Replace struct_tail and struct_lockstep_tails with variants handling normalization.

The old struct tail functions did not deal with `<T as Trait>::A` and `impl
Trait`, at least not explicitly. (We didn't notice this bug before because it
is only exposed when the tail (post deep normalization) is not `Sized`, so it
was a rare case to deal with.)

For post type-checking (i.e. during codegen), there is now
`struct_tail_erasing_lifetimes` and `struct_lockstep_tails_erasing_lifetimes`,
which each take an additional `ParamEnv` argument to drive normalization.

For pre type-checking cases where normalization is not needed, there is
`struct_tail_without_normalization`. (Currently, the only instance of this is
`Expectation::rvalue_hint`.)

All of these new entrypoints work by calling out to common helper routines.
The helpers are parameterized over a closure that handles the normalization.

src/librustc/ty/layout.rs

@@ -543,7 +543,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     return Ok(tcx.intern_layout(LayoutDetails::scalar(self, data_ptr)));
                 }
 
-                let unsized_part = tcx.struct_tail(pointee);
+                let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
                 let metadata = match unsized_part.sty {
                     ty::Foreign(..) => {
                         return Ok(tcx.intern_layout(LayoutDetails::scalar(self, data_ptr)));
@@ -1664,7 +1664,7 @@ impl<'tcx> SizeSkeleton<'tcx> {
             ty::Ref(_, pointee, _) |
             ty::RawPtr(ty::TypeAndMut { ty: pointee, .. }) => {
                 let non_zero = !ty.is_unsafe_ptr();
-                let tail = tcx.struct_tail(pointee);
+                let tail = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
                 match tail.sty {
                     ty::Param(_) | ty::Projection(_) => {
                         debug_assert!(tail.has_param_types() || tail.has_self_ty());
@@ -2015,7 +2015,7 @@ where
                     }));
                 }
 
-                match tcx.struct_tail(pointee).sty {
+                match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).sty {
                     ty::Slice(_) |
                     ty::Str => tcx.types.usize,
                     ty::Dynamic(_, _) => {

src/librustc/ty/util.rs

@@ -258,10 +258,42 @@ impl<'tcx> TyCtxt<'tcx> {
         false
     }
 
-    /// Returns the deeply last field of nested structures, or the same type,
-    /// if not a structure at all. Corresponds to the only possible unsized
-    /// field, and its type can be used to determine unsizing strategy.
-    pub fn struct_tail(self, mut ty: Ty<'tcx>) -> Ty<'tcx> {
+    /// Attempts to returns the deeply last field of nested structures, but
+    /// does not apply any normalization in its search. Returns the same type
+    /// if input `ty` is not a structure at all.
+    pub fn struct_tail_without_normalization(self, ty: Ty<'tcx>) -> Ty<'tcx>
+    {
+        let tcx = self;
+        tcx.struct_tail_with_normalize(ty, |ty| ty)
+    }
+
+    /// Returns the deeply last field of nested structures, or the same type if
+    /// not a structure at all. Corresponds to the only possible unsized field,
+    /// and its type can be used to determine unsizing strategy.
+    pub fn struct_tail_erasing_lifetimes(self,
+                                         ty: Ty<'tcx>,
+                                         param_env: ty::ParamEnv<'tcx>)
+                                         -> Ty<'tcx>
+    {
+        let tcx = self;
+        tcx.struct_tail_with_normalize(ty, |ty| tcx.normalize_erasing_regions(param_env, ty))
+    }
+
+    /// Returns the deeply last field of nested structures, or the same type if
+    /// not a structure at all. Corresponds to the only possible unsized field,
+    /// and its type can be used to determine unsizing strategy.
+    ///
+    /// This is parameterized over the normalization strategy (i.e. how to
+    /// handle `<T as Trait>::Assoc` and `impl Trait`); pass the identity
+    /// function to indicate no normalization should take place.
+    ///
+    /// See also `struct_tail_erasing_lifetimes`, which is what callers running
+    /// after type checking should use.
+    pub fn struct_tail_with_normalize(self,
+                                      mut ty: Ty<'tcx>,
+                                      normalize: impl Fn(Ty<'tcx>) -> Ty<'tcx>)
+                                      -> Ty<'tcx>
+    {
         loop {
             match ty.sty {
                 ty::Adt(def, substs) => {
@@ -282,6 +314,15 @@ impl<'tcx> TyCtxt<'tcx> {
                     }
                 }
 
+                ty::Projection(_) | ty::Opaque(..) => {
+                    let normalized = normalize(ty);
+                    if ty == normalized {
+                        return ty;
+                    } else {
+                        ty = normalized;
+                    }
+                }
+
                 _ => {
                     break;
                 }
@@ -295,10 +336,34 @@ impl<'tcx> TyCtxt<'tcx> {
     /// structure definitions.
     /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
     /// whereas struct_tail produces `T`, and `Trait`, respectively.
-    pub fn struct_lockstep_tails(self,
-                                 source: Ty<'tcx>,
-                                 target: Ty<'tcx>)
-                                 -> (Ty<'tcx>, Ty<'tcx>) {
+    ///
+    /// Must only be called after type-checking is complete; otherwise
+    /// normalization attempt may cause compiler bugs.
+    pub fn struct_lockstep_tails_erasing_lifetimes(self,
+                                                   source: Ty<'tcx>,
+                                                   target: Ty<'tcx>,
+                                                   param_env: ty::ParamEnv<'tcx>)
+                                                   -> (Ty<'tcx>, Ty<'tcx>)
+    {
+        let tcx = self;
+        tcx.struct_lockstep_tails_with_normalize(
+            source, target, |ty| tcx.normalize_erasing_regions(param_env, ty))
+    }
+
+    /// Same as applying struct_tail on `source` and `target`, but only
+    /// keeps going as long as the two types are instances of the same
+    /// structure definitions.
+    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
+    /// whereas struct_tail produces `T`, and `Trait`, respectively.
+    ///
+    /// See also struct_lockstep_tails_erasing_lifetimes, which
+    /// is what callers running after type checking should use.
+    pub fn struct_lockstep_tails_with_normalize(self,
+                                                source: Ty<'tcx>,
+                                                target: Ty<'tcx>,
+                                                normalize: impl Fn(Ty<'tcx>) -> Ty<'tcx>)
+                                                -> (Ty<'tcx>, Ty<'tcx>)
+    {
         let (mut a, mut b) = (source, target);
         loop {
             match (&a.sty, &b.sty) {
@@ -320,6 +385,22 @@ impl<'tcx> TyCtxt<'tcx> {
                         break;
                     }
                 },
+                (ty::Projection(_), _) | (ty::Opaque(..), _) |
+                (_, ty::Projection(_)) | (_, ty::Opaque(..)) => {
+                    // If either side is a projection, attempt to
+                    // progress via normalization. (Should be safe to
+                    // apply to both sides as normalization is
+                    // idempotent.)
+                    let a_norm = normalize(a);
+                    let b_norm = normalize(b);
+                    if a == a_norm && b == b_norm {
+                        break;
+                    } else {
+                        a = a_norm;
+                        b = b_norm;
+                    }
+                }
+
                 _ => break,
             }
         }

src/librustc_codegen_ssa/base.rs

@@ -128,7 +128,8 @@ pub fn unsized_info<'tcx, Cx: CodegenMethods<'tcx>>(
     target: Ty<'tcx>,
     old_info: Option<Cx::Value>,
 ) -> Cx::Value {
-    let (source, target) = cx.tcx().struct_lockstep_tails(source, target);
+    let (source, target) =
+        cx.tcx().struct_lockstep_tails_erasing_lifetimes(source, target, cx.param_env());
     match (&source.sty, &target.sty) {
         (&ty::Array(_, len), &ty::Slice(_)) => {
             cx.const_usize(len.unwrap_usize(cx.tcx()))

src/librustc_codegen_ssa/traits/type_.rs

@@ -77,11 +77,12 @@ pub trait DerivedTypeMethods<'tcx>: BaseTypeMethods<'tcx> + MiscMethods<'tcx> {
     }
 
     fn type_has_metadata(&self, ty: Ty<'tcx>) -> bool {
-        if ty.is_sized(self.tcx().at(DUMMY_SP), ty::ParamEnv::reveal_all()) {
+        let param_env = ty::ParamEnv::reveal_all();
+        if ty.is_sized(self.tcx().at(DUMMY_SP), param_env) {
             return false;
         }
 
-        let tail = self.tcx().struct_tail(ty);
+        let tail = self.tcx().struct_tail_erasing_lifetimes(ty, param_env);
         match tail.sty {
             ty::Foreign(..) => false,
             ty::Str | ty::Slice(..) | ty::Dynamic(..) => true,

src/librustc_mir/interpret/cast.rs

@@ -270,7 +270,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         dty: Ty<'tcx>,
     ) -> InterpResult<'tcx> {
         // A<Struct> -> A<Trait> conversion
-        let (src_pointee_ty, dest_pointee_ty) = self.tcx.struct_lockstep_tails(sty, dty);
+        let (src_pointee_ty, dest_pointee_ty) =
+            self.tcx.struct_lockstep_tails_erasing_lifetimes(sty, dty, self.param_env);
 
         match (&src_pointee_ty.sty, &dest_pointee_ty.sty) {
             (&ty::Array(_, length), &ty::Slice(_)) => {

src/librustc_mir/interpret/intern.rs

@@ -146,7 +146,9 @@ for
             let value = self.ecx.read_immediate(mplace.into())?;
             // Handle trait object vtables
             if let Ok(meta) = value.to_meta() {
-                if let ty::Dynamic(..) = self.ecx.tcx.struct_tail(referenced_ty).sty {
+                if let ty::Dynamic(..) =
+                    self.ecx.tcx.struct_tail_erasing_lifetimes(referenced_ty, self.param_env).sty
+                {
                     if let Ok(vtable) = meta.unwrap().to_ptr() {
                         // explitly choose `Immutable` here, since vtables are immutable, even
                         // if the reference of the fat pointer is mutable

src/librustc_mir/interpret/validity.rs

@@ -361,7 +361,8 @@ impl<'rt, 'mir, 'tcx, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                     "uninitialized data in fat pointer metadata", self.path);
                 let layout = self.ecx.layout_of(value.layout.ty.builtin_deref(true).unwrap().ty)?;
                 if layout.is_unsized() {
-                    let tail = self.ecx.tcx.struct_tail(layout.ty);
+                    let tail = self.ecx.tcx.struct_tail_erasing_lifetimes(layout.ty,
+                                                                          self.ecx.param_env);
                     match tail.sty {
                         ty::Dynamic(..) => {
                             let vtable = meta.unwrap();

src/librustc_mir/monomorphize/collector.rs

@@ -851,12 +851,13 @@ fn find_vtable_types_for_unsizing<'tcx>(
     target_ty: Ty<'tcx>,
 ) -> (Ty<'tcx>, Ty<'tcx>) {
     let ptr_vtable = |inner_source: Ty<'tcx>, inner_target: Ty<'tcx>| {
+        let param_env = ty::ParamEnv::reveal_all();
         let type_has_metadata = |ty: Ty<'tcx>| -> bool {
             use syntax_pos::DUMMY_SP;
-            if ty.is_sized(tcx.at(DUMMY_SP), ty::ParamEnv::reveal_all()) {
+            if ty.is_sized(tcx.at(DUMMY_SP), param_env) {
                 return false;
             }
-            let tail = tcx.struct_tail(ty);
+            let tail = tcx.struct_tail_erasing_lifetimes(ty, param_env);
             match tail.sty {
                 ty::Foreign(..) => false,
                 ty::Str | ty::Slice(..) | ty::Dynamic(..) => true,
@@ -866,7 +867,7 @@ fn find_vtable_types_for_unsizing<'tcx>(
         if type_has_metadata(inner_source) {
             (inner_source, inner_target)
         } else {
-            tcx.struct_lockstep_tails(inner_source, inner_target)
+            tcx.struct_lockstep_tails_erasing_lifetimes(inner_source, inner_target, param_env)
         }
     };
 

src/librustc_typeck/check/mod.rs

@@ -315,7 +315,7 @@ impl<'a, 'tcx> Expectation<'tcx> {
     /// See the test case `test/run-pass/coerce-expect-unsized.rs` and #20169
     /// for examples of where this comes up,.
     fn rvalue_hint(fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Expectation<'tcx> {
-        match fcx.tcx.struct_tail(ty).sty {
+        match fcx.tcx.struct_tail_without_normalization(ty).sty {
             ty::Slice(_) | ty::Str | ty::Dynamic(..) => {
                 ExpectRvalueLikeUnsized(ty)
             }

src/librustc_typeck/check/wfcheck.rs

@@ -366,7 +366,8 @@ fn check_item_type(
 
         let mut forbid_unsized = true;
         if allow_foreign_ty {
-            if let ty::Foreign(_) = fcx.tcx.struct_tail(item_ty).sty {
+            let tail = fcx.tcx.struct_tail_erasing_lifetimes(item_ty, fcx.param_env);
+            if let ty::Foreign(_) = tail.sty {
                 forbid_unsized = false;
             }
         }