Add undocumented outer attributes above StructExpr fields

[pushkine]

StructExprField may use outer attributes: https://replit.com/@pushkine/Attributes-in-Struct-Expressions#src/main.rs

Here's an occurrence in rust's compiler:

https://github.com/rust-lang/rust/blob/a00e130dae74a213338e2b095ec855156d8f3d8a/compiler/rustc_data_structures/src/sync.rs#L571-L577

[ehuss]

Thanks! Looks like this was implemented in rust-lang/rust#38814 and stabilized in rust-lang/rust#42656.

[pushkine]

@ehuss it's actually indirectly documented at the bottom of the expression index page
https://doc.rust-lang.org/reference/expressions.html#expression-attributes

Note that CallExpression's CallParams and StructExprTuple's members are also missing outer attributes in their reference syntax, I was not comfortable editing them myself since it requires introducing new pseudo AST nodes

[ehuss]

Sorry, I'm not sure I understand your comment. The Expression rule begins with OuterAttribute, with a marker to that section that specifies exactly where attributes are allowed. Thus something like CallParams doesn't need to specify OuterAttribute because it is part of Expression.

[pushkine]

From what I understand Expressions cannot have attributes on their own, it's only their position as the child of another specific node that grants them that ability

I think it is very confusing to put a blanket OuterAttribute* (...) around the syntax definition of all expressions with only a foot note listing the AST context that allows it to happen, I'd expect that information to be in the definition of each of those parents instead

[ehuss]

Ah, yea, I can understand that it can be confusing. The grammar in the reference defines the AST, which is essentially the parsed structure before certain validations are done. The reason it is done this way is primarily for macros, which work with AST fragments. For example, the following is valid:

macro_rules! expr {
    ($e:expr) => {};
}

fn main() {
    // Attributes are valid before any expression for `expr`.
    expr!(#[some_attr]1 + #[some_attr]2);
    expr!([
        #[some_attr]1,
        #[some_attr]2,
        #[some_attr]3,
    ]);
}

In this situation, attribute validation is done in a later phase.