Improve error message when using 'try'

[ghost]

As described here: #1587 (comment) when using 'try' in a function with that expects a non-error return type you get a confusing error message.

[ghost]

This is my first time looking at zig source code, please tell me if this is the wrong way/place to handle this

[andrewrk]

I think this is a huge improvement. There's a better place to put the ir_add_error_node call though, which I think is inside ir_analyze_instruction_return just before the call to ir_implicit_cast (which is what eventually is calling ir_add_error_node).

So you would check if value->value.type is an error, and that it could not be implicitly casted to ira->explicit_return_type (I think you can just check if it is an error union or an error set type), and in this case emit the error directly, without calling ir_implicit_cast.

There's a better way to get access to the function too, which is

        ZigFn *fn_entry = exec_fn_entry(ira->new_irb.exec);
        assert(fn_entry); // this assertion will be OK since we check earlier for `return` outside a function.

[andrewrk]

Note that this will change the compile error even if the user does return error.Foo; in a function that does not accept a possible error. I suggest either

• Check if instruction->value->id == IrInstructionIdUnwrapErrCode before doing this special error message, or
• (my preference) adjust the compile error message to be meaningful and correct for both cases - try when the return type cannot handle an error, and return error.Foo when the return type cannot handle the error.

[ghost]

I noticed that my changes do not apply to async functions and they require a different type of check

[andrewrk]

I wouldn't focus too much on the async function aspect of it yet. I'm in the middle of rewriting async functions in another branch. Maybe you can revisit it once that's merged.

[andrewrk]

This could hide a bug. Better to assert that your parameter is non-null.

[andrewrk]

Are you sure this case should be here? What would the zig source look like that uses this?

[andrewrk]

I solved this in a more generic way:

export fn foo() void {
    return true;
}

export fn bar() void {
    try baz();
}

fn baz() anyerror!void {}

/home/andy/downloads/zig/build/test.zig:2:12: error: expected type 'void', found 'bool'
    return true;
           ^
/home/andy/downloads/zig/build/test.zig:1:17: note: return type declared here
export fn foo() void {
                ^
/home/andy/downloads/zig/build/test.zig:6:5: error: expected type 'void', found 'anyerror'
    try baz();
    ^
/home/andy/downloads/zig/build/test.zig:5:17: note: return type declared here
export fn bar() void {
                ^