Rollup merge of #123564 - scottmcm:step-by-div-zero, r=joboet

Don't emit divide-by-zero panic paths in `StepBy::len`

I happened to notice today that there's actually two such calls emitted in the assembly: <https://rust.godbolt.org/z/1Wbbd3Ts6>

Since they're impossible, hopefully telling LLVM that will also help optimizations elsewhere.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/codegen/step_by-overflow-checks.rs b/tests/codegen/step_by-overflow-checks.rs
new file mode 100644
index 00000000000..43e8514a8b7
--- /dev/null
+++ b/tests/codegen/step_by-overflow-checks.rs
@@ -0,0 +1,26 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+use std::iter::StepBy;
+use std::slice::Iter;
+
+// The constructor for `StepBy` ensures we can never end up needing to do zero
+// checks on denominators, so check that the code isn't emitting panic paths.
+
+// CHECK-LABEL: @step_by_len_std
+#[no_mangle]
+pub fn step_by_len_std(x: &StepBy<Iter<i32>>) -> usize {
+    // CHECK-NOT: div_by_zero
+    // CHECK: udiv
+    // CHECK-NOT: div_by_zero
+    x.len()
+}
+
+// CHECK-LABEL: @step_by_len_naive
+#[no_mangle]
+pub fn step_by_len_naive(x: Iter<i32>, step_minus_one: usize) -> usize {
+    // CHECK: udiv
+    // CHECK: call{{.+}}div_by_zero
+    x.len() / (step_minus_one + 1)
+}