i8 literals and friends have incorrect ranges

[bstrie]

https://gist.github.com/2783543

The maximum legal value of signed integers of length x should be (2^(x-1)) - 1, but in most cases Rust allows signed literals of value 2^(x-1), but then interprets those literals as the minimum legal value (see the output in the gist above).

The exception is i16, which exhibits the same behavior as above, except that, bafflingly, x is 12 rather than 16.

EDIT: wrong equations

[msullivan]

The test case for i16 is "log(error, 2048i16);", so it isn't too surprising that it prints 2048...

[bstrie]

@msullivan Yes, the point is that log(error, 2049i16); fails with a literal out of range error. :)

[msullivan]

Ah. So yeah, that is just a straight up bug. I can fix it.

The other part is trickier: I think rust only considers positive literals, and when you write a negative number it is parsed as negation applied to a positive number. This means that if we had the proper positive bound on numbers, you wouldn't be able to write things like "-128i8"...

Argh.

[bstrie]

I don't think you need to mess with the parser, as long as the bounds check can be moved to after the negation is applied to the literal.

[msullivan]

I'm splitting the i16 part off into Issue #2462. This bug then is just the maximum value problem.

[bstrie]

Some more weirdness, for your perusal: using 128i8 to successfully index an array.

fn main() {
    let mut bar = [mut 0];
    vec::grow(bar, 128u, 0);
    bar[128] = 1;
    log(error, bar[128i8]);  // prints 1
}

[msullivan]

Split that last part into #2468

[catamorphism]

@msullivan or @bstrie -- could you clarify which part of this issue is still open?

[bstrie]

@catamorphism It depends on if you consider it a bug that the literal 128i8 is perfectly valid, despite the fact that i8 is within the range [-128, 127]. I just checked, and it's still the case that 128i8 is interpreted as -128.

[bstrie]

The more immediate problem was fixed, closing this in favor of #6958.