Tracking issue for `proc_macro_value`

Feature gate: #![feature(proc_macro_value)]

This feature provides methods on proc_macro::Literal to get their unescaped string equivalent.

Public API

For now, the only choice to get these values is to either copy-paste rustc's source code or use syn which has more or less the same code to achieve the same result.

After this change, one will be able to do:

// The one provided by `proc-macro`.
let literal_string = Literal::String { .. };

let unescaped_string_literal = literal_string.str_value().expect("failed to unescape");

Steps / History

• ACP
• Implementation
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Tests and/or shared code demonstrating that the new API parses the same set of (no more, no less) inputs with the same outputs as rustc would (cc https://github.com/rust-lang/rust/pull/154608#discussion_r3344111522).

Start with the proc_macro::Literal API described in the issue and review the implementation history, especially the linked discussion about parsing behavior. Add tests or shared code showing that the new API accepts exactly the same inputs and produces the same outputs as rustc, then verify the remaining FCP and stabilization steps are satisfied.