[error] Implement std::error::Error on errors

[joshlf]

While we're here, also relax Dispaly for AlignmentError<Src, Dst> to permit Dst: ?Sized in exchange for Dst: KnownLayout. This is an important relaxation since our APIs permit performing conversions into unsized destination types with runtime alignment checking.

Makes progress on #1297

[joshlf]

I added a SendSyncPhantomData type and tests to make sure that the errors are Send + Sync only dependent upon Src: Send + Sync, but regardless of Dst, which only exists at the type level.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.62791% with 23 lines in your changes missing coverage. Please review.

Project coverage is 87.31%. Comparing base (1bf5d4a) to head (fdde58e).

Files	Patch %	Lines
src/error.rs	87.95%	20 Missing ⚠️
src/util.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1298      +/-   ##
==========================================
- Coverage   87.42%   87.31%   -0.11%     
==========================================
  Files          15       15              
  Lines        5152     5289     +137     
==========================================
+ Hits         4504     4618     +114     
- Misses        648      671      +23     

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[jswrenn]

I went ahead and committed an example of how having a KnownLayout bound can be used to improve our diagnostics. Generally speaking, I think we should consistently have Src: Deref and Dst: ?Sized + KnownLayout on our Display and Error impls. We can always relax the bounds later.

I can't think, right now, of how it might be useful for ValidityError, but who knows — depending on how we evolve that trait, an opportunity might arise.

[joshlf]

Discussed offline with @jswrenn. Here's the plan:

• Debug impl is always verbose
• Conditional on debug_assertions:
  • On: Display impls are verbose (assume that this is in a non-prod context where there's no concern of data leak
  • Off: Display impls attempt to leak as little information as is reasonable
• Have some way of configuring this behavior (Permit configuring the verbosity of error type Display impls #1457); I can see two use cases:
  • Users enable debug_assertions in prod
  • Users want the output of Display to be stable regardless of debug_assertions
• Document this behavior at the crate root. Suggest that users who need a way of changing it chime in on Permit configuring the verbosity of error type Display impls #1457 so we know how much demand there is.