`try bikeshed`: What should the syntax be?

(Sub-issue of #149488)

Obviously bikeshed shouldn't literally be the syntax 🙃

Colon

Obviously the default choice for introducing types is a colon, like in let, leading to something like ``try : anyhow::Result<_> {`

However, the core trouble here is that let x: _ = and let x = are always the same, whereas try { and try: _ { would be different, as the former is homogeneous (https://github.com/rust-lang/rfcs/pull/3721) and the latter heterogeneous. That inconsistency isn't great.

Arrow

Another option would be the closure-like try -> anyhow::Result<_> {, similar to || -> anyhow::Result<_> {.

At least to me that looks aesthetically nicer than the colon, but it has a similar problem: || { … } and || -> _ { … } are the same, so it doesn't give the "and there's a conversion of some sort" happening intuition.

As

Niko originally hypothesized this feature in the form of try as anyhow::Result<_> { … } back in https://github.com/rust-lang/rust/issues/70941#issuecomment-612167041

That's nice because it's already the case that x and x as _ are different -- the latter introduces a conversion of sorts, and as _ as _ never type-checks -- which gives a similar intuition to how try { … } and try as _ { … } would be different. Plus try as _ { try as _ { … }? } also doesn't compile for the same reason that as _ as _ doesn't compile.

Other words

Because try is the keyword and the other form is try {, anything that's not { would work -- it doesn't need to be a keyword. (This is why bikeshed is working on nightly.) So we're not limited to existing reserved words (like in or whatever) if you have a good idea.

Start with the parent issue #149488, then compare the colon, arrow, as, and other syntax proposals described here, including RFC #3721 and the earlier discussion in #70941. Done means the project has chosen and documented a syntax for distinguishing homogeneous and heterogeneous try blocks, with the relevant examples and parsing or type-checking implications resolved.