Iterating through an unbounded range causes a panic

[jyn514]

I tried this code:

fn main() {
    for i in 2147483646.. {
        println!("{}", i);
    }
}

I expected to see this happen: The numbers 2147483646 and 2147483647 should be printed, then the program should exit.

Instead, this happened: The standard library panicked.

Note that it might make sense to panic on overflow instead, but at least 2147483647 should printed first, since it's within the range of i32.

Playground

Meta

Version: stable (1.41.0), but panic also occurs on nightly (1.43.0-nightly (2020-02-10 e6ec0d1))

Backtrace

... snip ...
  13: core::panicking::panic
             at src/libcore/panicking.rs:52
  14: <i32 as core::ops::arith::Add>::add
             at /rustc/e6ec0d125eba4074122b187032474b4174fb9d31/src/libcore/ops/arith.rs:94
  15: <i32 as core::iter::range::Step>::add_one
             at /rustc/e6ec0d125eba4074122b187032474b4174fb9d31/src/libcore/iter/range.rs:67
  16: core::iter::range::<impl core::iter::traits::iterator::Iterator for core::ops::range::RangeFrom<A>>::next
             at /rustc/e6ec0d125eba4074122b187032474b4174fb9d31/src/libcore/iter/range.rs:314
  17: playground::main
             at src/main.rs:2
... snip ...

[jyn514]

Note that the following code works as expected:

    for i in 2147483646..=2147483647 {
        println!("{}", i);
    }

[timvermeulen]

The RangeFrom docs specify that it will currently panic in debug mode, and that this behavior is subject to change.

I'm not sure how intentional it is that the last value is left out, but given that RangeFrom only has a start field, it can't currently represent the state of just having returned the maximum value from next.

[ollie27]

Duplicate of #25708