rust-analyzer macro expansion does not implement opaque ASTs

rust-analyzer version: (rust-analyzer version: 0.4.1888-standalone (6fce1d71d 2024-03-19)

rustc version: rustc 1.79.0-nightly (a7e4de13c 2024-03-19)

This was tested against a matrix of latest stable and nightly rustc, and stable and nightly rust-analyzer.

code snippet to reproduce:

macro_rules! a {
    ($arg:ident:Option<$type:ty>) => {
        "a"
    };
    ($arg:ident:$type:ty) => {
        "b"
    };
}

macro_rules! b {
    ($arg:ident:$type:ty) => {
       a!($arg:$type)
    }
}

a!(a:i32); // "b"
a!(a:Option<i32>); // "a"

b!(a:i32); // "b" in both rustc and rust-analyzer
b!(a:Option<i32>); // "b" in rustc, "a" in rust-analyzer

There is a discrepancy between the behavior of rustc and rust-analyzer when expanding this macro. I personally believe that the rust-analyzer behavior is more correct but it is up to rustc to decide this. Macro a works as expected for simple usage but when the input is passed indirectly through another macro this difference occurs. Opaqueness does not seem to be enforced in rust analyzer.

I also submitted a rustc issue: rust-lang/rust#122774

Start by reproducing the macro expansion discrepancy with the Rust snippet and the listed rustc and rust-analyzer versions. Trace rust-analyzer's handling of forwarded macro fragments and opaque ASTs, then compare it with rustc's behavior and the macros-by-example forwarding rule. Done means indirect expansion follows the intended opaqueness behavior consistently.