Pessimistically assume opaque types are !Freeze

Author: oli-obk

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

@@ -7,6 +7,7 @@ use rustc_hir::LangItem;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir;
 use rustc_middle::mir::*;
+use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
 use rustc_trait_selection::traits::{
     self, ImplSource, Obligation, ObligationCause, ObligationCtxt, SelectionContext,
@@ -91,7 +92,8 @@ 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)
+        // Pessimistically assume opaque types are `!Freeze`
+        ty.has_opaque_types() || !ty.is_freeze(cx.tcx, cx.param_env)
     }
 
     fn in_adt_inherently<'tcx>(

tests/ui/consts/const-fn-cycle-tait.rs

@@ -0,0 +1,17 @@
+#![feature(type_alias_impl_trait)]
+
+//! Test that we pessimistically assume the `Drop` impl of
+//! a hidden type is not const.
+
+pub struct Parser<H>(H);
+
+type Tait = impl Sized;
+
+const fn constrain() -> Tait {}
+
+pub const fn take(_: Tait) {}
+//~^ ERROR: destructor of `Tait` cannot be evaluated at compile-time
+
+fn main() {
+    println!("Hello, world!");
+}

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

@@ -0,0 +1,11 @@
+error[E0493]: destructor of `Tait` cannot be evaluated at compile-time
+  --> $DIR/const-fn-cycle-tait.rs:12:19
+   |
+LL | pub const fn take(_: Tait) {}
+   |                   ^         - value is dropped here
+   |                   |
+   |                   the destructor for this type cannot be evaluated in constant functions
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0493`.

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

@@ -1,11 +1,13 @@
-/// Discovered in https://github.com/rust-lang/rust/issues/112602.
-/// This caused a cycle error, which made no sense.
-/// Removing the `const` part of the `many` function would make the
-/// test pass again.
-/// The issue was that we were running const qualif checks on
-/// `const fn`s, but never using them. During const qualif checks we tend
-/// to end up revealing opaque types (the RPIT in `many`'s return type),
-/// which can quickly lead to cycles.
+//! Discovered in https://github.com/rust-lang/rust/issues/112602.
+//! This caused a cycle error, which made no sense.
+//! Removing the `const` part of the `many` function would make the
+//! test pass again.
+//! The issue was that we were running const qualif checks on
+//! `const fn`s, but never using them. During const qualif checks we tend
+//! 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);
 
@@ -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

@@ -1,39 +1,41 @@
-error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:13:12
+error[E0493]: destructor of `Foo` cannot be evaluated at compile-time
+  --> $DIR/const-promoted-opaque.rs:25:26
    |
-LL | type Foo = impl Sized;
-   |            ^^^^^^^^^^
-   |
-note: ...which requires borrow-checking `FOO`...
-  --> $DIR/const-promoted-opaque.rs:20:1
-   |
-LL | const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
-   | ^^^^^^^^^^^^^^
-note: ...which requires promoting constants in MIR for `FOO`...
-  --> $DIR/const-promoted-opaque.rs:20:1
+LL |     let _: &'static _ = &FOO;
+   |                          ^^^ the destructor for this type cannot be evaluated in constants
+...
+LL | };
+   | - value is dropped here
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/const-promoted-opaque.rs:25:26
    |
-LL | const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
-   | ^^^^^^^^^^^^^^
-note: ...which requires const checking `FOO`...
-  --> $DIR/const-promoted-opaque.rs:20:1
+LL |     let _: &'static _ = &FOO;
+   |            ----------    ^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+...
+LL | };
+   | - temporary value is freed at the end of this statement
+
+error[E0492]: constants cannot refer to interior mutable data
+  --> $DIR/const-promoted-opaque.rs:30:19
    |
-LL | const FOO: Foo = std::sync::atomic::AtomicU8::new(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 `Foo::{opaque#0}`, completing the cycle
-note: cycle used when checking item types in top-level module
-  --> $DIR/const-promoted-opaque.rs:2:1
+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:34:26
    |
-LL | / #![feature(type_alias_impl_trait)]
-LL | |
-LL | | //! Check that we do not cause cycle errors when trying to
-LL | | //! obtain information about interior mutability of an opaque type.
-...  |
-LL | |     let _: &'static _ = &FOO;
-LL | | }
-   | |_^
+LL |     let _: &'static _ = &FOO;
+   |            ----------    ^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0391`.
+Some errors have detailed explanations: E0492, E0493, E0716.
+For more information about an error, try `rustc --explain E0492`.

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

@@ -11,7 +11,6 @@
 //[unit] check-pass
 
 type Foo = impl Sized;
-//[string,atomic]~^ ERROR cycle detected
 
 #[cfg(string)]
 const FOO: Foo = String::new();
@@ -24,10 +23,14 @@ const FOO: Foo = ();
 
 const BAR: () = {
     let _: &'static _ = &FOO;
+    //[string,atomic]~^ ERROR: destructor of `Foo` cannot be evaluated at compile-time
+    //[string,atomic]~| ERROR: temporary value dropped while borrowed
 };
 
 const BAZ: &Foo = &FOO;
+//[string,atomic]~^ ERROR: constants cannot refer to interior mutable data
 
 fn main() {
     let _: &'static _ = &FOO;
+    //[string,atomic]~^ ERROR: temporary value dropped while borrowed
 }

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

@@ -1,39 +1,41 @@
-error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
-  --> $DIR/const-promoted-opaque.rs:13:12
+error[E0493]: destructor of `Foo` cannot be evaluated at compile-time
+  --> $DIR/const-promoted-opaque.rs:25:26
    |
-LL | type Foo = impl Sized;
-   |            ^^^^^^^^^^
-   |
-note: ...which requires borrow-checking `FOO`...
-  --> $DIR/const-promoted-opaque.rs:17:1
-   |
-LL | const FOO: Foo = String::new();
-   | ^^^^^^^^^^^^^^
-note: ...which requires promoting constants in MIR for `FOO`...
-  --> $DIR/const-promoted-opaque.rs:17:1
+LL |     let _: &'static _ = &FOO;
+   |                          ^^^ the destructor for this type cannot be evaluated in constants
+...
+LL | };
+   | - value is dropped here
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/const-promoted-opaque.rs:25:26
    |
-LL | const FOO: Foo = String::new();
-   | ^^^^^^^^^^^^^^
-note: ...which requires const checking `FOO`...
-  --> $DIR/const-promoted-opaque.rs:17:1
+LL |     let _: &'static _ = &FOO;
+   |            ----------    ^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+...
+LL | };
+   | - temporary value is freed at the end of this statement
+
+error[E0492]: constants cannot refer to interior mutable data
+  --> $DIR/const-promoted-opaque.rs:30:19
    |
-LL | const FOO: Foo = String::new();
-   | ^^^^^^^^^^^^^^
-   = 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 `Foo::{opaque#0}`, completing the cycle
-note: cycle used when checking item types in top-level module
-  --> $DIR/const-promoted-opaque.rs:2:1
+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:34:26
    |
-LL | / #![feature(type_alias_impl_trait)]
-LL | |
-LL | | //! Check that we do not cause cycle errors when trying to
-LL | | //! obtain information about interior mutability of an opaque type.
-...  |
-LL | |     let _: &'static _ = &FOO;
-LL | | }
-   | |_^
+LL |     let _: &'static _ = &FOO;
+   |            ----------    ^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0391`.
+Some errors have detailed explanations: E0492, E0493, E0716.
+For more information about an error, try `rustc --explain E0492`.