Only allow tait defining uses in function and method return position

compiler/rustc_typeck/src/check/check.rs

@@ -87,8 +87,26 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     let declared_ret_ty = fn_sig.output();
 
-    let revealed_ret_ty =
-        fcx.instantiate_opaque_types_from_value(fn_id, declared_ret_ty, decl.output.span());
+    let feature = match tcx.hir().get(fn_id) {
+        Node::Item(hir::Item { kind: ItemKind::Fn(..), .. }) |
+        Node::ImplItem(hir::ImplItem {
+            kind: hir::ImplItemKind::Fn(..), ..
+        }) => None,
+        // I don't know if TAIT uses in trait declarations make sense at all
+        Node::TraitItem(hir::TraitItem {
+            kind: hir::TraitItemKind::Fn(..),
+            ..
+        }) |
+        // Forbid TAIT in closure return position for now.
+        Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(..), .. }) => Some(sym::type_alias_impl_trait),
+        node => bug!("Item being checked wasn't a function/closure: {:?}", node),
+    };
+    let revealed_ret_ty = fcx.instantiate_opaque_types_from_value(
+        fn_id,
+        declared_ret_ty,
+        decl.output.span(),
+        feature,
+    );
     debug!("check_fn: declared_ret_ty: {}, revealed_ret_ty: {}", declared_ret_ty, revealed_ret_ty);
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(revealed_ret_ty)));
     fcx.ret_type_span = Some(decl.output.span());

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -26,11 +26,11 @@ use rustc_middle::ty::{
     self, AdtKind, CanonicalUserType, DefIdTree, GenericParamDefKind, ToPolyTraitRef, ToPredicate,
     Ty, UserType,
 };
-use rustc_session::lint;
-use rustc_span::hygiene::DesugaringKind;
+use rustc_session::{lint, parse::feature_err};
 use rustc_span::source_map::{original_sp, DUMMY_SP};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{self, BytePos, MultiSpan, Span};
+use rustc_span::{hygiene::DesugaringKind, Symbol};
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
@@ -362,6 +362,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         parent_id: hir::HirId,
         value: T,
         value_span: Span,
+        feature: Option<Symbol>,
     ) -> T {
         let parent_def_id = self.tcx.hir().local_def_id(parent_id);
         debug!(
@@ -380,7 +381,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let mut opaque_types = self.opaque_types.borrow_mut();
         let mut opaque_types_vars = self.opaque_types_vars.borrow_mut();
+
         for (ty, decl) in opaque_type_map {
+            if let Some(feature) = feature {
+                if let hir::OpaqueTyOrigin::TyAlias = decl.origin {
+                    if !self.tcx.features().enabled(feature) {
+                        feature_err(
+                            &self.tcx.sess.parse_sess,
+                            feature,
+                            value_span,
+                            "type alias impl trait is not permitted here",
+                        )
+                        .emit();
+                    }
+                }
+            }
             let _ = opaque_types.insert(ty, decl);
             let _ = opaque_types_vars.insert(decl.concrete_ty, decl.opaque_type);
         }

compiler/rustc_typeck/src/check/gather_locals.rs

@@ -4,7 +4,7 @@ use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::PatKind;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::ty::Ty;
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use rustc_trait_selection::traits;
 use std::mem;
 
@@ -58,11 +58,12 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
             Some(ref ty) => {
                 let o_ty = self.fcx.to_ty(&ty);
 
-                let revealed_ty = if self.fcx.tcx.features().impl_trait_in_bindings {
-                    self.fcx.instantiate_opaque_types_from_value(self.parent_id, o_ty, ty.span)
-                } else {
-                    o_ty
-                };
+                let revealed_ty = self.fcx.instantiate_opaque_types_from_value(
+                    self.parent_id,
+                    o_ty,
+                    ty.span,
+                    Some(sym::impl_trait_in_bindings),
+                );
 
                 let c_ty =
                     self.fcx.inh.infcx.canonicalize_user_type_annotation(UserType::Ty(revealed_ty));

compiler/rustc_typeck/src/check/mod.rs

@@ -121,9 +121,9 @@ use rustc_middle::ty::{self, RegionKind, Ty, TyCtxt, UserType};
 use rustc_session::config;
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
-use rustc_span::source_map::DUMMY_SP;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{self, BytePos, MultiSpan, Span};
+use rustc_span::{source_map::DUMMY_SP, sym};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
@@ -547,11 +547,12 @@ fn typeck_with_fallback<'tcx>(
             let expected_type = fcx.normalize_associated_types_in(body.value.span, expected_type);
             fcx.require_type_is_sized(expected_type, body.value.span, traits::ConstSized);
 
-            let revealed_ty = if tcx.features().impl_trait_in_bindings {
-                fcx.instantiate_opaque_types_from_value(id, expected_type, body.value.span)
-            } else {
-                expected_type
-            };
+            let revealed_ty = fcx.instantiate_opaque_types_from_value(
+                id,
+                expected_type,
+                body.value.span,
+                Some(sym::impl_trait_in_bindings),
+            );
 
             // Gather locals in statics (because of block expressions).
             GatherLocalsVisitor::new(&fcx, id).visit_body(body);

src/test/ui/feature-gates/feature-gate-type_alias_impl_trait.rs

@@ -13,7 +13,7 @@ fn define() -> Bar {
 type Foo2 = impl Debug;
 
 fn define2() {
-    let x = || -> Foo2 { 42 };
+    let x = || -> Foo2 { 42 }; //~ ERROR not permitted here
 }
 
 type Foo3 = impl Debug;
@@ -31,7 +31,7 @@ type Foo4 = impl Debug;
 
 fn define4() {
     let y: Foo4 = 42;
-    //~^ ERROR mismatched types
+    //~^ ERROR not permitted here
 }
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-type_alias_impl_trait.stderr

@@ -10,6 +10,15 @@ LL |     Bar(42)
    = note: expected opaque type `impl Debug`
                      found type `{integer}`
 
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/feature-gate-type_alias_impl_trait.rs:16:19
+   |
+LL |     let x = || -> Foo2 { 42 };
+   |                   ^^^^
+   |
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
+
 error[E0308]: mismatched types
   --> $DIR/feature-gate-type_alias_impl_trait.rs:23:18
    |
@@ -36,19 +45,14 @@ LL |     define3(42)
    = note: expected opaque type `impl Debug`
                      found type `{integer}`
 
-error[E0308]: mismatched types
-  --> $DIR/feature-gate-type_alias_impl_trait.rs:33:19
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/feature-gate-type_alias_impl_trait.rs:33:12
    |
-LL | type Foo4 = impl Debug;
-   |             ---------- the expected opaque type
-...
 LL |     let y: Foo4 = 42;
-   |            ----   ^^ expected opaque type, found integer
-   |            |
-   |            expected due to this
+   |            ^^^^
    |
-   = note: expected opaque type `impl Debug`
-                     found type `{integer}`
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+   = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
 
 error: could not find defining uses
   --> $DIR/feature-gate-type_alias_impl_trait.rs:5:12
@@ -68,6 +72,7 @@ error: could not find defining uses
 LL | type Foo4 = impl Debug;
    |             ^^^^^^^^^^
 
-error: aborting due to 7 previous errors
+error: aborting due to 8 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/generator/layout-error.min_tait.stderr

@@ -4,6 +4,37 @@ error[E0425]: cannot find value `Foo` in this scope
 LL |         let a = Foo;
    |                 ^^^ not found in this scope
 
-error: aborting due to previous error
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/layout-error.rs:31:27
+   |
+LL |     Task::spawn(&POOL, || cb());
+   |                           ^
+   |
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
+
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/layout-error.rs:30:28
+   |
+LL |     static POOL: Task<F> = Task::new();
+   |                            ^^^^^^^^^^^
+   |
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+   = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
+
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/layout-error.rs:31:24
+   |
+LL |     Task::spawn(&POOL, || cb());
+   |                        ^^^^^^^ expected `[type error]`, got `impl Future`
+   |
+note: previous use here
+  --> $DIR/layout-error.rs:30:5
+   |
+LL |     static POOL: Task<F> = Task::new();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0425`.
+Some errors have detailed explanations: E0425, E0658.
+For more information about an error, try `rustc --explain E0425`.

src/test/ui/generator/layout-error.rs

@@ -12,7 +12,7 @@ use std::future::Future;
 
 pub struct Task<F: Future>(F);
 impl<F: Future> Task<F> {
-    fn new() -> Self {
+    const fn new() -> Self {
         todo!()
     }
     fn spawn(&self, _: impl FnOnce() -> F) {
@@ -27,6 +27,7 @@ fn main() {
 
     type F = impl Future;
     // Check that statics are inhabited computes they layout.
-    static POOL: Task<F> = Task::new();
-    Task::spawn(&POOL, || cb());
+    static POOL: Task<F> = Task::new(); //[min_tait]~ ERROR not permitted here
+    Task::spawn(&POOL, || cb()); //[min_tait]~ ERROR type alias impl trait is not permitted here
+    //[min_tait]~^ ERROR concrete type differs from previous
 }

src/test/ui/generator/metadata-sufficient-for-layout.full_tait.stderr

@@ -1,5 +1,5 @@
 warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/metadata-sufficient-for-layout.rs:11:32
+  --> $DIR/metadata-sufficient-for-layout.rs:10:32
    |
 LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait, impl_trait_in_bindings))]
    |                                ^^^^^^^^^^^^^^^^^^^^^
@@ -8,12 +8,18 @@ LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait, impl_trait_in_binding
    = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
 
 warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/metadata-sufficient-for-layout.rs:11:55
+  --> $DIR/metadata-sufficient-for-layout.rs:10:55
    |
 LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait, impl_trait_in_bindings))]
    |                                                       ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
 
-warning: 2 warnings emitted
+error: fatal error triggered by #[rustc_error]
+  --> $DIR/metadata-sufficient-for-layout.rs:29:1
+   |
+LL | fn main() {}
+   | ^^^^^^^^^
+
+error: aborting due to previous error; 2 warnings emitted
 

src/test/ui/generator/metadata-sufficient-for-layout.min_tait.stderr

@@ -0,0 +1,24 @@
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/metadata-sufficient-for-layout.rs:22:23
+   |
+LL | static A: Option<F> = None;
+   |                       ^^^^
+   |
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+   = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
+
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/metadata-sufficient-for-layout.rs:25:1
+   |
+LL | fn f() -> F { metadata_sufficient_for_layout::g() }
+   | ^^^^^^^^^^^ expected `[type error]`, got `impl Generator`
+   |
+note: previous use here
+  --> $DIR/metadata-sufficient-for-layout.rs:22:1
+   |
+LL | static A: Option<F> = None;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/generator/metadata-sufficient-for-layout.rs

@@ -4,10 +4,9 @@
 // Regression test for #80998.
 //
 // aux-build:metadata-sufficient-for-layout.rs
-// check-pass
 
 // revisions: min_tait full_tait
-#![feature(min_type_alias_impl_trait)]
+#![feature(min_type_alias_impl_trait, rustc_attrs)]
 #![cfg_attr(full_tait, feature(type_alias_impl_trait, impl_trait_in_bindings))]
 //[full_tait]~^ WARN incomplete
 //[full_tait]~| WARN incomplete
@@ -21,7 +20,10 @@ type F = impl Generator<(), Yield = (), Return = ()>;
 
 // Static queries the layout of the generator.
 static A: Option<F> = None;
+//[min_tait]~^ ERROR not permitted here
 
 fn f() -> F { metadata_sufficient_for_layout::g() }
+//[min_tait]~^ ERROR concrete type differs
 
-fn main() {}
+#[rustc_error]
+fn main() {} //[full_tait]~ ERROR

src/test/ui/mir/issue-75053.full_tait.stderr

@@ -7,11 +7,15 @@ LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait))]
    = note: `#[warn(incomplete_features)]` on by default
    = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
 
-error: fatal error triggered by #[rustc_error]
-  --> $DIR/issue-75053.rs:51:1
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/issue-75053.rs:52:15
    |
-LL | fn main() {
-   | ^^^^^^^^^
+LL |     let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+   = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
 
 error: aborting due to previous error; 1 warning emitted
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/mir/issue-75053.in_bindings.stderr

@@ -8,7 +8,7 @@ LL | #![cfg_attr(in_bindings, feature(impl_trait_in_bindings))]
    = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
 
 error[E0282]: type annotations needed
-  --> $DIR/issue-75053.rs:53:38
+  --> $DIR/issue-75053.rs:52:38
    |
 LL |     type O;
    |     ------- `<Self as MyIndex<T>>::O` defined here

src/test/ui/mir/issue-75053.min_tait.stderr

@@ -1,8 +1,12 @@
-error: fatal error triggered by #[rustc_error]
-  --> $DIR/issue-75053.rs:51:1
+error[E0658]: type alias impl trait is not permitted here
+  --> $DIR/issue-75053.rs:52:15
    |
-LL | fn main() {
-   | ^^^^^^^^^
+LL |     let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+   = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/mir/issue-75053.rs

@@ -49,7 +49,6 @@ impl<T: MyFrom<Phantom2<DummyT<U>>>, U> MyIndex<Phantom1<T>> for Scope<U> {
 
 #[rustc_error]
 fn main() {
-    //[min_tait,full_tait]~^ ERROR rustc_error
-    let _pos: Phantom1<DummyT<()>> = Scope::new().my_index();
+    let _pos: Phantom1<DummyT<()>> = Scope::new().my_index(); //[min_tait,full_tait]~ ERROR not permitted here
     //[in_bindings]~^ ERROR type annotations needed
 }