Fix ICE with unsized type in const pattern

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -490,6 +490,17 @@ fn recur(
                 // convert the dereferenced constant to a pattern that is the sub-pattern of the
                 // deref pattern.
                 _ => {
+                    if !pointee_ty.is_sized(tcx.at(span), param_env) {
+                        // `tcx.deref_const()` below will ICE with an unsized type
+                        // (except slices, which are handled in a separate arm above).
+                        let msg = format!("cannot use unsized non-slice type `{}` in constant patterns", pointee_ty);
+                        if self.include_lint_checks {
+                            tcx.sess.span_err(span, &msg);
+                        } else {
+                            tcx.sess.delay_span_bug(span, &msg);
+                        }
+                        PatKind::Wild
+                    } else {
                         let old = self.behind_reference.replace(true);
                         // In case there are structural-match violations somewhere in this subpattern,
                         // we fall back to a const pattern. If we do not do this, we may end up with
@@ -502,6 +513,7 @@ fn recur(
                         self.behind_reference.set(old);
                         val
                     }
+                }
             },
             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::FnDef(..) => {
                 PatKind::Constant { value: cv }

src/test/ui/consts/issue-87046.rs

@@ -0,0 +1,34 @@
+// Regression test for the ICE described in #87046.
+
+#![crate_type="lib"]
+#![allow(unreachable_patterns)]
+#![feature(const_fn_union)]
+
+#[derive(PartialEq, Eq)]
+#[repr(transparent)]
+pub struct Username(str);
+
+pub const ROOT_USER: &Username = Username::from_str("root");
+
+impl Username {
+    pub const fn from_str(raw: &str) -> &Self {
+        union Transmute<'a> {
+            raw: &'a str,
+            typed: &'a Username,
+        }
+
+        unsafe { Transmute { raw }.typed }
+    }
+
+    pub const fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    pub fn is_root(&self) -> bool {
+        match self {
+            ROOT_USER => true,
+            //~^ ERROR: cannot use unsized non-slice type `Username` in constant patterns
+            _ => false,
+        }
+    }
+}

src/test/ui/consts/issue-87046.stderr

@@ -0,0 +1,8 @@
+error: cannot use unsized non-slice type `Username` in constant patterns
+  --> $DIR/issue-87046.rs:29:13
+   |
+LL |             ROOT_USER => true,
+   |             ^^^^^^^^^
+
+error: aborting due to previous error
+