Misplaced suggestion for missing generic parameter in macro invocation

[AuroransSolis]

Given the following code:

struct Example<T, U> {
    foo: T,
    bar: U,
}

macro_rules! impl_example {
    ($($t:ty)+) => {$(
        impl Example<$t> {
            fn baz() {
                println!(":)");
            }
        }
    )}
}

impl_example! { u8 }

The current output is:

error[E0107]: this struct takes 2 generic arguments but 1 generic argument was supplied
  --> src/lib.rs:8:14
   |
8  |         impl Example<$t> {
   |              ^^^^^^^ expected 2 generic arguments
...
16 | impl_example! { u8 }
   | --------------------
   | |               |
   | |               supplied 1 generic argument
   | in this macro invocation
   |
note: struct defined here, with 2 generic parameters: `T`, `U`
  --> src/lib.rs:1:8
   |
1  | struct Example<T, U> {
   |        ^^^^^^^ -  -
   = note: this error originates in the macro `impl_example` (in Nightly builds, run with -Z macro-backtrace for more info)
help: add missing generic argument
   |
16 | impl_example! { u8, U }
   |                   +++

Most of the error message is good up until the help section, whereupon following it (by passing, say, u8, u8 to the macro call) leads to another error:

error: no rules expected the token `,`
  --> src/lib.rs:16:19
   |
6  | macro_rules! impl_example {
   | ------------------------- when calling this macro
...
16 | impl_example! { u8, u8 }
   |                   ^ no rules expected this token in macro call

Asking for another generic parameter is good, but it's being asked for in the wrong place.

As for the versions affected, I've tried it on the stable (1.56.1), nightly (2021-11-02 18bc4bee9710b181b440), and beta (2021-11-01 708d57e288d051a6238e) versions available on the Playground as of the time of writing, and all of them are affected.