Do not try to reveal hidden types when trying to prove Freeze in the defining scope

Author: oli-obk

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -98,7 +98,33 @@ impl Qualif for HasMutInterior {
     }
 
     fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
-        !ty.is_freeze(cx.tcx, cx.param_env)
+        // Avoid selecting for simple cases, such as builtin types.
+        if ty.is_trivially_freeze() {
+            return false;
+        }
+
+        // We do not use `ty.is_freeze` here, because that requires revealing opaque types, which
+        // requires borrowck, which in turn will invoke mir_const_qualifs again, causing a cycle error.
+        // Instead we invoke an obligation context manually, and provide the opaque type inference settings
+        // that allow the trait solver to just error out instead of cycling.
+        let freeze_def_id = cx.tcx.require_lang_item(LangItem::Freeze, Some(cx.body.span));
+
+        let obligation = Obligation::new(
+            cx.tcx,
+            ObligationCause::dummy_with_span(cx.body.span),
+            cx.param_env,
+            ty::TraitRef::new(cx.tcx, freeze_def_id, [ty::GenericArg::from(ty)]),
+        );
+
+        let infcx = cx
+            .tcx
+            .infer_ctxt()
+            .with_opaque_type_inference(cx.body.source.def_id().expect_local())
+            .build();
+        let ocx = ObligationCtxt::new(&infcx);
+        ocx.register_obligation(obligation);
+        let errors = ocx.select_all_or_error();
+        !errors.is_empty()
     }
 
     fn in_adt_inherently<'tcx>(

compiler/rustc_middle/src/ty/util.rs

@@ -1232,7 +1232,7 @@ impl<'tcx> Ty<'tcx> {
     ///
     /// Returning true means the type is known to be `Freeze`. Returning
     /// `false` means nothing -- could be `Freeze`, might not be.
-    fn is_trivially_freeze(self) -> bool {
+    pub fn is_trivially_freeze(self) -> bool {
         match self.kind() {
             ty::Int(_)
             | ty::Uint(_)

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2382,13 +2382,18 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             }
 
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                // We can resolve the `impl Trait` to its concrete type,
-                // which enforces a DAG between the functions requiring
-                // the auto trait bounds in question.
-                match self.tcx().type_of_opaque(def_id) {
-                    Ok(ty) => t.rebind(vec![ty.instantiate(self.tcx(), args)]),
-                    Err(_) => {
-                        return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
+                if self.infcx.can_define_opaque_ty(def_id) {
+                    // We cannot possibly resolve this opaque type, because we are currently computing its hidden type.
+                    return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
+                } else {
+                    // We can resolve the `impl Trait` to its concrete type,
+                    // which enforces a DAG between the functions requiring
+                    // the auto trait bounds in question.
+                    match self.tcx().type_of_opaque(def_id) {
+                        Ok(ty) => t.rebind(vec![ty.instantiate(self.tcx(), args)]),
+                        Err(_) => {
+                            return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
+                        }
                     }
                 }
             }

tests/ui/const-generics/opaque_types.stderr

@@ -109,8 +109,6 @@ note: ...which requires const checking `main::{constant#0}`...
    |
 LL |     foo::<42>();
    |           ^^
-   = note: ...which requires computing whether `Foo` is freeze...
-   = note: ...which requires evaluating trait selection obligation `Foo: core::marker::Freeze`...
    = note: ...which again requires computing type of opaque `Foo::{opaque#0}`, completing the cycle
 note: cycle used when computing type of `Foo::{opaque#0}`
   --> $DIR/opaque_types.rs:3:12

tests/ui/impl-trait/rpit/const_check_false_cycle.current.stderr

@@ -1,13 +1,12 @@
-//! This test causes a cycle error when checking whether the
+//! This test caused a cycle error when checking whether the
 //! return type is `Freeze` during const checking, even though
 //! the information is readily available.
 
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
-//@[next] check-pass
+//@ check-pass
 
 const fn f() -> impl Eq {
-    //[current]~^ ERROR cycle detected
     g()
 }
 const fn g() {}

tests/ui/impl-trait/rpit/const_check_false_cycle.rs

@@ -3,7 +3,6 @@
 const fn test() -> impl ~const Fn() {
     //~^ ERROR `~const` can only be applied to `#[const_trait]` traits
     //~| ERROR `~const` can only be applied to `#[const_trait]` traits
-    //~| ERROR cycle detected
     const move || { //~ ERROR const closures are experimental
         let sl: &[u8] = b"foo";
 

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/ice-112822-expected-type-for-param.rs

@@ -1,5 +1,5 @@
 error[E0658]: const closures are experimental
-  --> $DIR/ice-112822-expected-type-for-param.rs:7:5
+  --> $DIR/ice-112822-expected-type-for-param.rs:6:5
    |
 LL |     const move || {
    |     ^^^^^
@@ -23,46 +23,15 @@ LL | const fn test() -> impl ~const Fn() {
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0277]: can't compare `&u8` with `&u8`
-  --> $DIR/ice-112822-expected-type-for-param.rs:12:17
+  --> $DIR/ice-112822-expected-type-for-param.rs:11:17
    |
 LL |                 assert_eq!(first, &b'f');
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `&u8 == &u8`
    |
    = help: the trait `~const PartialEq<&u8>` is not implemented for `&u8`
    = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0391]: cycle detected when computing type of opaque `test::{opaque#0}`
-  --> $DIR/ice-112822-expected-type-for-param.rs:3:20
-   |
-LL | const fn test() -> impl ~const Fn() {
-   |                    ^^^^^^^^^^^^^^^^
-   |
-note: ...which requires borrow-checking `test`...
-  --> $DIR/ice-112822-expected-type-for-param.rs:3:1
-   |
-LL | const fn test() -> impl ~const Fn() {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: ...which requires promoting constants in MIR for `test`...
-  --> $DIR/ice-112822-expected-type-for-param.rs:3:1
-   |
-LL | const fn test() -> impl ~const Fn() {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: ...which requires const checking `test`...
-  --> $DIR/ice-112822-expected-type-for-param.rs:3:1
-   |
-LL | const fn test() -> impl ~const Fn() {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   = note: ...which requires computing whether `test::{opaque#0}` is freeze...
-   = note: ...which requires evaluating trait selection obligation `test::{opaque#0}: core::marker::Freeze`...
-   = note: ...which again requires computing type of opaque `test::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `test::{opaque#0}`
-  --> $DIR/ice-112822-expected-type-for-param.rs:3:20
-   |
-LL | const fn test() -> impl ~const Fn() {
-   |                    ^^^^^^^^^^^^^^^^
-   = 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: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0277, E0391, E0658.
+Some errors have detailed explanations: E0277, E0658.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/ice-112822-expected-type-for-param.stderr

@@ -4,7 +4,6 @@
 #![feature(type_alias_impl_trait)]
 type Bar = impl Sized;
 //~^ ERROR: cycle
-//~| ERROR: cycle
 
 fn foo() -> Bar
 where

tests/ui/type-alias-impl-trait/in-where-clause.rs

@@ -10,7 +10,7 @@ note: ...which requires computing type of opaque `Bar::{opaque#0}`...
 LL | type Bar = impl Sized;
    |            ^^^^^^^^^^
 note: ...which requires type-checking `foo`...
-  --> $DIR/in-where-clause.rs:9:1
+  --> $DIR/in-where-clause.rs:8:1
    |
 LL | / fn foo() -> Bar
 LL | | where
@@ -25,26 +25,6 @@ LL | type Bar = impl Sized;
    |            ^^^^^^^^^^
    = 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 `Bar::{opaque#0}`
-  --> $DIR/in-where-clause.rs:5:12
-   |
-LL | type Bar = impl Sized;
-   |            ^^^^^^^^^^
-   |
-note: ...which requires type-checking `foo`...
-  --> $DIR/in-where-clause.rs:13:9
-   |
-LL |     [0; 1 + 2]
-   |         ^^^^^
-   = note: ...which requires evaluating trait selection obligation `Bar: core::marker::Send`...
-   = note: ...which again requires computing type of opaque `Bar::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `Bar::{opaque#0}`
-  --> $DIR/in-where-clause.rs:5:12
-   |
-LL | type Bar = impl Sized;
-   |            ^^^^^^^^^^
-   = 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: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0391`.