Auto merge of #123877 - ShE3py:expr-in-pats-2, r=fmease

Further improve diagnostics for expressions in pattern position

Follow-up of #118625, see #121697.

```rs
fn main() {
    match 'b' {
        y.0.0.1.z().f()? as u32 => {},
    }
}
```
Before:
```
error: expected one of `=>`, ``@`,` `if`, or `|`, found `.`
 --> src/main.rs:3:10
  |
3 |         y.0.0.1.z().f()? as u32 => {},
  |          ^ expected one of `=>`, ``@`,` `if`, or `|`
```
After:
```
error: expected a pattern, found an expression
 --> src/main.rs:3:9
  |
3 |         y.0.0.1.z().f()? as u32 => {},
  |         ^^^^^^^^^^^^^^^^^^^^^^^ arbitrary expressions are not allowed in patterns
  |
help: consider moving the expression to a match arm guard
  |
3 |         val if val == y.0.0.1.z().f()? as u32 => {},
  |         ~~~ +++++++++++++++++++++++++++++++++
help: consider extracting the expression into a `const`
  |
2 +     const VAL: /* Type */ = y.0.0.1.z().f()? as u32;
3 ~     match 'b' {
4 ~         VAL => {},
  |
help: consider wrapping the expression in an inline `const` (requires `#![feature(inline_const_pat)]`)
  |
3 |         const { y.0.0.1.z().f()? as u32 } => {},
  |         +++++++                         +
```

---

r? fmease
`@rustbot` label +A-diagnostics +A-parser +A-patterns +C-enhancement

0 files changed, 0 insertions, 0 deletions