Rollup merge of #87065 - FabianWolff:issue-87046, r=oli-obk

Fix ICE with unsized type in const pattern

Fixes #87046. The `deref_const()` query currently contains the following check:
e9a387d6cf/compiler/rustc_mir/src/const_eval/mod.rs (L191-L204)

i.e. this will cause an ICE for every unsized type except slices. An error is reported with my changes if such a type is used as a const pattern (this should not be a breaking change, since so far, this has caused an ICE).

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
+