add a temporary workaround for string_deref_patterns

Author: dianne

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

@@ -11,7 +11,7 @@ use rustc_abi::{FieldIdx, Integer};
 use rustc_errors::codes::*;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::pat_util::EnumerateAndAdjustIterator;
-use rustc_hir::{self as hir, RangeEnd};
+use rustc_hir::{self as hir, LangItem, RangeEnd};
 use rustc_index::Idx;
 use rustc_middle::mir::interpret::LitToConstInput;
 use rustc_middle::thir::{
@@ -650,7 +650,26 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         // the pattern's type will be `&[u8]` whereas the literal's type is `&[u8; 3]`; using the
         // pattern's type means we'll properly translate it to a slice reference pattern. This works
         // because slices and arrays have the same valtree representation.
-        let ct_ty = pat_ty.unwrap_or_else(|| self.typeck_results.node_type(expr.hir_id));
+        // HACK: As an exception, use the literal's type if `pat_ty` is `String`; this can happen if
+        // `string_deref_patterns` is enabled. There's a special case for that when lowering to MIR.
+        // FIXME(deref_patterns): This hack won't be necessary once `string_deref_patterns` is
+        // superseded by a more general implementation of deref patterns.
+        let ct_ty = match pat_ty {
+            Some(pat_ty)
+                if let ty::Adt(def, _) = *pat_ty.kind()
+                    && self.tcx.is_lang_item(def.did(), LangItem::String) =>
+            {
+                if !self.tcx.features().string_deref_patterns() {
+                    span_bug!(
+                        expr.span,
+                        "matching on `String` went through without enabling string_deref_patterns"
+                    );
+                }
+                self.typeck_results.node_type(expr.hir_id)
+            }
+            Some(pat_ty) => pat_ty,
+            None => self.typeck_results.node_type(expr.hir_id),
+        };
         let lit_input = LitToConstInput { lit: &lit.node, ty: ct_ty, neg };
         let constant = self.tcx.at(expr.span).lit_to_const(lit_input);
         self.const_to_pat(constant, ct_ty, expr.hir_id, lit.span).kind