This week was something else! There is a lot I’ve done this week (2023-04-17—2023-04-21). This is mostly a log for myself so that I remember that I did things, but you may be interested in reading it too, for whatever reason.

Assure everyone that has_type_flags is fast

This is funny. So I’ve been working on some type-flags-adjacent changes, in particular I’ve been looking at TypeVisitableExt::has_non_region_param. has_non_region_param calls has_type_flags from the same trait, which “visits” self with HasTypeFlagsVisitor.

At the first glance this looks very suspicious — a visitor normally recursively visits a structure, but this is not needed here! For example Ty (a type representing types in the compiler) has cached type flags, so you don’t need to visit it to compute them again! My first though was “how could such inefficient thing be used in the hot path??”, but of course I’ve just missed something.

HasTypeFlagsVisitor doesn’t actually recurse all the way down! If it visits a given type, it doesn’t go to the subtypes, it just uses the cached type flags. So this is in fact quite efficient and totally fine.

It turns out that I’m not the first one to stumble upon this and be confused, so I’ve made a PR that explains all of that in a comment, so that I’m hopefully the last one to be confused by that.

number_of_people_who_has_tripped_on_this += 1;

No transmutes of GenericArg and Ty

This was my “quest” of the week.

It all started from me changing how Ty is represented, trying to make a small optimization (yet to be completed, hopefully those changes will see the light of day next week…). Everything was good, until the compiler sigsegv’ed while compiling some core

At first I thought that my optimization itself was unsound or at least very broken. Which it kind of was — I did a little mistake, but it was not causing the sigsegv. Then I learned from @oli-obk that we have some places where we transmute GenericArg and Ty, which would be very broken for many reasons, including the fact that I changed the Ty representation.

Then I noticed in the compiler output, that it crashed inside _RNvMs7_NtCs8SrYHSYT6I_12rustc_middle2tyNtB5_2Ty13from_interned (rustc_middle::ty::Ty::from_interned) which was very suspicious, because from_interned is pretty much a trivial constructor, which should not cause sigsegvs. And then I noticed _RNvMse_NtNtCs8SrYHSYT6I_12rustc_middle2ty5substINtNtB7_4list4ListNtB5_10GenericArgE16try_as_type_list (rustc_middle::ty::subst::list::List::GenericArg::try_as_type_list), which, as you can guess from the name, transmutes between GenericArg and Ty, causing problems because I changed Ty’s layout…


As a side note: even before waffle’s changes, this was technically unsound, because both GenericArg and Ty are repr(Rust), which means that their layout is unstable. Yeah…

This was originally added as a clever optimization, because, well, we can! GenericArg is a tagged pointer that is basically a compressed version of Ty | Region | Const (the types of possible generic arguments — types, lifetimes and constants). Since we’ve chosen Ty’s tag to be 0b00, GenericArg and Ty has the same layout! In theory this allows us to store, hash and use types and generic args that are also types interchangeably.

In practice however everything is less nice:

  1. The transmutes are scattered across the code and are hard to maintain (as shown by my struggles with the sigsegv)
  2. This optimization actually doesn’t noticeably change the performance and memory usage of the compiler
  3. … While also blocking other optimizations (as, again, shown by my problems)

With all of that in mind I’ve made #110496 which removed the transmutes. Later I also followed it by #110599 and #110622 which removed other artifacts of this optimization.

deny(unsafe_op_in_unsafe_fn) in rustc_data_structures

This is the continuation of my improvements of tagged pointers, after their cleanup it made sense to deny unsafe_op_in_unsafe_fn lint in the crate.

unsafe_op_in_unsafe_fn lints against the following code:

unsafe fn dangerous() {}

unsafe fn something_unsafe() {
    //^-- this is allowed, by linted by `unsafe_op_in_unsafe_fn`

unsafe fn something_unsafe_fixed() {
    unsafe {
    //^-- this is the fix for the lint

The reason for this lint’s existence is that unsafe fn mainly conveys that it’s unsafe to call that function, but by default it also allows to use other unsafe functions inside it, without unsafe blocks. This might be a little foot-gun, at it places more code in unsafe context than necessary, potentially leading to missed safety requirements. With the lint and unsafe {} inside unsafe functions everything is more explicit and it’s less easy to make mistakes, which is very important for unsafe code.

Closure suggestions finally merged!

The pull request I’ve been working on on streams has been finally merged, yay!

Add impl_tag! macro to implement Tag for tagged pointer easily

As the title suggests, this is a PR that continues my work on tagged pointers in rustc, by adding a macro to implement the Tag trait. As a reminder this is basically how Tag looks:

unsafe trait Tag {
    /// The number of bits required
    const BITS: u32;

    fn into_usize(self) -> usize;

    unsafe fn from_usize(tag: usize) -> Self;

Implementing it correctly is a little bit tricky, since you need to correctly handle all the unsafe invariants, so adding a macro seems very nice. At the moment the macro usage looks something like the following, although I’ll probably remove the explicit tags later:

impl_tag! {
    impl Tag for Type;
    Type::A <=> 0,
    Type::B { v: false } <=> 1,
    Type::B { v: true } <=> 2,

I hope you agree that it’s a lot nicer than manually doing the implementation :)

As a side note, while working on this macro I’ve discovered #110613 which is annoying bug/feature of how macros and lints work, which was pretty sad. I won’t copy the issue description here, so go look at it if you want.

Other stuff

  • rust/110461 — Use Item::expect_* and ImplItem::expect_* more
  • rust-lang/libs-team/212Option::is_none_or (API change proposal)
  • rust/110545 — Add GenericArgKind::as_{type,const,region}
  • rust/110539 — Move around {Idx, IndexVec, IndexSlice} adjacent code
  • rust/110621 — (More) consistently use “region” terminology in rustc_middle
  • rust/110451 — Minor changes to IndexVec::ensure_contains_elem & related methods
  • teloxide/discussions/872 — How to add a tracing::span to every incoming message?
    • I answered this :>


  • rust/110394 — Various minor Idx-related tweaks
  • rust/110313 — allow repr(align = x) on inherent methods
  • rust/106934 — Add offset_of! macro (RFC 3308)
    • very happy with this one in particular, it was very nice to finally approve it!
  • rust/110438 — Allow transmute-ing for tuples with [u8; <const N: usize>]
    • this seems like a bad idea and so was basically rejected
  • rust/110473 — Assert that global cache values are stable on insertion
  • rust/108106 — Improve niche placement by trying two strategies and picking the better result
  • rust/110548 — Make impl Debug for Span not panic on not having session globals.
  • rust/110540 — Fix wrong comment in rustc_hir/src/
  • rust/110596 — Rename wasm32-wasi to wasm32-wasi-preview1


When a regression is found in the rust compiler it is helpful to run a bisection, which allows you to find which exact PR/commit/version introduced the regression. For rustc bisection is usually done via cargo bisect-rustc, which is pretty cool to look at, it runs a binary search, checking the regression and trying to find the place that changes the compilation result or script result, etc (it has multiple modes).

  • rust/110551 — Codegen regression involving assume/unreachable_unchecked
    • A codegen regression was found, which I helped to bisect/track (although finding the root cause and fixing was done by @saethlin)
    • For some reason I had a lot of problems bisecting this one, but I think I was just unlucky with my choice of upper/lower bounds on the bisection
  • 110457 — Incremental recompilation MIR ICE


This week I did not make the programming stream, and I probably won’t make them in the near future, following my concerns from the previous devlog


It feels like I did a lot this week, it feels great!! Hopefully I won’t lose all that momentum next week, heh. See you next time 0/