pattern lowering: make sure we never call user-defined PartialEq instances

Author: RalfJung

compiler/rustc_middle/src/thir.rs

@@ -783,16 +783,13 @@ pub enum PatKind<'tcx> {
     },
 
     /// One of the following:
-    /// * `&str` (represented as a valtree), which will be handled as a string pattern and thus
-    ///   exhaustiveness checking will detect if you use the same string twice in different
-    ///   patterns.
+    /// * `&str`/`&[u8]` (represented as a valtree), which will be handled as a string/slice pattern
+    ///   and thus exhaustiveness checking will detect if you use the same string/slice twice in
+    ///   different patterns.
     /// * integer, bool, char or float (represented as a valtree), which will be handled by
     ///   exhaustiveness to cover exactly its own value, similar to `&str`, but these values are
     ///   much simpler.
-    /// * Opaque constants (represented as `mir::ConstValue`), that must not be matched
-    ///   structurally. So anything that does not derive `PartialEq` and `Eq`.
-    ///
-    /// These are always compared with the matched place using (the semantics of) `PartialEq`.
+    /// * `String`, if `string_deref_patterns` is enabled.
     Constant {
         value: mir::Const<'tcx>,
     },

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

@@ -144,7 +144,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     && tcx.is_lang_item(def.did(), LangItem::String)
                 {
                     if !tcx.features().string_deref_patterns {
-                        bug!(
+                        span_bug!(
+                            test.span,
                             "matching on `String` went through without enabling string_deref_patterns"
                         );
                     }
@@ -432,40 +433,28 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
-        match *ty.kind() {
-            ty::Ref(_, deref_ty, _) => ty = deref_ty,
-            _ => {
-                // non_scalar_compare called on non-reference type
-                let temp = self.temp(ty, source_info.span);
-                self.cfg.push_assign(block, source_info, temp, Rvalue::Use(expect));
-                let ref_ty = Ty::new_imm_ref(self.tcx, self.tcx.lifetimes.re_erased, ty);
-                let ref_temp = self.temp(ref_ty, source_info.span);
-
-                self.cfg.push_assign(
-                    block,
-                    source_info,
-                    ref_temp,
-                    Rvalue::Ref(self.tcx.lifetimes.re_erased, BorrowKind::Shared, temp),
-                );
-                expect = Operand::Move(ref_temp);
-
-                let ref_temp = self.temp(ref_ty, source_info.span);
-                self.cfg.push_assign(
-                    block,
-                    source_info,
-                    ref_temp,
-                    Rvalue::Ref(self.tcx.lifetimes.re_erased, BorrowKind::Shared, val),
-                );
-                val = ref_temp;
+        // Figure out the type on which we are calling `PartialEq`. This involves an extra wrapping
+        // reference: we can only compare two `&T`, and then compare_ty will be `T`.
+        // Make sure that we do *not* call any user-defined code here.
+        // The only types that can end up here are string and byte literals,
+        // which have their comparison defined in `core`.
+        // (Interestingly this means that exhaustiveness analysis relies, for soundness,
+        // on the `PartialEq` impls for `str` and `[u8]` to b correct!)
+        let compare_ty = match *ty.kind() {
+            ty::Ref(_, deref_ty, _)
+                if deref_ty == self.tcx.types.str_ || deref_ty != self.tcx.types.u8 =>
+            {
+                deref_ty
             }
-        }
+            _ => span_bug!(source_info.span, "invalid type for non-scalar compare: {}", ty),
+        };
 
         let eq_def_id = self.tcx.require_lang_item(LangItem::PartialEq, Some(source_info.span));
         let method = trait_method(
             self.tcx,
             eq_def_id,
             sym::eq,
-            self.tcx.with_opt_host_effect_param(self.def_id, eq_def_id, [ty, ty]),
+            self.tcx.with_opt_host_effect_param(self.def_id, eq_def_id, [compare_ty, compare_ty]),
         );
 
         let bool_ty = self.tcx.types.bool;

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -462,7 +462,12 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                     // This is a box pattern.
                     ty::Adt(adt, ..) if adt.is_box() => Struct,
                     ty::Ref(..) => Ref,
-                    _ => bug!("pattern has unexpected type: pat: {:?}, ty: {:?}", pat, ty),
+                    _ => span_bug!(
+                        pat.span,
+                        "pattern has unexpected type: pat: {:?}, ty: {:?}",
+                        pat.kind,
+                        ty.inner()
+                    ),
                 };
             }
             PatKind::DerefPattern { .. } => {
@@ -518,7 +523,12 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                             .map(|ipat| self.lower_pat(&ipat.pattern).at_index(ipat.field.index()))
                             .collect();
                     }
-                    _ => bug!("pattern has unexpected type: pat: {:?}, ty: {:?}", pat, ty),
+                    _ => span_bug!(
+                        pat.span,
+                        "pattern has unexpected type: pat: {:?}, ty: {}",
+                        pat.kind,
+                        ty.inner()
+                    ),
                 }
             }
             PatKind::Constant { value } => {
@@ -663,7 +673,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                             }
                         }
                     }
-                    _ => bug!("invalid type for range pattern: {}", ty.inner()),
+                    _ => span_bug!(pat.span, "invalid type for range pattern: {}", ty.inner()),
                 };
                 fields = vec![];
                 arity = 0;
@@ -674,7 +684,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                         Some(length.eval_target_usize(cx.tcx, cx.param_env) as usize)
                     }
                     ty::Slice(_) => None,
-                    _ => span_bug!(pat.span, "bad ty {:?} for slice pattern", ty),
+                    _ => span_bug!(pat.span, "bad ty {} for slice pattern", ty.inner()),
                 };
                 let kind = if slice.is_some() {
                     SliceKind::VarLen(prefix.len(), suffix.len())