Proposal: Attribute noreturn functions

[riverar]

I propose that win32metadata emit an attribute (e.g. Windows.Win32.Interop.NoReturnAttribute) attached to functions and methods that do not return to the caller (e.g. FatalExit). This will allow languages, such as Rust (never, !), to generate smarter bindings, enabling better optimization of noreturn code paths, detection of dead code paths (that is, code that is mistakenly placed after the call of a noreturn function), and other benefits.

Some noreturn APIs are already annotated in the source material.

Continuing with the FatalExit example:

WINBASEAPI __analysis_noreturn VOID WINAPI FatalExit(
    _In_ int ExitCode
);

Some other forms found in the source material to be aware of:

_Analysis_noreturn_
DECLSPEC_NORETURN
__declspec(noreturn)

A theoretical example of how this could look in metadata:

[NoReturn]
[DllImport("KERNEL32", ExactSpelling = true)]
[SupportedOSPlatform("windows5.1.2600")]
public static extern void FatalExit([In] int ExitCode);

[AArnott]

Bonus points if the metadata can reuse the DoesNotReturnAttribute.

[riverar]

@AArnott Good catch. I was looking for an existing attribute and missed this one. I would definitely prefer this over a new one.

[mikebattista]

Assigning to @sotteson1 to see about scraping this from the headers and emitting as DoesNotReturnAttribute.

[sotteson1]

I'm wondering why a function that returns void needs an attribute to indicate it doesn't return anything. Isn't that what returning void means?

[kennykerr]

It's not that it doesn't return anything - it's that it doesn't return at all. Functions like abort, std::terminate, etc. will all result in some kind of process termination and compilers appreciate knowing this. Here's the C++ version of this:

https://en.cppreference.com/w/cpp/language/attributes/noreturn

[sotteson1]

Oh, got it.

[mikebattista]

https://task.ms/37680894

[kennykerr]

RaiseException is still lacking the DoesNotReturn attribute.