Resolve inline Clippy lints (comment explaining why it is OK or code changes)

[ozghimire]

As part of resolving issue #1363 , codebase needs a lot of inline allow clippy code. This is expected to touch all production code folders.

Solution : The resolution strategy is either a well written comment explaining why it's ok or necessary code change to remove inline allow.

[sffc]

@robertbastian will clean up low-hanging fruit, and create child issues for each component

[robertbastian]

These are all the remaining undocumented lints that I couldn't fix. Please fix them or tell me how to.

• [x] icu_calendar, @Manishearth
  • [x] Ethiopian::ethiopian_from_fixed
• [ ] icu_datetime, @zbraniecki ?
  • [ ] Parser::handle_generic_quoted_literal
  • [ ] Parser::collect_generic_segment
  • [ ] PatternPlurals::expect_pattern (looks like an intended panic)
  • [ ] skeleton::helpers::create_best_pattern_for_fields
  • [ ] skeleton::helpers::get_best_available_format_pattern (x2)
• [ ] icu_plurals, @zbraniecki ?
  • [ ] Lexer::advance_token
  • [ ] Rule
  • [ ] From<&Value> for u32
  • [ ] internal::to_icu4x_operands
• [x] fixed_decimal, @younies
  • [x] Sign
  • [x] FromStr for FixedDecimal (x3)
  • [x] DoublePrecision
• [x] zerovec, @Manishearth, @sffc
  • [ ] #1410

[Manishearth]

zerovec and calendars in #2435

[sffc]

@zbraniecki Assigning this to you since the remaining work is all in components you own.

[zbraniecki]

Why do we require comment to be added to panic!() which has content string?

It feels weird to have:

panic!("We panic because X");

have to be turned to:

panic!("We panic because of X"); // We panic because of X

[Manishearth]

@zbraniecki to me, the panic message is for the user, and the comment is for us. If they're redundant it feels fine to not comment it IMO.

[sffc]

@zbraniecki I've raised that concern before (also for expect) but the consensus of the group has been to require suppressions for all four (unwrap, expect, panic, and indexing_slicing).

[sffc]

For example, the following is the style we're going for: the comment and the panic message are disjoint

/// # Panics
/// Panics if idx is not in bounds
fn get(idx: usize) {
  if idx >= self.len() {
    #[allow(clippy::panic)] // documented
    panic!("index must be in bounds");
  }
  // ...
}