debug ice: `E0423` suggests assoc fns inside macro_rules

Code

use std::collections::HashMap;

macro_rules! all_sync_send {
    ($ctor:expr, $($iter:expr),+) => ({
        $(
            let mut x = $ctor;
            is_sync(x.$iter());
            let mut y = $ctor;
            is_send(y.$iter());
        )+
    })
}

fn main() {
    all_sync_send!(HashMap, HashMap);
}

Meta

rustc --version --verbose:

f967532a47eb728ada44473a5c4c2eca1a45fe30

Error output

This is the offending suggestion

error[E0423]: expected function, tuple struct or tuple variant, found struct `HashMap`
   --> C066.rs:9:23
    |
9   |             is_send(y.$iter());
    |                       ^^^^^^^
...
15  |     all_sync_send!(HashMap, HashMap);
    |     -------------------------------- in this macro invocation
    |
   ::: /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:213:1
    |
213 | pub struct HashMap<K, V, S = RandomState> {
    | ----------------------------------------- `HashMap` defined here
    |
    = note: this error originates in the macro `all_sync_send` (in Nightly builds, run with -Z macro-backtrace for more info)
help: you might have meant to use an associated function to build this type
    |
9   |             is_send(y.$iter()::new());
    |                              ~~~~~~~
9   |             is_send(y.$iter()::with_capacity(_));
    |                              ~~~~~~~~~~~~~~~~~~
9   |             is_send(y.$iter()::with_hasher(_));
    |                              ~~~~~~~~~~~~~~~~
9   |             is_send(y.$iter()::with_capacity_and_hasher(_, _));
    |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
help: consider using the `Default` trait
    |
9   |             is_send(y.$iter() as std::default::Default>::default()<HashMap);
    |                             + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

especially this is_send(y.$iter() as std::default::Default>::default()<HashMap); looks very weird

Start with the Rust reproducer in the issue and compare the diagnostic produced by the referenced nightly compiler. Trace the diagnostic suggestions for the macro_rules invocation, then verify that the completed fix no longer emits the malformed associated-function or Default-trait suggestions shown here.