@main: Attribute to add an entry point to a type. #30693
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When a type (class, enum, or struct) is annotated @main, it is required to provide a function with the following signature: static func main() -> () That function will be called when the executable the type is defined within is launched.
Previously, the function was being added to the list of top level decls in the source file as that list was being iterated, leading to iterator issues. Here, the function is instead added to the nominal type decl which is decorated with @main. Doing so requires calling the $main function with the metatype for the nominal type. The workaround for the iterator invalidation issue which had been applied previously, namely changing the type of the Decls member of SourceFile from std::vector<Decl *> to SmallVector<Decl *, 16> is reverted.
Previously the @main attribute could only be applied to NominalTypeDecls. Here, that limitation is lifted so that the attribute can be applied to ExtensionDecls.
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theblixguy
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swift evolution approved
SE-0281 was accepted with the modification that the main function should
be allowed to be throwing. Here support for enabling that is added.
Support is implemented in two steps:
(1) The $main wrapper function is modified to be throwing
static func $main() throws {
return try main()
}
whenever the main function is throwing (it remains non-throwing when
$main is not throwing).
(2) The @main entry point is modified to be
sil [ossa] @main : $@convention(c) (Int32, UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>) -> Int32 {
entry(%argc : $Int32, %argv : $UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>):
%the_main_type = metatype $@thin TheMainType.Type
%the_main_func = function_ref @`TheMainType.main()` : $@convention(method) (@thin TheMainType.Type) -> @error Error
try_apply %the_main_func(%the_main_type) : $@convention(method) (@thin TheMainType.Type) -> @error Error, normal success, error failure
success(%_ : $()):
%success_code_builtin_int32 = integer_literal $Builtin.Int32, 0
br bb1(%success_code_builtin_int32 : $Builtin.Int32)
failure(%error : @owned $Error):
%_ = builtin "errorInMain"(%error : $Error) : $()
end_lifetime %error : $Error
%error_code_builtin_int32 = integer_literal $Builtin.Int32, 1
br bb1(%error_code_builtin_int32 : $Builtin.Int32)
exit(%code_builtin_int32 : $Builtin.Int32):
%code = struct $Int32 (%code_builtin_int32 : $Builtin.Int32)
return %code : $Int32
}
whenever the main function is throwing (and consequently $main also is).
In the non-throwing case, (a) the try_apply instruction is replaced with an
apply instruction, (b) the body of the success block is appended to the
entry block, and (c) the success and failure blocks are removed.
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Here a new attribute @main is added. The type or extension that it is added to must have a static function
() -> Voidor() throws -> Voidvisible from the attribute. If it does, that function will be called at process launch. The same restrictions that apply to the existing@UIApplicationMainand@NSApplicationMainapply to the new attribute as well.