handle exponent without digits in numeric_literal

[y21]

Fixes #10912

The numeric literal util module didn't check for exponents with no digits.
So:

rust-clippy/clippy_utils/src/numeric_literal.rs

Lines 163 to 168 in 384cf37

	if let Some((separator, exponent)) = self.exponent {
	if exponent != "0" {
	output.push_str(separator);
	Self::group_digits(&mut output, exponent, group_size, true, false);
	}
	}

exponent here would be the empty string, which passed the != "0" check (when it shouldn't have, it should probably be treated as if the user wrote E0), then later fails when counting the digits and subtracting one (0 - 1 = overflow).

Also, interestingly I can't even write a test for this because exponents with no digits is some kind of error by itself and cargo dev fmt fails on it.

changelog: [unreadable_literal]: don't (debug) ICE on numeric literal with empty exponent

[rustbot]

r? @giraffate

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[giraffate]

Also, interestingly I can't even write a test for this because exponents with no digits is some kind of error by itself and cargo dev fmt fails on it.

Can we address it by putting ignore into rustfmt.toml, for example ignore = ["tests/ui/crashes/ice-10912.rs"]?
https://rust-lang.github.io/rustfmt/?version=master&search=#ignore

[giraffate]

@bors r+

Thanks!

[bors]

📌 Commit bbb9204 has been approved by giraffate

It is now in the queue for this repository.

[bors]

⌛ Testing commit bbb9204 with merge f93df98...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing f93df98 to master...