add: optional-fn-return-type

[xonx4l]

What does this PR do?

This PR implements optional function return type as discussed in #393

e.g., closes #393

How does it work, what questions do you have?

Functions can now be written fn foo() { } instead of requiringfn foo() -> () { }. When the return type is omitted, it defaults to the unit type ().

Added tests demonstrating that fn run() {trusted} parses identically tofn run() -> () {trusted}.

AI disclosure

• I did not use any AI tools

[rustbot]

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[nikomatsakis]

Nice work! I have one suggested improvement

View changes since this review

[nikomatsakis]

Suggested change

	let rest: Vec<TokenStream> =
	chars.map(|c| quote!(__p.expect_char(#c)?;)).collect();
	(quote!(__p.expect_char(#first).is_ok()), quote!(#(#rest)*))
	(
	quote!(__p.expect_char(#first).is_ok()),
	quote!(#(__p.expect_char(#chars)?;)*)
	)

I think maybe maybe you can just do this?

[nikomatsakis]

Hmm, I am not sure if we want a default for Ty that is always unit. It's convenient here but it might not be correct elsewhere. Let's have a new type like

struct OutputTy { ty: Ty }

impl Upcast<Ty> for OutputTy { }

impl Default for OutputTy { }

and then change output_ty: OutputTy

[xonx4l]

Ok , will do . Thanks!