Rollup merge of #116522 - bvanjoi:fix-115599, r=oli-obk

use `PatKind::Error` when an ADT const value has violation

Fixes #115599

Since the [to_pat](https://github.com/rust-lang/rust/pull/111913/files#diff-6d8d99538aca600d633270051580c7a9e40b35824ea2863d9dda2c85a733b5d9R126-R155) behavior has been changed in the #111913 update, the kind of `inlined_const_ast_pat` has transformed from `PatKind::Leaf { pattern: Pat { kind: Wild, ..} } ` to `PatKind::Constant`. This caused a scenario where there are no matched candidates, leading to a testing of the candidates. This process ultimately attempts to test the string const, triggering the `bug!` invocation finally.

r? ``@oli-obk``

Author: matthiaskrgr

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

@@ -198,6 +198,14 @@ impl<'tcx> ConstToPat<'tcx> {
                     // We errored. Signal that in the pattern, so that follow up errors can be silenced.
                     let kind = PatKind::Error(e);
                     return Box::new(Pat { span: self.span, ty: cv.ty(), kind });
+                } else if let ty::Adt(..) = cv.ty().kind() && matches!(cv, mir::Const::Val(..)) {
+                    // This branch is only entered when the current `cv` is `mir::Const::Val`.
+                    // This is because `mir::Const::ty` has already been handled by `Self::recur`
+                    // and the invalid types may be ignored.
+                    let err = TypeNotStructural { span: self.span, non_sm_ty };
+                    let e = self.tcx().sess.emit_err(err);
+                    let kind = PatKind::Error(e);
+                    return Box::new(Pat { span: self.span, ty: cv.ty(), kind });
                 } else if !self.saw_const_match_lint.get() {
                     if let Some(mir_structural_match_violation) = mir_structural_match_violation {
                         match non_sm_ty.kind() {

tests/ui/pattern/issue-115599.rs

@@ -0,0 +1,7 @@
+const CONST_STRING: String = String::new();
+
+fn main() {
+    let empty_str = String::from("");
+    if let CONST_STRING = empty_str {}
+    //~^ ERROR to use a constant of type `Vec<u8>` in a pattern, `Vec<u8>` must be annotated with `#[derive(PartialEq, Eq)]`
+}

tests/ui/pattern/issue-115599.stderr

@@ -0,0 +1,11 @@
+error: to use a constant of type `Vec<u8>` in a pattern, `Vec<u8>` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/issue-115599.rs:5:12
+   |
+LL |     if let CONST_STRING = empty_str {}
+   |            ^^^^^^^^^^^^
+   |
+   = note: the traits must be derived, manual `impl`s are not sufficient
+   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
+
+error: aborting due to previous error
+