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

Author: oli-obk

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -100,7 +100,33 @@ 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

@@ -1268,7 +1268,7 @@ 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

@@ -2385,13 +2385,18 @@ 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

@@ -122,8 +122,6 @@ 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/consts/const-fn-cycle.rs

@@ -7,6 +7,8 @@
 /// to end up revealing opaque types (the RPIT in `many`'s return type),
 /// which can quickly lead to cycles.
 
+//@ check-pass
+
 pub struct Parser<H>(H);
 
 impl<H, T> Parser<H>
@@ -18,7 +20,6 @@ where
     }
 
     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
-        //~^ ERROR: cycle detected
         Parser::new(|_| unimplemented!())
     }
 }

tests/ui/consts/const-fn-cycle.stderr

@@ -1,5 +1,5 @@
 error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
-  --> $DIR/const-promoted-opaque.rs:29:25
+  --> $DIR/const-promoted-opaque.rs:28:25
    |
 LL |     let _: &'static _ = &FOO;
    |                         ^^^^
@@ -9,7 +9,7 @@ LL |     let _: &'static _ = &FOO;
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0493]: destructor of `helper::Foo` cannot be evaluated at compile-time
-  --> $DIR/const-promoted-opaque.rs:29:26
+  --> $DIR/const-promoted-opaque.rs:28:26
    |
 LL |     let _: &'static _ = &FOO;
    |                          ^^^ the destructor for this type cannot be evaluated in constants
@@ -18,13 +18,13 @@ LL | };
    | - value is dropped here
 
 error[E0492]: constants cannot refer to interior mutable data
-  --> $DIR/const-promoted-opaque.rs:34:19
+  --> $DIR/const-promoted-opaque.rs:33:19
    |
 LL | const BAZ: &Foo = &FOO;
    |                   ^^^^ this borrow of an interior mutable value may end up in the final value
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/const-promoted-opaque.rs:38:26
+  --> $DIR/const-promoted-opaque.rs:37:26
    |
 LL |     let _: &'static _ = &FOO;
    |            ----------    ^^^ creates a temporary value which is freed while still in use
@@ -34,38 +34,7 @@ LL |
 LL | }
    | - temporary value is freed at the end of this statement
 
-error[E0391]: cycle detected when computing type of opaque `helper::Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:14:20
-   |
-LL |     pub type Foo = impl Sized;
-   |                    ^^^^^^^^^^
-   |
-note: ...which requires borrow-checking `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:21:5
-   |
-LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
-   |     ^^^^^^^^^^^^^^^^^^
-note: ...which requires promoting constants in MIR for `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:21:5
-   |
-LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
-   |     ^^^^^^^^^^^^^^^^^^
-note: ...which requires const checking `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:21:5
-   |
-LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
-   |     ^^^^^^^^^^^^^^^^^^
-   = note: ...which requires computing whether `helper::Foo` is freeze...
-   = note: ...which requires evaluating trait selection obligation `helper::Foo: core::marker::Freeze`...
-   = note: ...which again requires computing type of opaque `helper::Foo::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `helper::Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:14:20
-   |
-LL |     pub type Foo = 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 5 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0391, E0492, E0493, E0658, E0716.
-For more information about an error, try `rustc --explain E0391`.
+Some errors have detailed explanations: E0492, E0493, E0658, E0716.
+For more information about an error, try `rustc --explain E0492`.

tests/ui/consts/const-promoted-opaque.atomic.stderr

@@ -12,7 +12,6 @@
 
 mod helper {
     pub type Foo = impl Sized;
-    //[string,atomic]~^ ERROR cycle detected
 
     #[cfg(string)]
     pub const FOO: Foo = String::new();
@@ -28,11 +27,11 @@ use helper::*;
 const BAR: () = {
     let _: &'static _ = &FOO;
     //[string,atomic]~^ ERROR: destructor of `helper::Foo` cannot be evaluated at compile-time
-    //[string,atomic]~| ERROR: cannot borrow here
+    //[atomic]~| ERROR: cannot borrow here
 };
 
 const BAZ: &Foo = &FOO;
-//[string,atomic]~^ ERROR: constants cannot refer to interior mutable data
+//[atomic]~^ ERROR: constants cannot refer to interior mutable data
 
 fn main() {
     let _: &'static _ = &FOO;

tests/ui/consts/const-promoted-opaque.rs

@@ -1,30 +1,14 @@
-error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
-  --> $DIR/const-promoted-opaque.rs:29:25
-   |
-LL |     let _: &'static _ = &FOO;
-   |                         ^^^^
-   |
-   = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information
-   = help: add `#![feature(const_refs_to_cell)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
 error[E0493]: destructor of `helper::Foo` cannot be evaluated at compile-time
-  --> $DIR/const-promoted-opaque.rs:29:26
+  --> $DIR/const-promoted-opaque.rs:28:26
    |
 LL |     let _: &'static _ = &FOO;
    |                          ^^^ the destructor for this type cannot be evaluated in constants
 ...
 LL | };
    | - value is dropped here
 
-error[E0492]: constants cannot refer to interior mutable data
-  --> $DIR/const-promoted-opaque.rs:34:19
-   |
-LL | const BAZ: &Foo = &FOO;
-   |                   ^^^^ this borrow of an interior mutable value may end up in the final value
-
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/const-promoted-opaque.rs:38:26
+  --> $DIR/const-promoted-opaque.rs:37:26
    |
 LL |     let _: &'static _ = &FOO;
    |            ----------    ^^^ creates a temporary value which is freed while still in use
@@ -34,38 +18,7 @@ LL |
 LL | }
    | - temporary value is freed at the end of this statement
 
-error[E0391]: cycle detected when computing type of opaque `helper::Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:14:20
-   |
-LL |     pub type Foo = impl Sized;
-   |                    ^^^^^^^^^^
-   |
-note: ...which requires borrow-checking `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:18:5
-   |
-LL |     pub const FOO: Foo = String::new();
-   |     ^^^^^^^^^^^^^^^^^^
-note: ...which requires promoting constants in MIR for `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:18:5
-   |
-LL |     pub const FOO: Foo = String::new();
-   |     ^^^^^^^^^^^^^^^^^^
-note: ...which requires const checking `helper::FOO`...
-  --> $DIR/const-promoted-opaque.rs:18:5
-   |
-LL |     pub const FOO: Foo = String::new();
-   |     ^^^^^^^^^^^^^^^^^^
-   = note: ...which requires computing whether `helper::Foo` is freeze...
-   = note: ...which requires evaluating trait selection obligation `helper::Foo: core::marker::Freeze`...
-   = note: ...which again requires computing type of opaque `helper::Foo::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `helper::Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:14:20
-   |
-LL |     pub type Foo = 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 5 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0391, E0492, E0493, E0658, E0716.
-For more information about an error, try `rustc --explain E0391`.
+Some errors have detailed explanations: E0493, E0716.
+For more information about an error, try `rustc --explain E0493`.

tests/ui/consts/const-promoted-opaque.string.stderr

@@ -1,5 +1,5 @@
 error[E0599]: no method named `my_debug` found for opaque type `impl Debug` in the current scope
-  --> $DIR/call_method_on_inherent_impl_ref.rs:20:11
+  --> $DIR/call_method_on_inherent_impl_ref.rs:19:11
    |
 LL |     fn my_debug(&self);
    |        -------- the method is available for `&impl Debug` here
@@ -14,27 +14,6 @@ note: `MyDebug` defines an item `my_debug`, perhaps you need to implement it
 LL | trait MyDebug {
    | ^^^^^^^^^^^^^
 
-error[E0391]: cycle detected when computing type of opaque `my_foo::{opaque#0}`
-  --> $DIR/call_method_on_inherent_impl_ref.rs:15:16
-   |
-LL | fn my_foo() -> impl std::fmt::Debug {
-   |                ^^^^^^^^^^^^^^^^^^^^
-   |
-note: ...which requires type-checking `my_foo`...
-  --> $DIR/call_method_on_inherent_impl_ref.rs:20:9
-   |
-LL |         x.my_debug();
-   |         ^
-   = note: ...which requires evaluating trait selection obligation `my_foo::{opaque#0}: core::marker::Unpin`...
-   = note: ...which again requires computing type of opaque `my_foo::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `my_foo::{opaque#0}`
-  --> $DIR/call_method_on_inherent_impl_ref.rs:15:16
-   |
-LL | fn my_foo() -> impl std::fmt::Debug {
-   |                ^^^^^^^^^^^^^^^^^^^^
-   = 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
 
-Some errors have detailed explanations: E0391, E0599.
-For more information about an error, try `rustc --explain E0391`.
+For more information about this error, try `rustc --explain E0599`.

tests/ui/impl-trait/call_method_on_inherent_impl_ref.current.stderr

@@ -1,5 +1,5 @@
 error[E0282]: type annotations needed
-  --> $DIR/call_method_on_inherent_impl_ref.rs:18:13
+  --> $DIR/call_method_on_inherent_impl_ref.rs:17:13
    |
 LL |         let x = my_foo();
    |             ^
@@ -13,7 +13,7 @@ LL |         let x: /* Type */ = my_foo();
    |              ++++++++++++
 
 error[E0282]: type annotations needed for `&_`
-  --> $DIR/call_method_on_inherent_impl_ref.rs:28:13
+  --> $DIR/call_method_on_inherent_impl_ref.rs:27:13
    |
 LL |         let x = &my_bar();
    |             ^

tests/ui/impl-trait/call_method_on_inherent_impl_ref.next.stderr

@@ -13,7 +13,6 @@ where
 }
 
 fn my_foo() -> impl std::fmt::Debug {
-    //[current]~^ cycle
     if false {
         let x = my_foo();
         //[next]~^ type annotations needed