Error for RPIT if they are not defined during MIR borrowck

compiler/rustc_hir_analysis/messages.ftl

@@ -512,6 +512,8 @@ hir_analysis_typeof_reserved_keyword_used =
 hir_analysis_unconstrained_opaque_type = unconstrained opaque type
     .note = `{$name}` must be used in combination with a concrete type within the same {$what}
 
+hir_analysis_undefined_opaque_type = undefined opaque type
+
 hir_analysis_unnamed_fields_repr_field_defined = unnamed field defined here
 
 hir_analysis_unnamed_fields_repr_field_missing_repr_c =

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -8,7 +8,7 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::{sym, ErrorGuaranteed, DUMMY_SP};
 
-use crate::errors::{TaitForwardCompat, TypeOf, UnconstrainedOpaqueType};
+use crate::errors::{TaitForwardCompat, TypeOf, UnconstrainedOpaqueType, UndefinedOpaqueType};
 
 pub fn test_opaque_hidden_types(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
     let mut res = Ok(());
@@ -389,18 +389,9 @@ pub(super) fn find_opaque_ty_constraints_for_rpit<'tcx>(
             // the `concrete_opaque_types` table.
             Ty::new_error(tcx, guar)
         } else {
-            // Fall back to the RPIT we inferred during HIR typeck
-            if let Some(hir_opaque_ty) = hir_opaque_ty {
-                hir_opaque_ty.ty
-            } else {
-                // We failed to resolve the opaque type or it
-                // resolves to itself. We interpret this as the
-                // no values of the hidden type ever being constructed,
-                // so we can just make the hidden type be `!`.
-                // For backwards compatibility reasons, we fall back to
-                // `()` until we the diverging default is changed.
-                Ty::new_diverging_default(tcx)
-            }
+            // Error if we couldn't define the RPIT during MIR borrowck
+            let err = tcx.dcx().emit_err(UndefinedOpaqueType { span: tcx.def_span(def_id) });
+            Ty::new_error(tcx, err)
         }
     }
 }

compiler/rustc_hir_analysis/src/errors.rs

@@ -380,6 +380,13 @@ pub struct UnconstrainedOpaqueType {
     pub what: &'static str,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_undefined_opaque_type)]
+pub struct UndefinedOpaqueType {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_tait_forward_compat)]
 #[note]

tests/ui/borrowck/opaque-types-deadcode.rs

@@ -0,0 +1,33 @@
+//@ compile-flags:-Zverbose-internals
+
+#![feature(rustc_attrs)]
+#![rustc_hidden_type_of_opaques]
+
+trait CallMeMaybe<'a, 'b> {
+    fn mk() -> Self;
+    fn subtype<T>(self, x: &'b T) -> &'a T;
+}
+
+struct Foo<'a, 'b: 'a>(&'a (), &'b ());
+impl<'a, 'b> CallMeMaybe<'a, 'b> for Foo<'a, 'b> {
+    fn mk() -> Self {
+        Foo(&(), &())
+    }
+
+    fn subtype<T>(self, x: &'b T) -> &'a T {
+        x
+    }
+}
+
+fn good_bye() -> ! {
+    panic!();
+}
+
+fn foo<'a, 'b: 'a>() -> impl CallMeMaybe<'a, 'b> {
+    //~^ ERROR: {type error}
+    //~| ERROR: undefined opaque type
+    good_bye();
+    Foo(&(), &())
+}
+
+fn main() {}

tests/ui/borrowck/opaque-types-deadcode.stderr

@@ -0,0 +1,14 @@
+error: undefined opaque type
+  --> $DIR/opaque-types-deadcode.rs:26:25
+   |
+LL | fn foo<'a, 'b: 'a>() -> impl CallMeMaybe<'a, 'b> {
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: {type error}
+  --> $DIR/opaque-types-deadcode.rs:26:25
+   |
+LL | fn foo<'a, 'b: 'a>() -> impl CallMeMaybe<'a, 'b> {
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/delegation/not-supported.rs

@@ -70,12 +70,14 @@ mod opaque {
 
         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ ERROR undefined opaque type
     }
     reuse to_reuse::opaque_arg;
     //~^ ERROR delegation with early bound generics is not supported yet
 
     trait ToReuse {
         fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ ERROR undefined opaque type
     }
 
     // FIXME: Inherited `impl Trait`s create query cycles when used inside trait impls.

tests/ui/delegation/not-supported.stderr

@@ -107,62 +107,74 @@ LL |         reuse Trait::foo2 { &self.0 }
    |                      ^^^^
 
 error: delegation with early bound generics is not supported yet
-  --> $DIR/not-supported.rs:74:21
+  --> $DIR/not-supported.rs:75:21
    |
 LL |         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
    |         --------------------------------------- callee defined here
 ...
 LL |     reuse to_reuse::opaque_arg;
    |                     ^^^^^^^^^^
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`
-  --> $DIR/not-supported.rs:83:25
+error: undefined opaque type
+  --> $DIR/not-supported.rs:79:28
+   |
+LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
+   |                            ^^^^^^^^^^
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:84:5: 84:24>::{synthetic#0}`
+  --> $DIR/not-supported.rs:85:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:83:25
+  --> $DIR/not-supported.rs:85:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>` is well-formed
-  --> $DIR/not-supported.rs:82:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:84:5: 84:24>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:84:5: 84:24>` is well-formed
+  --> $DIR/not-supported.rs:84:5
    |
 LL |     impl ToReuse for u8 {
    |     ^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`
-  --> $DIR/not-supported.rs:86:24
+error: undefined opaque type
+  --> $DIR/not-supported.rs:72:32
+   |
+LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+   |                                ^^^^^^^^^^
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:87:5: 87:25>::{synthetic#0}`
+  --> $DIR/not-supported.rs:88:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:86:24
+  --> $DIR/not-supported.rs:88:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>` is well-formed
-  --> $DIR/not-supported.rs:85:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:87:5: 87:25>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:87:5: 87:25>` is well-formed
+  --> $DIR/not-supported.rs:87:5
    |
 LL |     impl ToReuse for u16 {
    |     ^^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 error: recursive delegation is not supported yet
-  --> $DIR/not-supported.rs:99:22
+  --> $DIR/not-supported.rs:101:22
    |
 LL |         pub reuse to_reuse2::foo;
    |                              --- callee defined here
 ...
 LL |     reuse to_reuse1::foo;
    |                      ^^^
 
-error: aborting due to 16 previous errors
+error: aborting due to 18 previous errors
 
 Some errors have detailed explanations: E0049, E0195, E0391.
 For more information about an error, try `rustc --explain E0049`.

tests/ui/impl-trait/divergence.rs

@@ -1,6 +1,5 @@
-//@ check-pass
-
 fn foo() -> impl MyTrait {
+    //~^ ERROR undefined opaque type
     panic!();
     MyStruct
 }

tests/ui/impl-trait/divergence.stderr

@@ -0,0 +1,8 @@
+error: undefined opaque type
+  --> $DIR/divergence.rs:1:13
+   |
+LL | fn foo() -> impl MyTrait {
+   |             ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/impl-trait/impl-fn-hrtb-bounds.rs

@@ -18,6 +18,7 @@ fn c() -> impl for<'a> Fn(&'a u8) -> (impl Debug + '_) {
 
 fn d() -> impl Fn() -> (impl Debug + '_) {
     //~^ ERROR missing lifetime specifier
+    //~| ERROR undefined opaque type
     || ()
 }
 

tests/ui/impl-trait/impl-fn-hrtb-bounds.stderr

@@ -46,7 +46,13 @@ note: lifetime declared here
 LL | fn c() -> impl for<'a> Fn(&'a u8) -> (impl Debug + '_) {
    |                    ^^
 
-error: aborting due to 4 previous errors
+error: undefined opaque type
+  --> $DIR/impl-fn-hrtb-bounds.rs:19:25
+   |
+LL | fn d() -> impl Fn() -> (impl Debug + '_) {
+   |                         ^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0106, E0657.
 For more information about an error, try `rustc --explain E0106`.

tests/ui/impl-trait/impl-fn-parsing-ambiguities.rs

@@ -9,6 +9,7 @@ fn a() -> impl Fn(&u8) -> impl Debug + '_ {
 
 fn b() -> impl Fn() -> impl Debug + Send {
     //~^ ERROR ambiguous `+` in a type
+    //~| ERROR undefined opaque type
     || ()
 }
 

tests/ui/impl-trait/impl-fn-parsing-ambiguities.stderr

@@ -22,6 +22,12 @@ note: lifetime declared here
 LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
    |                   ^
 
-error: aborting due to 3 previous errors
+error: undefined opaque type
+  --> $DIR/impl-fn-parsing-ambiguities.rs:10:24
+   |
+LL | fn b() -> impl Fn() -> impl Debug + Send {
+   |                        ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0657`.

tests/ui/impl-trait/impl_fn_associativity.rs

@@ -1,16 +1,19 @@
-//@ run-pass
 #![feature(impl_trait_in_fn_trait_return)]
 use std::fmt::Debug;
 
 fn f_debug() -> impl Fn() -> impl Debug {
+    //~^ ERROR undefined opaque type
     || ()
 }
 
 fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+    //~^ ERROR undefined opaque type
+    //~| ERROR undefined opaque type
     || f_debug()
 }
 
 fn multi() -> impl Fn() -> (impl Debug + Send) {
+    //~^ ERROR undefined opaque type
     || ()
 }
 

tests/ui/impl-trait/impl_fn_associativity.stderr

@@ -0,0 +1,26 @@
+error: undefined opaque type
+  --> $DIR/impl_fn_associativity.rs:4:30
+   |
+LL | fn f_debug() -> impl Fn() -> impl Debug {
+   |                              ^^^^^^^^^^
+
+error: undefined opaque type
+  --> $DIR/impl_fn_associativity.rs:9:31
+   |
+LL | fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: undefined opaque type
+  --> $DIR/impl_fn_associativity.rs:9:44
+   |
+LL | fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+   |                                            ^^^^^^^^^^
+
+error: undefined opaque type
+  --> $DIR/impl_fn_associativity.rs:15:29
+   |
+LL | fn multi() -> impl Fn() -> (impl Debug + Send) {
+   |                             ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+

tests/ui/impl-trait/impl_trait_projections.rs

@@ -10,20 +10,21 @@ fn path_parametrized_type_is_allowed() -> option::Option<impl Debug> {
 }
 
 fn projection_is_disallowed(x: impl Iterator) -> <impl Iterator>::Item {
-//~^ ERROR `impl Trait` is not allowed in path parameters
-//~| ERROR `impl Trait` is not allowed in path parameters
+    //~^ ERROR `impl Trait` is not allowed in path parameters
+    //~| ERROR `impl Trait` is not allowed in path parameters
     x.next().unwrap()
 }
 
-fn projection_with_named_trait_is_disallowed(mut x: impl Iterator)
-    -> <impl Iterator as Iterator>::Item
+fn projection_with_named_trait_is_disallowed(
+    mut x: impl Iterator,
+) -> <impl Iterator as Iterator>::Item
 //~^ ERROR `impl Trait` is not allowed in path parameters
 {
     x.next().unwrap()
 }
 
 fn projection_with_named_trait_inside_path_is_disallowed()
-    -> <::std::ops::Range<impl Debug> as Iterator>::Item
+-> <::std::ops::Range<impl Debug> as Iterator>::Item
 //~^ ERROR `impl Trait` is not allowed in path parameters
 //~| ERROR `impl Debug: Step` is not satisfied
 {
@@ -32,8 +33,9 @@ fn projection_with_named_trait_inside_path_is_disallowed()
 }
 
 fn projection_from_impl_trait_inside_dyn_trait_is_disallowed()
-    -> <dyn Iterator<Item = impl Debug> as Iterator>::Item
+-> <dyn Iterator<Item = impl Debug> as Iterator>::Item
 //~^ ERROR `impl Trait` is not allowed in path parameters
+//~| ERROR undefined opaque type
 {
     panic!()
 }