Assert well-formedness of spans

[estebank]

rustc currently has peppered throughout the codebase some checks for "backwards spans", spans where their end is earlier than their start. I believe that the current approach of actively checking for this and not fail is very reasonable in beta and stable rustc, but in nightly builds I would like to have an assertion so that we expose this incorrectly formatted spans so that the underlying cause can be fixed.

This might take the form of a simple assert and ICEing (not my preference, but it would certainly bring attention to the problem) to generating a diagnostic error and emitting it, allowing the compiler to continue working, but making the problem visible to anyone using the nightly compiler, with text prompting a report in this issue tracker. If we go down the later route, it probably should be a warning, so that anyone using the nightly compiler in production isn't stopped from using it due to a rustc bug.

[mfaqiri]

@rustbot claim

[ammarbinfaisal]

I would like to work on this.
Should the assertion be in functions returning a Span or even in functions accepting Spans as arguments?

[quinnwerks]

@rustbot claim

[Noratrieb]

Also relevant: #106192

if someone wants to pick that up, that would be very welcome

[ariscript]

@rustbot claim

I want to work on this, but I'll need some pointers on how to approach it.

[Noratrieb]

open a thread on zulip in t-compiler/help (link)

[nnethercote]

I remove the E-easy tag, because #122418 shows that this is harder than it first appears.