odd issue with #fmt and floats

[erickt]

I've run into a rather weird error with #fmt and floats:

use std;
import std::io;

fn main() {
    let f = -0.550153;
    let s = #fmt("%f", f);
    io::println(s);
}

Incorrectly prints out 0. But if we instead write:

use std;
import std::io;

fn main() {
    let f = -0.550153;
    let s = #fmt("%f", f);
    io::println(s);
    io::println("before " + s + " after");
}

It prints out:

-0.550152
before -0.550152 after

[erickt]

There is definitely something weird going on. Probably some memory corruption somewhere. Here's another example:

use std;
import std::io;

fn foo(num: float) -> str {
    assert num < 0.0;
    ret "";
}

fn main() {
    let f = -0.550153;
    log(error, f);
    assert f < 0.0;
    let s = foo(f);
    io::println(s);
    io::println("before " + s + " after");
}

Prints out this scary message:

rust: -0.550153
rust: 0
rust: upcall fail 'Assertion x < 0.0 failed', foo.rs:6
rust: domain main @0x10082a200 root task failed

Even spookier, if you comment out one of the io::println, this bug goes away:

use std;
import std::io;

fn foo(x: float) -> str {
    log(error, x);
    assert x < 0.0;
    ret "";
}

fn main() {
    let f = -0.550153;
    log(error, f);
    assert f < 0.0;
    let s = foo(f);
    io::println(s);
    //io::println("before " + s + " after");
}

Prints out:

rust: -0.550153
rust: -0.550153

[graydon]

This is stack-growth related. The bug goes away when you run with RUST_MIN_STACK=0x1000 ./testcase

Will dig into why that is true tomorrow.