Promote Certain Lints to Errors in Rust 2021 Edition

[m-ou-se]

Tracking issue for turning lints into errors in Rust 2021.

• BARE_TRAIT_OBJECTS
• ELLIPSIS_INCLUSIVE_RANGE_PATTERNS

cc @rylev

[camelid]

• MACRO_USE_EXTERN_CRATE

I'm not sure if this one should be deny-by-default; this is what the lint's page says:

This lint is "allow" by default because this is a stylistic choice that has not been settled, see issue #52043 for more information.

[camelid]

Also, MACRO_USE_EXTERN_CRATE is not warn-by-default – it's actually allow-by-default.

[m-ou-se]

@rylev GitHub won't let me assign you to this issue; you wanted to work on this, right? I think you can @​rustbot claim to claim the issue.

[camelid]

@rustbot assign @rylev

[rustbot]

Error: Parsing assign command in comment failed: ...'sign rylev' | error: user should start with @ at >| ''...

Please let @rust-lang/release know if you're having trouble with this bot.

[camelid]

Hmm, I wonder why GitHub won't let us assign rylev directly – they are an org member...

[rylev]

@rustbot claim

[taiki-e]

• UNUSED_EXTERN_CRATES

• EXPLICIT_OUTLIVES_REQUIREMENTS
• UNREACHABLE_PUB
• MACRO_USE_EXTERN_CRATE

I think these lints belong to a similar category as unused lint. I think it probably makes sense to warn by default, but I don't think we need to deny it by default.

• ELLIPSIS_INCLUSIVE_RANGE_PATTERNS
• ELIDED_LIFETIMES_IN_PATHS

The other two (ELLIPSIS_INCLUSIVE_RANGE_PATTERNS and ELIDED_LIFETIMES_IN_PATHS) are related to compatibility or deprecation, and it seems reasonable to deny them in the next edition. (ELIDED_LIFETIMES_IN_PATHS is actually already rejected on async functions.)

[jyn514]

unreachable_pub still has bugs after three years: #64762. I don't think it should be even warn-by-default until the false positives are fixed.