Rollup merge of rust-lang#75886 - erikdesjardins:index, r=nikic

Test that bounds checks are elided for [..index] after .position()

Closes rust-lang#73396.

This was fixed by the LLVM 11 update in rust-lang#73526.

src/test/codegen/issue-73396-bounds-check-after-position.rs

@@ -0,0 +1,78 @@
+// 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 `position()` or `rposition()`.
+
+// CHECK-LABEL: @position_slice_to_no_bounds_check
+#[no_mangle]
+pub fn position_slice_to_no_bounds_check(s: &[u8]) -> &[u8] {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().position(|b| *b == b'\\') {
+        &s[..idx]
+    } else {
+        s
+    }
+}
+
+// CHECK-LABEL: @position_slice_from_no_bounds_check
+#[no_mangle]
+pub fn position_slice_from_no_bounds_check(s: &[u8]) -> &[u8] {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().position(|b| *b == b'\\') {
+        &s[idx..]
+    } else {
+        s
+    }
+}
+
+// CHECK-LABEL: @position_index_no_bounds_check
+#[no_mangle]
+pub fn position_index_no_bounds_check(s: &[u8]) -> u8 {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().position(|b| *b == b'\\') {
+        s[idx]
+    } else {
+        42
+    }
+}
+// CHECK-LABEL: @rposition_slice_to_no_bounds_check
+#[no_mangle]
+pub fn rposition_slice_to_no_bounds_check(s: &[u8]) -> &[u8] {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().rposition(|b| *b == b'\\') {
+        &s[..idx]
+    } else {
+        s
+    }
+}
+
+// CHECK-LABEL: @rposition_slice_from_no_bounds_check
+#[no_mangle]
+pub fn rposition_slice_from_no_bounds_check(s: &[u8]) -> &[u8] {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().rposition(|b| *b == b'\\') {
+        &s[idx..]
+    } else {
+        s
+    }
+}
+
+// CHECK-LABEL: @rposition_index_no_bounds_check
+#[no_mangle]
+pub fn rposition_index_no_bounds_check(s: &[u8]) -> u8 {
+    // CHECK-NOT: panic
+    // CHECK-NOT: slice_index_len_fail
+    if let Some(idx) = s.iter().rposition(|b| *b == b'\\') {
+        s[idx]
+    } else {
+        42
+    }
+}