use PatKind::wild when an ADT const value has violation

Author: bvanjoi

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

@@ -200,6 +200,13 @@ impl<'tcx> ConstToPat<'tcx> {
                         value: mir::Const::Ty(ty::Const::new_error(self.tcx(), e, cv.ty())),
                     };
                     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 };
+                    self.tcx().sess.emit_err(err);
+                    return Box::new(Pat { span: self.span, ty: cv.ty(), kind: PatKind::Wild });
                 } 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 +1,8 @@
+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)]`
+    //~| WARN irrefutable `if let` pattern
+}

tests/ui/pattern/issue-115599.stderr

@@ -0,0 +1,21 @@
+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
+
+warning: irrefutable `if let` pattern
+  --> $DIR/issue-115599.rs:5:8
+   |
+LL |     if let CONST_STRING = empty_str {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
+   = note: `#[warn(irrefutable_let_patterns)]` on by default
+
+error: aborting due to previous error; 1 warning emitted
+