remove the bug invoke during compare string const

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -236,10 +236,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
             TestKind::Eq { value, ty } => {
                 let tcx = self.tcx;
-                if let ty::Adt(def, _) = ty.kind() && Some(def.did()) == tcx.lang_items().string() {
-                    if !tcx.features().string_deref_patterns {
-                        bug!("matching on `String` went through without enabling string_deref_patterns");
-                    }
+                if let ty::Adt(def, _) = ty.kind()
+                    && Some(def.did()) == tcx.lang_items().string()
+                    && tcx.features().string_deref_patterns
+                {
                     let re_erased = tcx.lifetimes.re_erased;
                     let ref_string = self.temp(Ty::new_imm_ref(tcx,re_erased, ty), test.span);
                     let ref_str_ty = Ty::new_imm_ref(tcx,re_erased, tcx.types.str_);

tests/ui/pattern/issue-115599.rs

@@ -0,0 +1,10 @@
+// check-pass
+
+const CONST_STRING: String = String::new();
+
+fn main() {
+    let empty_str = String::from("");
+    if let CONST_STRING = empty_str {}
+    //~^ WARN must be annotated with `#[derive(PartialEq, Eq)]`
+    //~| WARN this was previously accepted
+}

tests/ui/pattern/issue-115599.stderr

@@ -0,0 +1,14 @@
+warning: to use a constant of type `Vec<u8>` in a pattern, `Vec<u8>` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/issue-115599.rs:7:12
+   |
+LL |     if let CONST_STRING = empty_str {}
+   |            ^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/62411>
+   = 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
+   = note: `#[warn(indirect_structural_match)]` on by default
+
+warning: 1 warning emitted
+