add #[rustc_layout(debug)]

[RalfJung]

@eddyb recently told me about the #[rustc_layout] attribute, and I think it would be very useful if it could be used to print all the layout information Rust has about a type. When working with layouts (e.g. in Miri), it is often not clear how certain surface language features get represented internally. I have some awful hacks locally to be able to dump this debug information; with this attribute I could get it on the playground which is so much better. :)

[rust-highfive]

r? @cramertj

(rust_highfive has picked a reviewer for you, use r? to override)

[jonas-schievink]

Cool! This will be helpful for testing generator layout optimizations in the future.

[RalfJung]

I figured it would be useful for tons of stuff. :) We have to think of some ways to make sure that the right people even know about it... like finding a good place in the rustc dev guide to mention it. I also plan to write a blog post.

[eddyb]

cc @rust-lang/compiler

[cramertj]

r? @eddyb

[RalfJung]

@eddyb I made this work on type definitions, not just type aliases.

[eddyb]

@bors r+ Thanks!

[bors]

📌 Commit c62e36b has been approved by eddyb

[eddyb]

@bors r- (failed because formatting)

[RalfJung]

Fixed that.

@bors r=eddyb

[bors]

📌 Commit 7b49678 has been approved by eddyb

[Centril]

Failed in #70203 (comment), @bors r-

[RalfJung]

Looks like "preferred alignment" differs for some platforms:

---- [ui] ui/layout/debug.rs stdout ----
diff of stderr:

37	                        pow2: 0,
189	        },
190	        pref: Align {
-	            pow2: 3,
+	            pow2: 2,
192	        },
193	    },
194	    size: Size {

245	                        pow2: 2,
246	                    },
247	                    pref: Align {
-	                        pow2: 3,
+	                        pow2: 2,
249	                    },
250	                },
251	                size: Size {

275	                        pow2: 2,
276	                    },
277	                    pref: Align {
-	                        pow2: 3,
+	                        pow2: 2,
279	                    },
280	                },
281	                size: Size {

319	            pow2: 2,
320	        },
321	        pref: Align {
-	            pow2: 3,
+	            pow2: 2,
323	        },
324	    },
325	    size: Size {

@eddyb should I just normalize all pow2: [0-9]+ to pow2: $ALIGN to skip this problems?

[eddyb]

@RalfJung Does this work? pref: Align \{\n *pow2: [1-3],\n *\} to pref: $PREF_ALIGN.

[RalfJung]

Yes that works, nice.

@bors r=eddyb

[bors]

📌 Commit e548df7 has been approved by eddyb

[elichai]

That's awesome!
I was just looking for exactly that for C++ in the last few days :)