add link to stabilization PR and explain why the code has UB

[web.git] / personal / _posts / 2020-04-04-layout-debugging.md

diff --git a/personal/_posts/2020-04-04-layout-debugging.md b/personal/_posts/2020-04-04-layout-debugging.md
index 627a45fda3cc955b7bf6acefb64af7c2f8c2de08..66000e079bdf10a7935edbade4cd9630bc975cba 100644 (file)
--- a/personal/_posts/2020-04-04-layout-debugging.md
+++ b/personal/_posts/2020-04-04-layout-debugging.md
@@ -1,10 +1,11 @@
 ---
 title: "Debugging rustc type layouts"
 categories: rust
 ---
 title: "Debugging rustc type layouts"
 categories: rust

+forum: https://internals.rust-lang.org/t/psa-debugging-rustc-type-layouts/12082

 ---
 
 This post is a "public service announcement" for people working on the guts of rustc.
 ---
 
 This post is a "public service announcement" for people working on the guts of rustc.

-I wish I had know about this a year ago, so I hope this post can make this feature more widely known.
+I wish I had known about this a year ago, so I hope this post can make this feature more widely known.

 
 <!-- MORE -->
 
 
 <!-- MORE -->
 


@@ -28,7 +29,7 @@ The (permanently) unstable `rustc_layout` attribute [can now be used](https://gi
 In this case, it prints:
 
 ```
 In this case, it prints:
 
 ```

-error: layout debugging: Layout {
+error: layout_of((u8, u16)) = Layout {

     fields: Arbitrary {
         offsets: [
             Size {
     fields: Arbitrary {
         offsets: [
             Size {


@@ -82,6 +83,8 @@ the fields are at offsets 0 and 2, the type has alignment 2 (but preferred align
 We can also see that it uses the `ScalarPair` abi which is relevant for Miri and when passing data as arguments to another function.
 To learn more about what all this information means, see [the `Layout` type docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.Layout.html).
 
 We can also see that it uses the `ScalarPair` abi which is relevant for Miri and when passing data as arguments to another function.
 To learn more about what all this information means, see [the `Layout` type docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/struct.Layout.html).
 

+**Update:** After a suggestions by @jschievink, this can now also be used to print the [underlying type and layout](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=1de2bed0c0d0f9171bfb41969f5028fb) of named opaque types, which is particularly useful [for generators](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=de99ab78a4d77bceee6760021b19de7d). **/Update**
+

 So the next time you work with `Layout` and wonder how exactly the niche gets represented, or whether an `enum` can have `ScalarPair` abi (hint: yes it can), you can easily look at a few examples to see how rustc thinks about this type internally.
 This is basically the type-level equivalent of `--emit mir`.
 I have wanted this since forever, so much that some time ago I wrote an awful hack for this based on rustc debug tracing.
 So the next time you work with `Layout` and wonder how exactly the niche gets represented, or whether an `enum` can have `ScalarPair` abi (hint: yes it can), you can easily look at a few examples to see how rustc thinks about this type internally.
 This is basically the type-level equivalent of `--emit mir`.
 I have wanted this since forever, so much that some time ago I wrote an awful hack for this based on rustc debug tracing.