Rollup merge of #122793 - compiler-errors:deref-pat-syntax, r=Nadrieril

Implement macro-based deref!() syntax for deref patterns

Stop using `box PAT` syntax for deref patterns, and instead use a perma-unstable macro.

Blocked on #122222

r? `@Nadrieril`

compiler/rustc_middle/src/thir.rs

@@ -1179,7 +1179,7 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 write!(f, "{subpattern}")
             }
             PatKind::DerefPattern { ref subpattern } => {
-                write!(f, "k#deref {subpattern}")
+                write!(f, "deref!({subpattern})")
             }
             PatKind::Constant { value } => write!(f, "{value}"),
             PatKind::InlineConstant { def: _, ref subpattern } => {