Rollup merge of #97675 - nvzqz:unsized-needs-drop, r=dtolnay

Make `std::mem::needs_drop` accept `?Sized`

This change attempts to make `needs_drop` work with types like `[u8]` and `str`.

This enables code in types like `Arc<T>` that was not possible before, such as #97676.

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -567,7 +567,7 @@ pub mod intrinsics {
  pub fn copy<T>(src: *const T, dst: *mut T, count: usize);
  pub fn transmute<T, U>(e: T) -> U;
  pub fn ctlz_nonzero<T>(x: T) -> T;
- pub fn needs_drop<T>() -> bool;
+ pub fn needs_drop<T: ?::Sized>() -> bool;
  pub fn bitreverse<T>(x: T) -> T;
  pub fn bswap<T>(x: T) -> T;
  pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);

compiler/rustc_codegen_cranelift/example/mini_core_hello_world.rs

@@ -55,6 +55,11 @@ struct NoisyDrop {
  inner: NoisyDropInner,
 }
 
+struct NoisyDropUnsized {
+ inner: NoisyDropInner,
+ text: str,
+}
+
 struct NoisyDropInner;
 
 impl Drop for NoisyDrop {
@@ -170,7 +175,9 @@ fn main() {
  assert_eq!(intrinsics::min_align_of_val(&a) as u8, intrinsics::min_align_of::<&str>() as u8);
 
  assert!(!intrinsics::needs_drop::<u8>());
+ assert!(!intrinsics::needs_drop::<[u8]>());
  assert!(intrinsics::needs_drop::<NoisyDrop>());
+ assert!(intrinsics::needs_drop::<NoisyDropUnsized>());
 
  Unique {
  pointer: NonNull(1 as *mut &str),

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -514,7 +514,7 @@ pub mod intrinsics {
  pub fn copy<T>(src: *const T, dst: *mut T, count: usize);
  pub fn transmute<T, U>(e: T) -> U;
  pub fn ctlz_nonzero<T>(x: T) -> T;
- pub fn needs_drop<T>() -> bool;
+ pub fn needs_drop<T: ?::Sized>() -> bool;
  pub fn bitreverse<T>(x: T) -> T;
  pub fn bswap<T>(x: T) -> T;
  pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);

compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs

@@ -47,6 +47,11 @@ struct NoisyDrop {
  inner: NoisyDropInner,
 }
 
+struct NoisyDropUnsized {
+ inner: NoisyDropInner,
+ text: str,
+}
+
 struct NoisyDropInner;
 
 impl Drop for NoisyDrop {
@@ -184,7 +189,9 @@ fn main() {
  assert_eq!(intrinsics::min_align_of_val(&a) as u8, intrinsics::min_align_of::<&str>() as u8);
 
  assert!(!intrinsics::needs_drop::<u8>());
+ assert!(!intrinsics::needs_drop::<[u8]>());
  assert!(intrinsics::needs_drop::<NoisyDrop>());
+ assert!(intrinsics::needs_drop::<NoisyDropUnsized>());
 
  Unique {
  pointer: 0 as *const &str,

library/core/src/intrinsics.rs

@@ -1162,7 +1162,7 @@ extern "rust-intrinsic" {
  ///
  /// The stabilized version of this intrinsic is [`mem::needs_drop`](crate::mem::needs_drop).
  #[rustc_const_stable(feature = "const_needs_drop", since = "1.40.0")]
- pub fn needs_drop<T>() -> bool;
+ pub fn needs_drop<T: ?Sized>() -> bool;
 
  /// Calculates the offset from a pointer.
  ///

library/core/src/mem/mod.rs

@@ -592,7 +592,7 @@ pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
 #[stable(feature = "needs_drop", since = "1.21.0")]
 #[rustc_const_stable(feature = "const_mem_needs_drop", since = "1.36.0")]
 #[rustc_diagnostic_item = "needs_drop"]
-pub const fn needs_drop<T>() -> bool {
+pub const fn needs_drop<T: ?Sized>() -> bool {
  intrinsics::needs_drop::<T>()
 }