Uses of "do catch" suggest non-working "try" instead

[RalfJung]

I have code like this

    let action : Result<Action, Error> = do catch {
        let signed_action = base64::decode(form.signed_action.as_str())?;
        let signed_action: SignedAction = deserialize_from_slice(signed_action.as_slice())?;
        signed_action.verify(&config.secrets.action_signing_key)?
    };

after updating nightly, this does not compile any more. It says

error: found removed `do catch` syntax
   --> src/routes.rs:164:42
    |
164 |     let action : Result<Action, Error> = do catch {
    |                                          ^^
    |
    = help: Following RFC #2388, the new non-placeholder syntax is `try`

So I tried

    let action : Result<Action, Error> = try {
        let signed_action = base64::decode(form.signed_action.as_str())?;
        let signed_action: SignedAction = deserialize_from_slice(signed_action.as_slice())?;
        signed_action.verify(&config.secrets.action_signing_key)?
    };

but that does not compile either

error: expected identifier, found keyword `let`
   --> src/routes.rs:164:9
    |
163 |     let action : Result<Action, Error> = try {
    |                                          --- while parsing this struct
164 |         let signed_action = base64::decode(form.signed_action.as_str())?;
    |         ^^^ expected identifier, found keyword

I have also tried try catch, to no avail.

At the very least, the error message about "do catch" needs to be improved, as its advise is clearly incorrect. But also, I wonder what I should do now :D I guess I'll use an immediately-applied closure again...

[Centril]

cc @scottmcm

[scottmcm]

@RalfJung Oh, right, the error message should mention that you need to be in the 2018 edition.

(I could also make do catch or do try work temporarily in 2015, I suppose...)

[durka]

So is do catch going to be removed from 2015 with no replacement? That's not very nice.

[Centril]

@durka It was never the plan to stabilize do catch; it was always a temporary syntax.

[scottmcm]

@durka Do you have a case where you have nightly 2015 code that cannot migrate to nightly 2018?

[jonhoo]

I'm confused. Even with

#![feature(rust_2018_preview)]

the syntax

let foo: Result<_, _> = try {
    // ...
};

still does not work?

[jonhoo]

Ahh, it looks like you also need edition = 2018 in your Cargo.toml as outlined in the transition guide.

[varkor]

This was very confusing to run into (for some reason this error occurs on Travis, but not locally for me). It'd make much more sense to me to allow do catch before the 2018 edition.

[scottmcm]

I've closed this now that there's an additional message about the edition on try. That's not amazing, but this is also a nightly-only thing.

Have you perchance been able to use try{} in nightly 2018 edition now, @RalfJung?

[RalfJung]

Have you perchance been able to use try{} in nightly 2018 edition now, @RalfJung?

Yes, they seem to work as intended. :)