Results of Integer fallback investigation

I made the following changes:

• Require array repeat counts to be uints ([expr, ..count])
• Make << and >> operators require uint for the shift count when applied to integral types
  • current behavior is: accept any integral type at all
• Improve type inferencer so that things like 3 as u8 or 0 as *T get inferred
  • Basically track downward the type that an expression is being cast to
  • A bit more subtle than existing inference mechanism
• Added warning for int fallback

Code at https://github.com/nikomatsakis/rust/tree/remove-int-fallback

After those changes, the warnings that remain fall into 4 categories:

• Pointless casts (2)
• Enum discriminants (29)
• Range (3)
• Counters (14)
• ISAAC (24, see below)

Pointless casts (2)

Occasionally, people add casts that aren't needed. For example:

-    fn size(&self) -> int { (1 << self.log_size) as int }
+    fn size(&self) -> int { 1 << self.log_size }

These generate a lack of constraints, but I'd say the code reads nicer the new way anyhow.

Enum discriminants (20)

For something like

enum Color { Red = 0, Green = 1, Blue = 2 }

we generate no particular constraint on the types of those constants, then default to int. I propose we just say that these constants must be typed as int (i.e., writing Red = 0_i64 would be an error). Or special case this to retain current behavior just in this instance (enum discriminants are so weird).

Range (3)

Some code does:

for _ in range(0, 10) { ... }

Naturally this is unconstrained. In such cases, we would have to either:

1. add another iterator (like times) that requires a uint and encourage people to use that
2. change the code to range(0u, 10)

Counters (14)

There are bits of code that do:

let mut c = 0;
loop {
    ...;
    c += 1;
}
println!("{}", c);

The counter c here is unconstrained and we pick int. In most cases I examined, this seems...slightly wrong, in that uint would have been a better choice.

ISAAC

ISAAC does some odd stuff using macros that I can't quite categorize. It winds up with a lot of duplicate error reports because the same macro blats the same code 4 or 5 times, with distinct constants. Example:

                rngstep!(i + 0, 13);
                rngstep!(i + 1, -6);
                rngstep!(i + 2, 2);
                rngstep!(i + 3, -16);

where rngstep! is:

        macro_rules! rngstep(
            ($j:expr, $shift:expr) => {{
                let base = $j;
                let mix = if $shift < 0 {
                    a >> -$shift as uint
                } else {
                    a << $shift as uint
                };

                let x = self.mem[base  + mr_offset];
                a = (a ^ mix) + self.mem[base + m2_offset];
                let y = ind!(x) + a + b;
                self.mem[base + mr_offset] = y;

                b = ind!(y >> RAND_SIZE_LEN) + x;
                self.rsl[base + mr_offset] = b;
            }}
        );

Note that $shift < 0 expands to something like 17 < 0 or whatever, which leaves the type of 17 and 0 unconstrained.