StrSearcher does not behave correctly with empty strings as needle/haystack

[Luro02]

I tried this code:

#![feature(pattern)]
use core::str::pattern::{Pattern, Searcher, SearchStep};

fn main() {
    let mut searcher = "".into_searcher("");

    assert_eq!(searcher.next(), SearchStep::Match(0, 0));
    assert_eq!(searcher.next(), SearchStep::Done);
    assert_eq!(searcher.next(), SearchStep::Done); // <- panics here
    assert_eq!(searcher.next(), SearchStep::Done);
}

I expected to see this happen:

The searcher should not return matches after SearchStep::Done, because the entire haystack has been already covered.

To quote from the documentation:

Returns Done if every byte of the haystack has been visited.

The stream of Match and Reject values up to a Done will contain index ranges that are adjacent, non-overlapping, covering the whole haystack, and laying on utf8 boundaries.

https://doc.rust-lang.org/std/str/pattern/trait.Searcher.html

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 2.10s
     Running `target/debug/playground`
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Match(0, 0)`,
 right: `Done`', src/main.rs:9:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Meta

rustc --version --verbose:

rustc 1.54.0-nightly (79e50bf77 2021-05-10)
binary: rustc
commit-hash: 79e50bf77928f374921a6bcafee3fcff1915f062
commit-date: 2021-05-10
host: x86_64-unknown-linux-gnu
release: 1.54.0-nightly
LLVM version: 12.0.1

Backtrace

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 1.91s
     Running `target/debug/playground`
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Match(0, 0)`,
 right: `Done`', src/main.rs:9:5
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4e3e6db011c5b482d2bef8ba02274657f93b5e0d/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/4e3e6db011c5b482d2bef8ba02274657f93b5e0d/library/core/src/panicking.rs:92:14
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
             at /rustc/4e3e6db011c5b482d2bef8ba02274657f93b5e0d/library/core/src/panicking.rs:117:5
   4: playground::main
             at ./src/main.rs:9:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/4e3e6db011c5b482d2bef8ba02274657f93b5e0d/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[SkiFire13]

This also means that core::str::Matches (and maybe other str iterators too) is breaking the contract of its FusedIterator implementation

fn check_is_really_fused(mut it: impl core::iter::FusedIterator) {
    for _ in &mut it {}
    assert!(it.next().is_none()); // fails
}

fn main() {
    check_is_really_fused("".matches(""));
}

playground link

[poliorcetics]

@rustbot claim