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)

8 files changed, 53 insertions, 9 deletions

diff --git a/tests/run-make/amdgpu-kd/foo.rs b/tests/run-make/amdgpu-kd/foo.rs
index a0d803ab813..a097e9211a7 100644
--- a/tests/run-make/amdgpu-kd/foo.rs
+++ b/tests/run-make/amdgpu-kd/foo.rs
@@ -4,8 +4,14 @@
 #![no_std]
 
 // This is needed because of #![no_core]:
+#[lang = "pointee_sized"]
+trait PointeeSized {}
+
+#[lang = "meta_sized"]
+trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+trait Sized: MetaSized {}
 
 #[no_mangle]
 extern "gpu-kernel" fn kernel() {}
diff --git a/tests/run-make/atomic-lock-free/atomic_lock_free.rs b/tests/run-make/atomic-lock-free/atomic_lock_free.rs
index e8bbd420cc4..f5c3b360ee8 100644
--- a/tests/run-make/atomic-lock-free/atomic_lock_free.rs
+++ b/tests/run-make/atomic-lock-free/atomic_lock_free.rs
@@ -16,8 +16,14 @@ pub enum AtomicOrdering {
 #[rustc_intrinsic]
 unsafe fn atomic_xadd<T, const ORD: AtomicOrdering>(dst: *mut T, src: T) -> T;
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 #[lang = "copy"]
 trait Copy {}
 #[lang = "freeze"]
diff --git a/tests/run-make/avr-rjmp-offset/avr-rjmp-offsets.rs b/tests/run-make/avr-rjmp-offset/avr-rjmp-offsets.rs
index c91cd695cee..07fda1f3f4a 100644
--- a/tests/run-make/avr-rjmp-offset/avr-rjmp-offsets.rs
+++ b/tests/run-make/avr-rjmp-offset/avr-rjmp-offsets.rs
@@ -24,14 +24,20 @@ pub fn main() -> ! {
 
 // FIXME: replace with proper minicore once available (#130693)
 mod minicore {
+    #[lang = "pointee_sized"]
+    pub trait PointeeSized {}
+
+    #[lang = "meta_sized"]
+    pub trait MetaSized: PointeeSized {}
+
     #[lang = "sized"]
-    pub trait Sized {}
+    pub trait Sized: MetaSized {}
 
     #[lang = "copy"]
     pub trait Copy {}
     impl Copy for u32 {}
     impl Copy for &u32 {}
-    impl<T: ?Sized> Copy for *mut T {}
+    impl<T: PointeeSized> Copy for *mut T {}
 
     pub mod ptr {
         #[inline]
diff --git a/tests/run-make/cross-lang-lto-riscv-abi/riscv-xlto.rs b/tests/run-make/cross-lang-lto-riscv-abi/riscv-xlto.rs
index c31cf27f9ae..dbccf8cea03 100644
--- a/tests/run-make/cross-lang-lto-riscv-abi/riscv-xlto.rs
+++ b/tests/run-make/cross-lang-lto-riscv-abi/riscv-xlto.rs
@@ -2,8 +2,12 @@
 #![feature(no_core, lang_items)]
 #![no_core]
 
+#[lang = "pointee_sized"]
+trait PointeeSized {}
+#[lang = "meta_sized"]
+trait MetaSized: PointeeSized {}
 #[lang = "sized"]
-trait Sized {}
+trait Sized: MetaSized {}
 
 #[no_mangle]
 pub fn hello() {}
diff --git a/tests/run-make/min-global-align/min_global_align.rs b/tests/run-make/min-global-align/min_global_align.rs
index cd1ef8cb351..fd6f8357030 100644
--- a/tests/run-make/min-global-align/min_global_align.rs
+++ b/tests/run-make/min-global-align/min_global_align.rs
@@ -9,8 +9,14 @@ pub static mut STATIC_MUT_BOOL: bool = true;
 const CONST_BOOL: bool = true;
 pub static CONST_BOOL_REF: &'static bool = &CONST_BOOL;
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+pub trait Sized: MetaSized {}
 
 #[lang = "copy"]
 trait Copy {}
diff --git a/tests/run-make/raw-dylib-cross-compilation/lib.rs b/tests/run-make/raw-dylib-cross-compilation/lib.rs
index d3b7fd23ca8..8d53ac04485 100644
--- a/tests/run-make/raw-dylib-cross-compilation/lib.rs
+++ b/tests/run-make/raw-dylib-cross-compilation/lib.rs
@@ -4,8 +4,12 @@
 #![crate_type = "lib"]
 
 // This is needed because of #![no_core]:
+#[lang = "pointee_sized"]
+trait PointeeSized {}
+#[lang = "meta_sized"]
+trait MetaSized: PointeeSized {}
 #[lang = "sized"]
-trait Sized {}
+trait Sized: MetaSized {}
 
 #[link(name = "extern_1", kind = "raw-dylib")]
 extern "C" {
diff --git a/tests/run-make/simd-ffi/simd.rs b/tests/run-make/simd-ffi/simd.rs
index 9ea8eb8cf88..1cd961ff87e 100644
--- a/tests/run-make/simd-ffi/simd.rs
+++ b/tests/run-make/simd-ffi/simd.rs
@@ -52,8 +52,14 @@ pub fn bar(a: i32x4, b: i32x4) -> i32x4 {
     unsafe { integer(a, b) }
 }
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-pub trait Sized {}
+pub trait Sized: MetaSized {}
 
 #[lang = "copy"]
 pub trait Copy {}
diff --git a/tests/run-make/target-specs/foo.rs b/tests/run-make/target-specs/foo.rs
index 22939e87912..aead76dff87 100644
--- a/tests/run-make/target-specs/foo.rs
+++ b/tests/run-make/target-specs/foo.rs
@@ -4,8 +4,14 @@
 #[lang = "copy"]
 trait Copy {}
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-trait Sized {}
+trait Sized: MetaSized {}
 
 #[lang = "freeze"]
 auto trait Freeze {}