regression: hard error on ? in type

[Mark-Simulacrum]

Crater picked up this issue in:

• https://crater-reports.s3.amazonaws.com/beta-1.60-1/beta-2022-02-22/reg/fn_block-0.2.1/log.txt

[INFO] [stdout] error: invalid `?` in type
[INFO] [stdout]   --> src/tests.rs:65:26
[INFO] [stdout]    |
[INFO] [stdout] 65 |     let foo = fn_expr!{ o?.when(|&i| i > 0)?.when(|&i| i%2 == 0) };
[INFO] [stdout]    |                          ^ `?` is only allowed on expressions, not types
[INFO] [stdout]    |
[INFO] [stdout] help: if you meant to express that the type might not contain a value, use the `Option` wrapper type
[INFO] [stdout]    |
[INFO] [stdout] 65 |     let foo = fn_expr!{ Option<o>.when(|&i| i > 0)?.when(|&i| i%2 == 0) };
[INFO] [stdout]    |                         +++++++ ~
[INFO] [stdout] 

cc @estebank likely introduced by #92746

[estebank]

Oh that sucks... :(

Feel free to revert and backport. It likely only affects macros and can be fixed easily, but don't have the time now to look for it.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-medium

[Dylan-DPC]

working on the revert if nobody has got to it yet

#94574