Re: [PATCH] Suppress warning in rust-ast-lower-type.h ASTLowerGenericParam.visit.

[Marc <dkm@kataplop.net>]

Mark Wielaard <mark@klomp.org> writes:

> On Tue, Jun 29, 2021 at 12:47:07AM +0200, Mark Wielaard wrote:
>> On Tue, Jun 29, 2021 at 12:06:56AM +0200, Marc wrote:
>> > Hi,
>> > 
>> > > Translating the AST LifetimeType to the HIR LifetimeType causes a warning:
>> > > warning: ‘ltt’ may be used uninitialized
>> > 
>> > Was wondering why this is needed as the switch case covers all enum
>> > variants, how can ltt be uninitialized ? I have the same fix locally but
>> > was thinking something else was causing the error...
>> 
>> LifetimeType is a plain enum, which aren't really their own types,
>> they are really just ints with fancy names. We could make them enum
>> class, which is a strong type. Then the compiler would know the switch
>> really covers all enum (class) variants. But then we have to provide
>> the right scope/type everywhere we use them in the code (which might
>> be a good idea, but is more typing).
>
> I just tried to make LifetimeType an enum class and that doesn't help.
> So I was wrong. I don't know why the compiler doesn't see this? It
> should know since if not all switch cases were covered, -Wswitch
> (enabled by -Wall) gives us a warning... So, I don't fully understand
> why gcc needs the default gcc_unreachable case. It is what is used in
> the rest of the code though.

I thought maybe that's a C++ vs C diff, or something caused by the
Lifetime being returned by a function call, but I can't reproduce it, so
that must be something else:

https://godbolt.org/z/sjbcWEqdj

Anyway, Philipp wants to have these enum shared between AST and HIR, so
this kind of 'if(AST::Foo) t=HIR::Foo' can be removed.

I'm still curious about why the warning is raised in this case...

Marc