Support importing path-segment keyword with renaming

[mu001999]

Reference PR:

• Clarify use $crate restriction reference#2010

Description:

• Make imports consistent, self/super/crate/$crate can be imported with renaming now
  • Allow use self/super/crate/$crate as name; and also use super/crate/$crate::{self as name};
  • Deny use self/super/crate/$crate; and also use super/crate/$crate::{self};
  • Deny use ::self;, use ::{self};, use ::self as name; and use ::{self as name};
• Proper error messages, and E0430 and E0431 are no longer emitted now
  • [E0430]: self import can only appear once in an import list was emitted before when std::fmt::{self, self}, but the emitted [E0252]: the name fmt is defined multiple times is good enough
  • [E0431]: self import can only appear in an import list with a non-empty prefix was emiited before when use {self as name};, now use {self as name}; is allowed; for use {self};, it's denied because it lacks renaming and will emit imports need to be explicitly named with suggesting renaming.

Fixes #146967

r? petrochenkov

[rustbot]

Failed to set assignee to [petrochenkov]: invalid assignee

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[mu001999]

@rustbot ready

[petrochenkov]

This is not good, super is only supposed to be used in start position, or after another super.
super, super::super, super::super::super - ok, foobar::super - not ok, self::super - not ok even if accepted on stable.

It does have a reasonable meaning (technically we can support self and super in all positions in paths), but relaxing the rules here is probably a different issue than what this PR tries to address.

[petrochenkov]

I've focused on the new tests for now.
Could you extend them as commented, and submit as a separate PR, to make a snapshot of the current compiler's behavior, before making changes.
(It won't require any lang team process unlike this PR.)
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[mu001999]

@rustbot ready