Forbid lifetime elision in let position impl Trait

This is consistent with types.

src/librustc_ast_lowering/lib.rs

@@ -1619,15 +1619,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 visitor.visit_ty(ty);
             }
         }
-        let parent_def_id = self.current_hir_id_owner.last().unwrap().0;
         let ty = l.ty.as_ref().map(|t| {
             self.lower_ty(
                 t,
                 if self.sess.features_untracked().impl_trait_in_bindings {
-                    ImplTraitContext::OpaqueTy(
-                        Some(parent_def_id.to_def_id()),
-                        hir::OpaqueTyOrigin::Misc,
-                    )
+                    ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Binding)
                 } else {
                     ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
                 },

src/librustc_hir/hir.rs

@@ -2018,7 +2018,9 @@ pub enum OpaqueTyOrigin {
     FnReturn,
     /// `async fn`
     AsyncFn,
-    /// Impl trait in bindings, consts, statics, bounds.
+    /// `let _: impl Trait = ...`
+    Binding,
+    /// Impl trait in type aliases, consts, statics, bounds.
     Misc,
 }
 

src/librustc_resolve/late/lifetimes.rs

@@ -258,6 +258,9 @@ enum Elide {
     Exact(Region),
     /// Less or more than one lifetime were found, error on unspecified.
     Error(Vec<ElisionFailureInfo>),
+    /// Forbid lifetime elision inside of a larger scope that does. For
+    /// example, in let position impl trait.
+    Forbid,
 }
 
 #[derive(Clone, Debug)]
@@ -566,7 +569,14 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     // This arm is for `impl Trait` in the types of statics, constants and locals.
                     hir::ItemKind::OpaqueTy(hir::OpaqueTy { impl_trait_fn: None, .. }) => {
                         intravisit::walk_ty(self, ty);
-                        intravisit::walk_item(this, opaque_ty);
+
+                        // Elided lifetimes are not allowed in non-return
+                        // position impl Trait
+                        let scope = Scope::Elision { elide: Elide::Forbid, s: self.scope };
+                        self.with(scope, |_, this| {
+                            intravisit::walk_item(this, opaque_ty);
+                        });
+
                         return;
                     }
                     // RPIT (return position impl trait)
@@ -2332,6 +2342,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                             }
                             break Some(e);
                         }
+                        Elide::Forbid => break None,
                     };
                     for lifetime_ref in lifetime_refs {
                         self.insert_lifetime(lifetime_ref, lifetime);

src/librustc_trait_selection/opaque_types.rs

@@ -413,7 +413,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }
             // These opaque type inherit all lifetime parameters from their
             // parent.
-            hir::OpaqueTyOrigin::Misc => 0,
+            hir::OpaqueTyOrigin::Binding | hir::OpaqueTyOrigin::Misc => 0,
         };
 
         let span = tcx.def_span(def_id);
@@ -569,7 +569,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             hir::OpaqueTyOrigin::AsyncFn => return false,
 
             // Otherwise, generate the label we'll use in the error message.
-            hir::OpaqueTyOrigin::TypeAlias
+            hir::OpaqueTyOrigin::Binding
             | hir::OpaqueTyOrigin::FnReturn
             | hir::OpaqueTyOrigin::Misc => "impl Trait",
         };

src/librustc_typeck/collect/type_of.rs

@@ -100,26 +100,17 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     let substs = InternalSubsts::identity_for_item(tcx, def_id);
                     tcx.mk_adt(def, substs)
                 }
+                ItemKind::OpaqueTy(OpaqueTy { origin: hir::OpaqueTyOrigin::Binding, .. }) => {
+                    let_position_impl_trait_type(tcx, def_id.expect_local())
+                }
                 ItemKind::OpaqueTy(OpaqueTy { impl_trait_fn: None, .. }) => {
                     find_opaque_ty_constraints(tcx, def_id.expect_local())
                 }
                 // Opaque types desugared from `impl Trait`.
-                ItemKind::OpaqueTy(OpaqueTy { impl_trait_fn: Some(owner), origin, .. }) => {
-                    let concrete_types = match origin {
-                        OpaqueTyOrigin::FnReturn | OpaqueTyOrigin::AsyncFn => {
-                            &tcx.mir_borrowck(owner.expect_local()).concrete_opaque_types
-                        }
-                        OpaqueTyOrigin::Misc => {
-                            // We shouldn't leak borrowck results through impl trait in bindings.
-                            // For example, we shouldn't be able to tell if `x` in
-                            // `let x: impl Sized + 'a = &()` has type `&'static ()` or `&'a ()`.
-                            &tcx.typeck_tables_of(owner.expect_local()).concrete_opaque_types
-                        }
-                        OpaqueTyOrigin::TypeAlias => {
-                            span_bug!(item.span, "Type alias impl trait shouldn't have an owner")
-                        }
-                    };
-                    let concrete_ty = concrete_types
+                ItemKind::OpaqueTy(OpaqueTy { impl_trait_fn: Some(owner), .. }) => {
+                    let concrete_ty = tcx
+                        .mir_borrowck(owner.expect_local())
+                        .concrete_opaque_types
                         .get(&def_id)
                         .map(|opaque| opaque.concrete_type)
                         .unwrap_or_else(|| {
@@ -148,13 +139,6 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                             }
                         });
                     debug!("concrete_ty = {:?}", concrete_ty);
-                    if concrete_ty.has_erased_regions() {
-                        // FIXME(impl_trait_in_bindings) Handle this case.
-                        tcx.sess.span_fatal(
-                            item.span,
-                            "lifetimes in impl Trait types in bindings are not currently supported",
-                        );
-                    }
                     concrete_ty
                 }
                 ItemKind::Trait(..)
@@ -589,6 +573,50 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
     }
 }
 
+fn let_position_impl_trait_type(tcx: TyCtxt<'_>, opaque_ty_id: LocalDefId) -> Ty<'_> {
+    let scope = tcx.hir().get_defining_scope(tcx.hir().as_local_hir_id(opaque_ty_id));
+    let scope_def_id = tcx.hir().local_def_id(scope);
+
+    let opaque_ty_def_id = opaque_ty_id.to_def_id();
+
+    let owner_tables = tcx.typeck_tables_of(scope_def_id);
+    let concrete_ty = owner_tables
+        .concrete_opaque_types
+        .get(&opaque_ty_def_id)
+        .map(|opaque| opaque.concrete_type)
+        .unwrap_or_else(|| {
+            tcx.sess.delay_span_bug(
+                DUMMY_SP,
+                &format!(
+                    "owner {:?} has no opaque type for {:?} in its tables",
+                    scope_def_id, opaque_ty_id
+                ),
+            );
+            if let Some(ErrorReported) = owner_tables.tainted_by_errors {
+                // Some error in the owner fn prevented us from populating the
+                // `concrete_opaque_types` table.
+                tcx.types.err
+            } else {
+                // We failed to resolve the opaque type or it resolves to
+                // itself. Return the non-revealed type, which should result in
+                // E0720.
+                tcx.mk_opaque(
+                    opaque_ty_def_id,
+                    InternalSubsts::identity_for_item(tcx, opaque_ty_def_id),
+                )
+            }
+        });
+    debug!("concrete_ty = {:?}", concrete_ty);
+    if concrete_ty.has_erased_regions() {
+        // FIXME(impl_trait_in_bindings) Handle this case.
+        tcx.sess.span_fatal(
+            tcx.hir().span(tcx.hir().as_local_hir_id(opaque_ty_id)),
+            "lifetimes in impl Trait types in bindings are not currently supported",
+        );
+    }
+    concrete_ty
+}
+
 fn infer_placeholder_type(
     tcx: TyCtxt<'_>,
     def_id: LocalDefId,

src/test/ui/impl-trait/issue-60473.rs

@@ -5,13 +5,11 @@
 
 struct A<'a>(&'a ());
 
-trait Trait<T> {
-}
+trait Trait<T> {}
 
-impl<T> Trait<T> for () {
-}
+impl<T> Trait<T> for () {}
 
 fn main() {
-    let x: impl Trait<A> = (); // FIXME: The error doesn't seem correct.
-    //~^ ERROR: opaque type expands to a recursive type
+    let x: impl Trait<A> = ();
+    //~^ ERROR: missing lifetime specifier
 }

src/test/ui/impl-trait/issue-60473.stderr

@@ -1,11 +1,15 @@
-error[E0720]: opaque type expands to a recursive type
-  --> $DIR/issue-60473.rs:15:12
+error[E0106]: missing lifetime specifier
+  --> $DIR/issue-60473.rs:13:23
    |
-LL |     let x: impl Trait<A> = (); // FIXME: The error doesn't seem correct.
-   |            ^^^^^^^^^^^^^ expands to a recursive type
+LL |     let x: impl Trait<A> = ();
+   |                       ^ expected named lifetime parameter
+   |
+help: consider introducing a named lifetime parameter
+   |
+LL | fn main<'a>() {
+LL |     let x: impl Trait<A<'a>> = ();
    |
-   = note: type resolves to itself
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0720`.
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/impl-trait/issue-67166.rs

@@ -4,8 +4,8 @@
 #![allow(incomplete_features)]
 
 pub fn run() {
-    let _foo: Box<impl Copy + '_> = Box::new(()); // FIXME: The error doesn't much make sense.
-    //~^ ERROR: opaque type expands to a recursive type
+    let _foo: Box<impl Copy + '_> = Box::new(());
+    //~^ ERROR: missing lifetime specifier
 }
 
 fn main() {}

src/test/ui/impl-trait/issue-67166.stderr

@@ -1,11 +1,15 @@
-error[E0720]: opaque type expands to a recursive type
-  --> $DIR/issue-67166.rs:7:19
+error[E0106]: missing lifetime specifier
+  --> $DIR/issue-67166.rs:7:31
    |
-LL |     let _foo: Box<impl Copy + '_> = Box::new(()); // FIXME: The error doesn't much make sense.
-   |                   ^^^^^^^^^^^^^^ expands to a recursive type
+LL |     let _foo: Box<impl Copy + '_> = Box::new(());
+   |                               ^^ expected named lifetime parameter
+   |
+help: consider introducing a named lifetime parameter
+   |
+LL | pub fn run<'a>() {
+LL |     let _foo: Box<impl Copy + 'a> = Box::new(());
    |
-   = note: type resolves to itself
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0720`.
+For more information about this error, try `rustc --explain E0106`.