Consider changing the way unevaluated constants are printed

[oli-obk]

cc #60742 (comment)

Right now when we encounter a ConstValue::Unevaluated during pretty printing, we fall back to printing the constant's source code or _ if that also fails. Alternative options are:

• try to eagerly evaluate (and fall back to one of the following)
• always print _
• print something like {{unevaluated: {}}} where {} is the source code
• print just {unevaluated} (seems useless for users, so maybe not a good idea)

[varkor]

I think it would be ideal if we could print the unevaluated expression itself. (Though we might have to point out specifically that it's unevaluated in diagnostics if this proves confusing.)

[oli-obk]

that's what we are doing right now, but you end up with errors like expected 3 got 3usize. If we eagerly evaluated we'd even get expected 3usize got 3usize

These errors may just disappear if we manage to normalize before doing whatever causes the error.

[varkor]

Ah, I was specifically referring to the case when _ is printed at the moment, which has been encountered quite frequently with const generics.

I agree the cases when it does print the expression, and they look the same as what's expected is confusing, but these seem more indicative of existing issues, like normalisation, than printing. Maybe the diagnostics, rather than the pretty printing, could be adjusted for now to address this.