Auto merge of rust-lang#81354 - SkiFire13:binary-search-assume, r=nagisa

Instruct LLVM that binary_search returns a valid index

This allows removing bound checks when the return value of `binary_search` is used to index into the slice it was call on. I also added a codegen test for this, not sure if it's the right thing to do (I didn't find anything on the dev guide), but it felt so.

library/core/src/slice/mod.rs

@@ -2204,6 +2204,8 @@ impl<T> [T] {
             } else if cmp == Greater {
                 right = mid;
             } else {
+                // SAFETY: same as the `get_unchecked` above
+                unsafe { crate::intrinsics::assume(mid < self.len()) };
                 return Ok(mid);
             }
 

src/test/codegen/binary-search-index-no-bound-check.rs

@@ -0,0 +1,19 @@
+// min-llvm-version: 11.0.0
+// compile-flags: -O
+// ignore-debug: the debug assertions get in the way
+#![crate_type = "lib"]
+
+// Make sure no bounds checks are emitted when slicing or indexing
+// with an index from `binary_search`.
+
+// CHECK-LABEL: @binary_search_index_no_bounds_check
+#[no_mangle]
+pub fn binary_search_index_no_bounds_check(s: &[u8]) -> u8 {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Ok(idx) = s.binary_search(&b'\\') {
+        s[idx]
+    } else {
+        42
+    }
+}