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Prevent Error::type_id overrides #60902

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Merged
merged 2 commits into from
May 17, 2019
Merged

Prevent Error::type_id overrides #60902

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e836a4c
Prevent Error::type_id overrides
sfackler May 17, 2019
686a611
Update src/libstd/error.rs
sfackler May 17, 2019
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12 changes: 10 additions & 2 deletions src/libstd/error.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -201,11 +201,19 @@ pub trait Error: Debug + Display {
#[unstable(feature = "error_type_id",
reason = "this is memory unsafe to override in user code",
issue = "60784")]
fn type_id(&self) -> TypeId where Self: 'static {
fn type_id(&self, _: private::Internal) -> TypeId where Self: 'static {
TypeId::of::<Self>()
}
}

mod private {
// This is a hack to prevent `type_id` from being overridden by `Error`
// implementations, since that can enable unsound downcasting.
#[unstable(feature = "error_type_id", issue = "60784")]
#[derive(Debug)]
pub struct Internal;
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Would pub(crate) be a better option?

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No, that violates the private types in public interfaces check.

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Hypothetically, what would happen if somebody fixed this technical loophole? Would the fix then be blocked on the grounds of this workaround-hack?

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cc @petrochenkov

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Fixed which loophole? Allowing such a pseudo public type in an exported interface? Fixing that would likely require a new edition, as many libraries already take advantage.

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This is not a loophole - it was the explicit design of the relevant RFC: https://github.com/rust-lang/rfcs/blob/26197104b7bb9a5a35db243d639aee6e46d35d75/text/0136-no-privates-in-public.md#when-is-an-item-public

}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
/// Converts a type of [`Error`] into a box of dyn [`Error`].
Expand Down Expand Up @@ -575,7 +583,7 @@ impl dyn Error + 'static {
let t = TypeId::of::<T>();

// Get TypeId of the type in the trait object
let boxed = self.type_id();
let boxed = self.type_id(private::Internal);

// Compare both TypeIds on equality
t == boxed
Expand Down