduced in the first place, because Zoxc did it together with the work on compiler parallelization.

[bors]

☔ The latest upstream changes (presumably #76574) made this pull request unmergeable. Please resolve the merge conflicts.

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[Dylan-DPC-zz]

Closing this as I don't think it's being worked on at the moment