Add barest-bones deref patterns

Co-authored-by: Deadbeef <ent3rm4n@gmail.com>

compiler/rustc_middle/src/thir.rs

@@ -647,6 +647,7 @@ impl<'tcx> Pat<'tcx> {
             AscribeUserType { subpattern, .. }
             | Binding { subpattern: Some(subpattern), .. }
             | Deref { subpattern }
+            | DerefPattern { subpattern }
             | InlineConstant { subpattern, .. } => subpattern.walk_(it),
             Leaf { subpatterns } | Variant { subpatterns, .. } => {
                 subpatterns.iter().for_each(|field| field.pattern.walk_(it))
@@ -762,6 +763,11 @@ pub enum PatKind<'tcx> {
         subpattern: Box<Pat<'tcx>>,
     },
 
+    /// Deref pattern, written `box P` for now.
+    DerefPattern {
+        subpattern: Box<Pat<'tcx>>,
+    },
+
     /// One of the following:
     /// * `&str` (represented as a valtree), which will be handled as a string pattern and thus
     ///   exhaustiveness checking will detect if you use the same string twice in different
@@ -1172,6 +1178,9 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 }
                 write!(f, "{subpattern}")
             }
+            PatKind::DerefPattern { ref subpattern } => {
+                write!(f, "k#deref {subpattern}")
+            }
             PatKind::Constant { value } => write!(f, "{value}"),
             PatKind::InlineConstant { def: _, ref subpattern } => {
                 write!(f, "{} (from inline const)", subpattern)

compiler/rustc_middle/src/thir/visit.rs

@@ -229,6 +229,7 @@ pub fn walk_pat<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
     match &pat.kind {
         AscribeUserType { subpattern, ascription: _ }
         | Deref { subpattern }
+        | DerefPattern { subpattern }
         | Binding {
             subpattern: Some(subpattern),
             mutability: _,