E0277 explanation when using optional chaining can be improved

[learnopengles]

For this error:

 error[E0277]: the trait bound `std::option::Option<&Symbol>: std::ops::FnOnce<()>` is not satisfied
    --> JackCompiler.rs:302:19
     |
 302 |         let b = a.or_else(self.subroutine_symbols.get(name));
     |                   ^^^^^^^ the trait `std::ops::FnOnce<()>` is not implemented for `std::option::Option<&Symbol>`
 
 error: aborting due to previous error

The compiler can suggest adding a closure here. I'm honestly not sure what the issue is myself and I find optional and result chaining very difficult to use in Rust as compared to a language like Swift. However, changing it to a closure by adding "|| " fixed the error for me.

[estebank]

The traits Fn:

rust/src/libcore/ops/function.rs

Line 70 in fefe816

pub trait Fn<Args> : FnMut<Args> {

FnOnce:

rust/src/libcore/ops/function.rs

Line 216 in fefe816

pub trait FnOnce<Args> {

and FnMut:

rust/src/libcore/ops/function.rs

Line 143 in fefe816

pub trait FnMut<Args> : FnOnce<Args> {

need to have a #[rustc_on_unimplemented] attribute added, with a note suggesting the use of a closure and a short explanation of why the attempted code doesn't work.

[lorian7392]

i'm a noob, but I would like to take this on if nobody else is working on that.

[lorian7392]

Hello @estebank , I am sorry that I have not worked before, because this time is working overtime.Now I built the compiler and tried adding the #[rustc_on_unimplemented] property, but I have some questions.
How does (crate_local, label='xxxx', note='xxx') in #[rustc_on_unimplemented] work? Is there any relevant information? Because I use the method in the example, but it doesn't work. Thank you very much for your help.

[estebank]

crate_local filtering was added here. The machinery that parses rustc_on_unmplemented now lives here.

[crate_local is a flag added in error_reporting.rs only if the current evaluated scope is part of the local crate](

rust/src/librustc/traits/error_reporting.rs

Lines 394 to 396 in 0aa8d03

	if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
	flags.push(("crate_local".to_string(), None));
	}

being compiled (in other words, your code). This means that if that flag is present, and crate_local rule was in rustc_on_unimplemented, then the following message/label/note will be shown. This is done so we can suggest code changes that are only valid if you have access to the source and we don't suggest, for example, changing code in Diesel, Serde or another dependency. These flags, IIRC, only work with on, as in #[rustc_on_unimlpemented(on(crate_local, note="the note"))].

Try first annotating with #[rustc_on_unimplemented(note="test")] to see if anything actually comes up, as I am now worried that there might be some special handling of closure traits.

Regardless, you can filter on any of the type arguments (including Self) as well, in the following way: #[rustc_on_unimlpemented(note="regular note", on(Args="()", note="the note if Fn<()>"))].

The documentation for the feature is at https://doc.rust-lang.org/unstable-book/language-features/on-unimplemented.html, but it is a bit out of date.

[estebank]

Extended the documentation a bit in #53279.

[lorian7392]

hi @estebank . i try first annotating with #[rustc_on_unimplemented(note="test")] , but it not work. and
#[rustc_on_unimplemented = "test"] can work

[estebank]

@LuGuoHuas I took a look to see what was needed and arrived at this #53296