"Manual" crater run for edition migration

[nikomatsakis]

Before doing the public test, we should do a manual crater run. Here are the crates.io top-downloaded crates:

• rand
• syn
• libc
• rand_core
• quote
• unicode-xid
• proc-macro2
• cfg-if
• serde
• bitflags

[rylev]

Here is a running list of issues found in manual crater testing the edition migrations:

• ICE when running cargo +nightly fix --edition on tokio #86753
• Failure to update dyn usage in Rust 2018 to 2021 transition #86751

[rylev]

If you'd like to run this "mini" crater yourself, please take a look at my absolutely horrendous Ruby code here.

[rylev]

Closing this now as we've done the run and found some issues. I think we're unlikely to find more issues in this controlled path, and we should focus on the full crater run and public testing.