`str::split_terminator` is never finished

[WaffleLapkin]

I tried this code:

let mut iter = "A..B..".split_terminator('.');
dbg!(&iter); // finished: false (expected)
    
iter.by_ref().for_each(drop);
dbg!(&iter); // finished: false (unexpected)
    
dbg!(iter.next());
dbg!(&iter); // finished: false (unexpected)

output

[src/main.rs:3] &iter = SplitTerminator(
    SplitInternal {
        start: 0,
        end: 6,
        matcher: CharSearcher {
            haystack: "A..B..",
            finger: 0,
            finger_back: 6,
            needle: '.',
            utf8_size: 1,
            utf8_encoded: [
                46,
                0,
                0,
                0,
            ],
        },
        allow_trailing_empty: false,
        finished: false,
    },
)
[src/main.rs:6] &iter = SplitTerminator(
    SplitInternal {
        start: 6,
        end: 6,
        matcher: CharSearcher {
            haystack: "A..B..",
            finger: 6,
            finger_back: 6,
            needle: '.',
            utf8_size: 1,
            utf8_encoded: [
                46,
                0,
                0,
                0,
            ],
        },
        allow_trailing_empty: false,
        finished: false,
    },
)
[src/main.rs:8] iter.next() = None
[src/main.rs:9] &iter = SplitTerminator(
    SplitInternal {
        start: 6,
        end: 6,
        matcher: CharSearcher {
            haystack: "A..B..",
            finger: 6,
            finger_back: 6,
            needle: '.',
            utf8_size: 1,
            utf8_encoded: [
                46,
                0,
                0,
                0,
            ],
        },
        allow_trailing_empty: false,
        finished: false,
    },
)

I've expected the finished flag to be set when the iterator is exhausted, however it isn't.

Meta

Tested on playground's stable 1.51.0 and nightly 1.54.0-nightly (2021-05-05 bacf770f2983a52f31e3)

Likely cause

It seems this behaviour is caused by allow_trailing_empty: false: SplitInternal only sets the finished flag if allow_trailing_empty: true or there is something to return.

Should the flag be set unconditionally? Maybe like this?

if !self.finished {
    self.finished = true;

    if self.allow_trailing_empty || self.end - self.start > 0 {
        return unsafe { ... } 
    }
}

None

[nagisa]

What's the actual issue? Does the iterator produce the wrong results?

The debug output here is printing out internal implementation details and it is perfectly fine for the implementation to utilize them in whatever way it wishes to do so.

[WaffleLapkin]

The issue actually showed up while I was changing as_str split methods (see #77998 (comment)).

The new method returns None if self.finished and Some(...) otherwise. So for exhausted SplitTerminator the method returned Some(""), instead of None.