translate-c generates zig code with unnecessary _ = foo; statements for unused local variables and parameters

[andrewrk]

void foo(void) {
    int x;
}

This code translates to:

pub export fn foo() void {
    var x: c_int = undefined;
}

If you try to compile it with self-hosted:

$ ./zig build-obj test.zig
test.zig:2:9: error: unused local variable
    var x: c_int = undefined;
        ^

Instead it should generate:

pub export fn foo() void {
    var x: c_int = undefined;
    _ = x;
}

However ideally it would keep track of which variables and parameters are used and not generate unnecessary _ = foo; statements.

This is blocking #9191.

[ehaas]

I think the _ = x; solution would be pretty quick to implement as an unblocker, then maybe we could circle back and make it smarter about not emitting those for used variables?

[andrewrk]

Sounds good to me!

[ehaas]

Cool, can have that to you later tonight

[ehaas]

@andrewrk mostly done; however __USER_LABEL_PREFIX__ is causing a problem on this branch. It gets injected automatically by clang https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

It translates to: pub const __USER_LABEL_PREFIX__ = _;

Which causes

test.zig:273:35: error: '_' may not be used as an identifier
pub const __USER_LABEL_PREFIX__ = _;
                                  ^

Even when the symbol is not otherwise referred to.

Seems like I could either special case that particular macro (skip it) or look for ones that have a single _ identifier as the definition.

[ehaas]

Actually, one other issue which is the removal of packed/extern enums. I have a PR for that already - #9164 - should I bring that in as well?

[ehaas]

OK, PR for this one here: #9209

Assuming it looks good, I can rebase #9164 onto stage1-astcheck after this one gets merged.

[andrewrk]

Seems like I could either special case that particular macro (skip it) or look for ones that have a single _ identifier as the definition.

The intended solution for this is to use @"_" to refer to an identifier named _. Let me know if you hit any bugs related to that.

[ehaas]

The intended solution for this is to use @"_" to refer to an identifier named _. Let me know if you hit any bugs related to that.

Is that expected to work now? I get the same error for pub const FOO = @"_";

test.zig:1:17: error: '_' may not be used as an identifier
pub const FOO = @"_";
                ^

[andrewrk]

Is that expected to work now? I get the same error for pub const FOO = @"_";

Oops, forgot to answer this question. I believe I fixed it in abfee12.

[ehaas]

Is that expected to work now? I get the same error for pub const FOO = @"_";

Oops, forgot to answer this question. I believe I fixed it in abfee12.

I'm able to declare a public symbol with that commit (eg. pub const @"_": c_int = 5; works) however I'm not able to declare a variable:

pub export fn main() void {
    var @"_": c_int = 5;
    _ = @"_";
}

./test.zig:2:5: error: `_` is not a declarable symbol
    var @"_": c_int = 5;
    ^
./test.zig:3:9: error: `_` may only be used to assign things to
    _ = @"_";
        ^