Deprecate try! macro in favor of ?

I propose to mark the try! macro as deprecated.

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Reasons for it:

• ? was stabilized in 1.13, November 2016. 2+years ago
• there is an interest in having a lint for replacing try! with ? for 2+years
• try is a keyword in rust 2018 and needs to be written as r#try!
• ""The Rust Programming Language", 2nd edition (book) and "Rust by Example" have both already removed all mentions of try!."
• the doc of try! literally names the definition of deprecation

The ? operator was added to replace try! and should be used instead.

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There are no real downsides 😃
(I am happy to wait for #37939 ;)

[Centril]

I did some investigation on the playground and it seems that the reason r#try! isn't deprecated yet is because macro_rules! cannot be deprecated at all. cc @petrochenkov

Thanks for the response @Centril .
Seems like this is true.

I thought you could deprecate any struct/function/etc 😬

[petrochenkov]

because macro_rules! cannot be deprecated at all

The issue is #49912, someone (not me) needs to do the work.

Deprecation and stability checking is actually done after macro expansion

So I assume clippy can't check this either, or am I wrong?

Otherwise we could make a general clippy lint for #[deprecated] on macros
(which would solve #49912 as well).

[petrochenkov]

Deprecation and stability checking is actually done after macro expansion

It can work at any point if name resolution results are available.
Stability checking is already implemented for macros and deprecation checking can be done by analogy.

[tesuji]

I take this in #62672 .