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Move to using an extern type for opaqueness
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This prevents accidental dereferences and so forth of the Void type, as well as
cleaning up the error message to reference Opaque rather than the more
complicated PhantomData type.
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Mark-Simulacrum committed Feb 17, 2020
1 parent 6c45e45 commit 34ef8f5
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Showing 2 changed files with 12 additions and 24 deletions.
16 changes: 4 additions & 12 deletions src/libcore/fmt/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -238,16 +238,8 @@ pub struct Formatter<'a> {
// NB. Argument is essentially an optimized partially applied formatting function,
// equivalent to `exists T.(&T, fn(&T, &mut Formatter<'_>) -> Result`.

struct Void {
_priv: (),
/// Erases all oibits, because `Void` erases the type of the object that
/// will be used to produce formatted output. Since we do not know what
/// oibits the real types have (and they can have any or none), we need to
/// take the most conservative approach and forbid all oibits.
///
/// It was added after #45197 showed that one could share a `!Sync`
/// object across threads by passing it into `format_args!`.
_oibit_remover: PhantomData<*mut dyn Fn()>,
extern "C" {
type Opaque;
}

/// This struct represents the generic "argument" which is taken by the Xprintf
Expand All @@ -259,8 +251,8 @@ struct Void {
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
#[doc(hidden)]
pub struct ArgumentV1<'a> {
value: &'a Void,
formatter: fn(&Void, &mut Formatter<'_>) -> Result,
value: &'a Opaque,
formatter: fn(&Opaque, &mut Formatter<'_>) -> Result,
}

impl<'a> ArgumentV1<'a> {
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20 changes: 8 additions & 12 deletions src/test/ui/fmt/send-sync.stderr
Original file line number Diff line number Diff line change
@@ -1,34 +1,30 @@
error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
error[E0277]: `core::fmt::Opaque` cannot be shared between threads safely
--> $DIR/send-sync.rs:8:5
|
LL | fn send<T: Send>(_: T) {}
| ---- ---- required by this bound in `send`
...
LL | send(format_args!("{:?}", c));
| ^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
| ^^^^ `core::fmt::Opaque` cannot be shared between threads safely
|
= help: within `[std::fmt::ArgumentV1<'_>]`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
= note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
= note: required because it appears within the type `core::fmt::Void`
= note: required because it appears within the type `&core::fmt::Void`
= help: within `[std::fmt::ArgumentV1<'_>]`, the trait `std::marker::Sync` is not implemented for `core::fmt::Opaque`
= note: required because it appears within the type `&core::fmt::Opaque`
= note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
= note: required because it appears within the type `[std::fmt::ArgumentV1<'_>]`
= note: required because of the requirements on the impl of `std::marker::Send` for `&[std::fmt::ArgumentV1<'_>]`
= note: required because it appears within the type `std::fmt::Arguments<'_>`

error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
error[E0277]: `core::fmt::Opaque` cannot be shared between threads safely
--> $DIR/send-sync.rs:9:5
|
LL | fn sync<T: Sync>(_: T) {}
| ---- ---- required by this bound in `sync`
...
LL | sync(format_args!("{:?}", c));
| ^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
| ^^^^ `core::fmt::Opaque` cannot be shared between threads safely
|
= help: within `std::fmt::Arguments<'_>`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
= note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
= note: required because it appears within the type `core::fmt::Void`
= note: required because it appears within the type `&core::fmt::Void`
= help: within `std::fmt::Arguments<'_>`, the trait `std::marker::Sync` is not implemented for `core::fmt::Opaque`
= note: required because it appears within the type `&core::fmt::Opaque`
= note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
= note: required because it appears within the type `[std::fmt::ArgumentV1<'_>]`
= note: required because it appears within the type `&[std::fmt::ArgumentV1<'_>]`
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