d9c4b8d475
Avoid superfluous UB checks in `IndexRange` `IndexRange::len` is justified as an overall invariant, and `take_prefix` and `take_suffix` are justified by local branch conditions. A few more UB-checked calls remain in cases that are only supported locally by `debug_assert!`, which won't do anything in distributed builds, so those UB checks may still be useful. We generally expect core's `#![rustc_preserve_ub_checks]` to optimize away in user's release builds, but the mere presence of that extra code can sometimes inhibit optimization, as seen in #131563.
The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.
One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:
// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT: emitted-only-for-bbb
// bbb-NOT: emitted-only-for-aaa
// bbb-SAME: emitted-only-for-bbb