Rename and rewrite the "question mark operator" #1931
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This renames the "question mark operator" section to the "try propagation expression", and also drops the grammar name "error propagation expression". The "error propagation expression" isn't really accurate, since `?` isn't just for errors. The intent is to describe the expression by what it does (as most other expressions do) instead of how it looks. It also felt a little awkward to call it the "question mark expression".
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This moves the examples to the intro. It also adds a ControlFlow example. The intent here is to prepare for changing the rest of the text, and the examples won't quite fit like the used to. I also prefer the idea of having an example near every intro section. Edited-by: TC
This rewrites the section to say that it uses the `Try` trait. I felt it was a bit too awkward to avoid it (since it is unstable). This keeps an explicit list of the types that the `Try` trait is implemented for, and adds the missing types (like `ControlFlow`). The intent is that when `Try` is stabilized that the desuggaring note moves to a normative rule, the `expr.try.restricted-types` rule is removed (and instead direct the user to the standard library documentation). I'm still holding out hope that they change the terminology, since I think the current choice is going to be confusing. This has a hack to allow `feature(try_trait_v2)` by abusing the style checker by adding some spaces. I really wanted to keep this example tested to ensure that it is kept up-to-date. However, I may regret this in the future. Fixes rust-lang#1927 Edited-by: TC
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This renames the "question mark operator" section, and rewrites it to reference the
Trytrait. See individual commits for a more detailed description.Fixes #1927