Add String::extend_from_within

[WaffleLapkin]

This PR adds String::extend_from_within function under the string_extend_from_within feature gate similar to the Vec::extend_from_within function.

// String
pub fn extend_from_within<R>(&mut self, src: R)
where
    R: RangeBounds<usize>;

[rust-highfive]

r? @joshtriplett

(rust-highfive has picked a reviewer for you, use r? to override)

[pickfire]

If panic happens, how does one know if it panics because of start or end?

[WaffleLapkin]

I think it should be possible by checking the line number in the panic message. It isn't convenient, but it is possible.

Also, note that the same pattern appears multiple times in this file:

rust/library/alloc/src/string.rs

Lines 1643 to 1644 in 23f9b92

	assert!(self.is_char_boundary(start));
	assert!(self.is_char_boundary(end));

rust/library/alloc/src/string.rs

Lines 1688 to 1700 in 23f9b92

	let start = range.start_bound();
	match start {
	Included(&n) => assert!(self.is_char_boundary(n)),
	Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
	Unbounded => {}
	};
	// WARNING: Inlining this variable would be unsound (#81138)
	let end = range.end_bound();
	match end {
	Included(&n) => assert!(self.is_char_boundary(n + 1)),
	Excluded(&n) => assert!(self.is_char_boundary(n)),
	Unbounded => {}
	};

[pickfire]

Maybe we should have a separate issue for this.

[pickfire]

Wait, it's not necessary. The current panic error message is good enough. (but still without the function name, like drain)

thread 'main' panicked at 'assertion failed: self.is_char_boundary(start)', /rustc/9bc8c42bb2f19e745a63f3445f1ac248fb015e53/library/alloc/src/string.rs:1571:9

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4e29eec2c640b0a42b054f58b1f6753d

[WaffleLapkin]

Ah, so when called without a message, assert! prints the passed expression. That's nice.

[joshtriplett]

Seems reasonable, and I appreciate that it calls Vec::extend_from_within rather than doing anything unsafe itself.

@bors r+

[bors]

📌 Commit 23f9b92 has been approved by joshtriplett