Span that can point to closing delimiter of a delimited group

[dtolnay]

The proc_macro::TokenTree data structure currently look like this.

enum TokenTree {
    Group(Group),
    Ident(Ident),
    Punct(Punct),
    Literal(Literal),
}

impl Group {
    fn span(&self) -> Span;
}

Every delimited group like (...) [...] {...} has only a single Span that covers the full source location from opening delimiter to closing delimiter. This makes it impossible for a procedural macro to trigger an error pointing to just the opening or closing delimiter. The Rust compiler does not seem to have the same limitation:

mod m {
    type T =
}

error: expected type, found `}`
 --> src/main.rs:3:1
  |
3 | }
  | ^

The best things we can do in a proc macro are point to the last token inside the block, point to the entire block, or point to the next token after the block, none of which is really what you want for an error like above.

[alexcrichton]

The recent restructuring of proc_macro has enabled this issue to be implemented, so the OP isn't as accurate as it necessarily was before. This is still unimplemented, however, so I'm leaving it open

[parched]

Related to this (or should I raise a separate issue?) is creating a span that points to a subset of the characters of an existing span. For example, when you have a template string like in format! I want the error span to just point to the actual error in the template string, not the whole string. The compiler seems to be able to do this internally.