Make namelookup not allergic to async main #40577
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etcwilde
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Expecting this to fail on macOS too. |
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The typesystem was avoiding the async main function if any synchronous main was available. This is because the decl contexts where the main function is looked up from aren't async contexts (extension contexts, generic type contexts), so if a synchronous main function exists anywhere, it will be a better "fit" for the context. Since we're the main function, we decide on whether or not we want to be async, so looking at the decl-context's asyncness is really the wrong thing to do. Plumbing this info through the constraint system seems risky at best, catastrophic at worst, so instead, I'm just grabbing the buffer of resolved decls and doing my own scoring and selection. We want to ensure that we are picking the main function that is closest to the decl marked with `@main` in the protocol conformance chain. This means that if your `@main` struct directly conforms to a protocol that implements a main function, you should probably use it. If that protocol implements two main functions, one sync and one async, we look at whether we support async before selecting it. Things get a little bit weird if you have implemented an async main in the closest protocol, but are targeting something that doesn't support concurrency. The `@available` checking should handle this with a nice error saying you're doing bad things, but if you circumvent that checking for bad reasons, it will still fall back on a synchronous main function if one is available.
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We won't look for async functions that can't be used due to availability unless the availability checking is disabled. Need to disable the availability checking due to the minimum deploy target being too low for concurrency.
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Fixing testing for different platforms. MacOS and iOS have backdeployment and availability to consider. Other platforms don't need to worry about that because they need to include the swift runtime in the installation package anyway, so the concurrency bits can ship with the installation.
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Feb 26, 2022
This is effectively reverting swiftlang#40577. The mechanism that I tried to use didn't respect `where` clauses on extensions and, as a result, could select a totally unrelated and incorrect main function. This results in beautiful explosions in the rest of the typesystem.
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The typesystem was avoiding the async main function if any synchronous
main was available. This is because the decl contexts where the main
function is looked up from aren't async contexts (extension contexts,
generic type contexts), so if a synchronous main function exists
anywhere, it will be a better "fit" for the context.
Since we're the main function, we decide on whether or not we want to be
async, so looking at the decl-context's asyncness is really the wrong
thing to do. Plumbing this info through the constraint system seems
risky at best, catastrophic at worst, so instead, I'm just grabbing the
buffer of resolved decls and doing my own scoring and selection.
We want to ensure that we are picking the main function that is closest
to the decl marked with
@mainin the protocol conformance chain. Thismeans that if your
@mainstruct directly conforms to a protocol thatimplements a main function, you should probably use it. If that protocol
implements two main functions, one sync and one async, we look at
whether we support async before selecting it.
Things get a little bit weird if you have implemented an async main in
the closest protocol, but are targeting something that doesn't support
concurrency. The
@availablechecking should handle this with a niceerror saying you're doing bad things, but if you circumvent that
checking for bad reasons, it will still fall back on a synchronous main
function if one is available.
https://bugs.swift.org/browse/SR-15211
rdar://83316060