Provide access to item attributes

[celinval]

A definition in Rust can have attributes attached to them, .e.g., tool attributes. They are used by different tools to add metadata to the user code that can provide more information on how the code must be analyzed.

StableMIR should provide an API to retrieve the attributes of a crate definition. We can probably add a new method to the CrateDef that returns a list of Attributes attached to them.

[adwinwhite]

@rustbot claim

[adwinwhite]

rustc_ast::ast::Attribute's spec is quite complicated so we probably don't want to mirror it.

My plan:

• define stable Attribute similar to Span, merely as an index to internal struct.
• add two most needed methods value_str and path for our stable Attribute.

Is this design okay?

[celinval]

That's a great start! Maybe add span() too.

I would like to eventually expose the tokens() method that returns proc_macro::TokenStream, so users can use crates such as syn to extract the contents of the attribute. That said, I haven't done any thorough investigation to see if that would be feasible and a reasonable implementation.

[celinval]

Or we could provide a to_string() for now (https://github.com/rust-lang/rust/blob/c290e9de32e8ba6a673ef125fde40eadd395d170/compiler/rustc_ast_pretty/src/pprust/mod.rs#L75)

[adwinwhite]

Good idea. Perhaps I should start with span() and to_string() for now. String can be easily parsed into TokenStream if needed.

tokens() could save us some parsing, but it would also introduce rustc_ast as a dependency to stable_mir which has basically no dependency for now.

[celinval]

Let's resolve this for now. We can always add token() later. Thanks again @adwinwhite