Add StrictSendableMetatypes to require Sendable requirements on metatypes #79352
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…ypes
Introduce a new experimental feature StrictSendableMetatypes that stops
treating all metatypes as `Sendable`. Instead, metatypes of generic
parameters and existentials are only considered Sendable if their
corresponding instance types are guaranteed to be Sendable.
Start with enforcing this property within region isolation. Track
metatype creation instructions and put them in the task's isolation
domain, so that transferring them into another isolation domain
produces a diagnostic. As an example:
func f<T: P>(_: T.Type) {
let x: P.Type = T.self
Task.detached {
x.someStaticMethod() // oops, T.Type is not Sendable
}
}
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…ries Keep track of all of the type parameters and archetypes that are captured by a local function or closure. Use that information to diagnose cases where a non-Sendable metatype crosses an isolation boundary.
Introduce a marker protocol SendableMetatype that is used to indicate when the metatype of a type will conform to Sendable. Specifically, `T: SendableMetatype` implies `T.Type: Sendable`. When strict metatype sendability is enabled, metatypes are only sendable when `T: SendableMetatype`. All nominal types implicitly conform to `SendableMetatype`, as do the various builtin types, function types, etc. The `Sendable` marker protocol now inherits from `SendableMetatype`, so that `T: Sendable` implies `T.Type: Sendable`. Thank you Slava for the excellent idea!
DougGregor
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glessard
added a commit
to glessard/swift
that referenced
this pull request
Feb 15, 2025
This change is a partial cherry-pick from commit e24598b found in swiftlang#79352
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Introduce a new experimental feature StrictSendableMetatypes that stops treating all metatypes as
Sendable. Instead, metatypes of generic parameters and existentials are only considered Sendable if their corresponding instance types are guaranteed to be Sendable.Enforce this property within region isolation. Track metatype creation instructions and put them in the task's isolation domain, so that transferring them into another isolation domain produces a diagnostic. As an example:
At the type checker level, handle direct attempts to capture metatypes by tracking generic type captures within closures, so that we can diagnose, e.g.,
Model the sendability of a type's metatype with the new marker protocol
SendableMetatype, whichSendablenow inherits from and which is used to determine when a metatype is Sendable. Specifically,T: SendableMetatypeimpliesT.Type: Sendable, on which this analysis is based. Thank you to @slavapestov for the insight into this approach!