Auto merge of rust-lang#8250 - pr2502:fix_repeat_underflow, r=giraffate

Fix underflow in `manual_split_once` lint

Hi, a friend found clippy started crashing on a suspiciously large allocation of `u64::MAX` memory on their code.

The mostly minimized repro is:
```rust
fn _f01(title: &str) -> Option<()> {
    let _ = title[1..].splitn(2, '[').next()?;
    Some(())
}
```

The underflow happens in this case on line 57 of the patch but I've changed the other substraction to saturating as well since it could potentially cause the same issue.

I'm not sure where to put a regression test, or if it's even worth for such a thing.

Aside, has it been considered before to build clippy with overflow checks enabled?

changelog: fix ICE of underflow in `manual_split_once` lint

Author: bors

clippy_lints/src/methods/str_splitn.rs

@@ -45,16 +45,16 @@ pub(super) fn check_manual_split_once(
         IterUsageKind::Next | IterUsageKind::Second => {
             let self_deref = {
                 let adjust = cx.typeck_results().expr_adjustments(self_arg);
-                if adjust.is_empty() {
+                if adjust.len() < 2 {
                     String::new()
                 } else if cx.typeck_results().expr_ty(self_arg).is_box()
                     || adjust
                         .iter()
                         .any(|a| matches!(a.kind, Adjust::Deref(Some(_))) || a.target.is_box())
                 {
-                    format!("&{}", "*".repeat(adjust.len() - 1))
+                    format!("&{}", "*".repeat(adjust.len().saturating_sub(1)))
                 } else {
-                    "*".repeat(adjust.len() - 2)
+                    "*".repeat(adjust.len().saturating_sub(2))
                 }
             };
             if matches!(usage.kind, IterUsageKind::Next) {

tests/ui/crashes/ice-8250.rs

@@ -0,0 +1,6 @@
+fn _f(s: &str) -> Option<()> {
+    let _ = s[1..].splitn(2, '.').next()?;
+    Some(())
+}
+
+fn main() {}

tests/ui/crashes/ice-8250.stderr

@@ -0,0 +1,18 @@
+error: manual implementation of `split_once`
+  --> $DIR/ice-8250.rs:2:13
+   |
+LL |     let _ = s[1..].splitn(2, '.').next()?;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `s[1..].split_once('.').map_or(s[1..], |x| x.0)`
+   |
+   = note: `-D clippy::manual-split-once` implied by `-D warnings`
+
+error: unnecessary use of `splitn`
+  --> $DIR/ice-8250.rs:2:13
+   |
+LL |     let _ = s[1..].splitn(2, '.').next()?;
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: try this: `s[1..].split('.')`
+   |
+   = note: `-D clippy::needless-splitn` implied by `-D warnings`
+
+error: aborting due to 2 previous errors
+