[beta] ICE fixes for newly stabilized features

[sinkuu]

(Do not merge yet - #49830 is still waiting to be merged into master.)

This PR beta-backports PRs for fixing ICEs caused by the features that will be stabilized in the next release.

• Fix ICE with main's return type containing lifetimes #49692: fn main() -> Result<(), Box<std::error::Error>> causes ICE
• Fix ICE with impl Trait #49730: under some circumstances, returning a closure from -> impl Trait functions causes ICE
• Fix ICE by disallowing impl Trait in unsupported position #49830: placing impl Trait in invalid positions causes ICE

[rust-highfive]

r? @pnkfelix

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[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[TimNN]

@nikomatsakis / @cramertj: You beta-nominated the PR's backported here, have they already been accepted or are they still waiting on a decision on whether to actually backport?

[sinkuu]

Superseded by 50127.