Auto merge of #137944 - davidtwco:sized-hierarchy, r=oli-obk

Sized Hierarchy: Part I

This patch implements the non-const parts of rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`. See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to `feature(sized_hierarchy)`. These traits are not behind `cfg`s as this would make implementation unfeasible, there would simply be too many `cfg`s required to add the necessary bounds everywhere. So, like `Sized`, these traits are automatically implemented by the compiler.

RFC 3729 describes changes which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- `?Sized` is rewritten as `MetaSized`
- `MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.

There are no edition migrations implemented in this,  as these are primarily required for the constness parts of the RFC and prior to stabilisation of this (and so will come in follow-up PRs alongside the const parts). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `MetaSized`) unless the `sized_hierarchy` feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax `Deref::Target` (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

**Notes:**

- Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
- This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
  - Each commit has a short description describing its purpose.
  - This patch is large but it's primarily in the test suite.
- I've worked on the performance of this patch and a few optimisations are implemented so that the performance impact is neutral-to-minor.
- `PointeeSized` is a different name from the RFC just to make it more obvious that it is different from `std::ptr::Pointee` but all the names are yet to be bikeshed anyway.
- `@nikomatsakis` has confirmed [that this can proceed as an experiment from the t-lang side](https://rust-lang.zulipchat.com/#narrow/channel/435869-project-goals/topic/SVE.20and.20SME.20on.20AArch64.20.28goals.23270.29/near/506196491)
- FCP in https://github.com/rust-lang/rust/pull/137944#issuecomment-2912207485

Fixes rust-lang/rust#79409.

r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)

12 files changed, 87 insertions, 13 deletions

diff --git a/tests/codegen/abi-x86-sse.rs b/tests/codegen/abi-x86-sse.rs
index 90757e601af..68d2acfb527 100644
--- a/tests/codegen/abi-x86-sse.rs
+++ b/tests/codegen/abi-x86-sse.rs
@@ -17,7 +17,13 @@
 #![crate_type = "lib"]
 
 #[lang = "sized"]
-trait Sized {}
+trait Sized: MetaSized {}
+
+#[lang = "meta_sized"]
+trait MetaSized: PointeeSized {}
+
+#[lang = "pointee_sized"]
+trait PointeeSized {}
 
 #[lang = "copy"]
 trait Copy {}
diff --git a/tests/codegen/dst-offset.rs b/tests/codegen/dst-offset.rs
index 7177a960432..2cf5fa9fac6 100644
--- a/tests/codegen/dst-offset.rs
+++ b/tests/codegen/dst-offset.rs
@@ -3,8 +3,9 @@
 //@ compile-flags: -C no-prepopulate-passes -Copt-level=0
 
 #![crate_type = "lib"]
-#![feature(extern_types)]
+#![feature(extern_types, sized_hierarchy)]
 
+use std::marker::PointeeSized;
 use std::ptr::addr_of;
 
 // Hack to get the correct type for usize
@@ -12,7 +13,7 @@ use std::ptr::addr_of;
 #[no_mangle]
 pub fn helper(_: usize) {}
 
-struct Dst<T: ?Sized> {
+struct Dst<T: PointeeSized> {
     x: u32,
     y: u8,
     z: T,
diff --git a/tests/codegen/emscripten-catch-unwind-js-eh.rs b/tests/codegen/emscripten-catch-unwind-js-eh.rs
index 3ab4b5c9c63..f43869cf218 100644
--- a/tests/codegen/emscripten-catch-unwind-js-eh.rs
+++ b/tests/codegen/emscripten-catch-unwind-js-eh.rs
@@ -9,8 +9,14 @@
 #![no_std]
 #![no_core]
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 #[lang = "freeze"]
 trait Freeze {}
 #[lang = "copy"]
diff --git a/tests/codegen/emscripten-catch-unwind-wasm-eh.rs b/tests/codegen/emscripten-catch-unwind-wasm-eh.rs
index d0571e4df08..b0750d52268 100644
--- a/tests/codegen/emscripten-catch-unwind-wasm-eh.rs
+++ b/tests/codegen/emscripten-catch-unwind-wasm-eh.rs
@@ -8,8 +8,14 @@
 #![no_std]
 #![no_core]
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 #[lang = "freeze"]
 trait Freeze {}
 #[lang = "copy"]
diff --git a/tests/codegen/terminating-catchpad.rs b/tests/codegen/terminating-catchpad.rs
index 17d88796300..a2ec19871d1 100644
--- a/tests/codegen/terminating-catchpad.rs
+++ b/tests/codegen/terminating-catchpad.rs
@@ -15,8 +15,14 @@
 #![no_std]
 #![no_core]
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 unsafe extern "C-unwind" {
     safe fn unwinds();
diff --git a/tests/codegen/unwind-abis/aapcs-unwind-abi.rs b/tests/codegen/unwind-abis/aapcs-unwind-abi.rs
index 0d9c7757883..ecace722e0d 100644
--- a/tests/codegen/unwind-abis/aapcs-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/aapcs-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=armv7-unknown-linux-gnueabihf --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `aapcs` and
 // `aapcs-unwind` extern functions. `aapcs-unwind` functions MUST NOT have this attribute. We
diff --git a/tests/codegen/unwind-abis/fastcall-unwind-abi.rs b/tests/codegen/unwind-abis/fastcall-unwind-abi.rs
index 4c7b2856e2e..7df46813ed1 100644
--- a/tests/codegen/unwind-abis/fastcall-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/fastcall-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `fastcall` and
 // `fastcall-unwind` extern functions. `fastcall-unwind` functions MUST NOT have this attribute. We
diff --git a/tests/codegen/unwind-abis/stdcall-unwind-abi.rs b/tests/codegen/unwind-abis/stdcall-unwind-abi.rs
index ffc11d1faef..cc06ee12549 100644
--- a/tests/codegen/unwind-abis/stdcall-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/stdcall-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `stdcall` and `stdcall-unwind`
 // extern functions. `stdcall-unwind` functions MUST NOT have this attribute. We disable
diff --git a/tests/codegen/unwind-abis/sysv64-unwind-abi.rs b/tests/codegen/unwind-abis/sysv64-unwind-abi.rs
index c869ca7e2b8..69bfaf80b4b 100644
--- a/tests/codegen/unwind-abis/sysv64-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/sysv64-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `sysv64` and
 // `sysv64-unwind` extern functions. `sysv64-unwind` functions MUST NOT have this attribute. We
diff --git a/tests/codegen/unwind-abis/thiscall-unwind-abi.rs b/tests/codegen/unwind-abis/thiscall-unwind-abi.rs
index 19b23ee47ba..05f6b8b70e1 100644
--- a/tests/codegen/unwind-abis/thiscall-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/thiscall-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `thiscall` and
 // `thiscall-unwind` extern functions. `thiscall-unwind` functions MUST NOT have this attribute. We
diff --git a/tests/codegen/unwind-abis/vectorcall-unwind-abi.rs b/tests/codegen/unwind-abis/vectorcall-unwind-abi.rs
index b420f67ca9b..d001a16b32a 100644
--- a/tests/codegen/unwind-abis/vectorcall-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/vectorcall-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items, abi_vectorcall)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `vectorcall` and
 // `vectorcall-unwind` extern functions. `vectorcall-unwind` functions MUST NOT have this attribute.
diff --git a/tests/codegen/unwind-abis/win64-unwind-abi.rs b/tests/codegen/unwind-abis/win64-unwind-abi.rs
index 2697d3cbcd6..257f00b54e4 100644
--- a/tests/codegen/unwind-abis/win64-unwind-abi.rs
+++ b/tests/codegen/unwind-abis/win64-unwind-abi.rs
@@ -2,8 +2,15 @@
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
 #![feature(no_core, lang_items)]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 // Test that `nounwind` attributes are correctly applied to exported `win64` and
 // `win64-unwind` extern functions. `win64-unwind` functions MUST NOT have this attribute. We