seq: compute correct width for -0e0

[jfinkels]

This adds code similar to the code in print_seq_integers() that correctly supports negative zero as a starting number when the negative zero is given in the input in scientific notation.

Before this change:

$ ./target/debug/coreutils seq -w -0e0 1
00
01

After this change:

$ ./target/debug/coreutils seq -w -0e0 1
-0
01

That matches GNU seq:

$ seq -w -0e0 1
-0
01

[jfinkels]

I don't know why the CI is complaining about this new test. It works on my machine (Ubuntu 18.04, rustc 1.55.0):

$ cargo run seq -w -0e0 1
    Finished dev [unoptimized + debuginfo] target(s) in 0.22s
     Running `target/debug/coreutils seq -w -0e0 1`
-0
01
$ cargo test test_seq::test_width_negative_zero_scientific_notation
[...]
running 1 test
test test_seq::test_width_negative_zero_scientific_notation ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 1400 filtered out; finished in 0.00s

[jfinkels]

I see. The CI build is using an older version of rustc. I found that rustc v1.52.0 causes this new test to fail:

$ rustup toolchain install 1.52.0 && cargo +1.52.0 test test_seq::test_width_negative_zero_scientific_notation
[...]
---- test_seq::test_width_negative_zero_scientific_notation stdout ----
current_directory_resolved: 
run: /home/jeffrey/src/coreutils/target/debug/coreutils seq -w -0e0 1
thread 'test_seq::test_width_negative_zero_scientific_notation' panicked at 'assertion failed: `(left == right)`

Diff < left / right > :
<"-0\n1\n"
>"-0\n01\n"

', tests/common/util.rs:229:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

And v1.53.0 causes the test to pass:

$ rustup toolchain install 1.53.0 && cargo +1.53.0 test test_seq::test_width_negative_zero_scientific_notation
[...]
running 1 test
test test_seq::test_width_negative_zero_scientific_notation ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 1412 filtered out; finished in 0.00s

I'll try to figure out what change I can make to get this code to work.

[jfinkels]

Okay, I've figured it out. Consider this Rust program:

fn main() {
    println!("{}", (-0.0f64).to_string());
}

When compiled with rustc v1.52.1 and rustc v1.53.0, we get different outputs:

$ cargo +1.52.1 run
   Compiling negativezero v0.1.0 (/tmp/negativezero)
    Finished dev [unoptimized + debuginfo] target(s) in 0.92s
     Running `target/debug/negativezero`
0
$ cargo +1.53.0 run
   Compiling negativezero v0.1.0 (/tmp/negativezero)
    Finished dev [unoptimized + debuginfo] target(s) in 0.89s
     Running `target/debug/negativezero`
-0

I believe this is due to this change rust-lang/rust#78618, which appears in v1.53.0: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1530-2021-06-17

Since my other pull request, #2613, replaces this part of the code with something that doesn't suffer from this problem, I'm just going to close this pull request and make sure the unit test is included in that other branch.