Bound checking elided for `[index]` but not for `[..index]`

[tesuji]

I tried this code: https://rust.godbolt.org/z/ihbmwB

pub fn foo(s: &[u8]) -> &[u8] {
    if let Some(idx) = s.iter().position(|b| *b == b'\\') {
        &s[..idx]
    } else {
        s
    }
}

I expected to see this happen: There is no bound check for s[..idx] in line 3.

Instead, this happened: There is bound check.

Note using s[idx] bound check is elided:

pub fn bar(s: &[u8]) -> u8 {
    if let Some(idx) = s.iter().position(|b| *b == b'\\') {
        s[idx]
    } else {
        42
    }
}

cc #30112

Meta

rustc --version --verbose: rustc 1.46.0-nightly (4fb54ed 2020-06-14)

rustc 1.46.0-nightly (4fb54ed48 2020-06-14)
binary: rustc
commit-hash: 4fb54ed484e2239a3e9eff3be17df00d2a162be3
commit-date: 2020-06-14
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

[leonardo-m]

Apparently here LLVM removes the bound tests:

pub fn foo2(s: &[u8]) -> &[u8] {
    let mut idx = 0;
    while idx < s.len() {
        if s[idx] == b'\\' {
            return &s[.. idx];
        }
        idx += 1;
    }
    s
}

[nikic]

  %_4.i.i.i = icmp ult i64 %.sroa.3.0.i.i.i, %s.1, !dbg !66
  tail call void @llvm.assume(i1 %_4.i.i.i) #2, !dbg !74, !noalias !75
  %_8.i.i.i = icmp ugt i64 %.sroa.3.0.i.i.i, %s.1, !dbg !80
  br i1 %_8.i.i.i, label %bb5.i.i.i, label %bb6, !dbg !87

This is https://bugs.llvm.org/show_bug.cgi?id=40149.

[nikic]

https://reviews.llvm.org/D82717