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 https://github.com/rust-lang/rust/pull/97676.

6 files changed, 18 insertions, 4 deletions

diff --git a/compiler/rustc_codegen_cranelift/example/mini_core.rs b/compiler/rustc_codegen_cranelift/example/mini_core.rs
index 8da705e0cb0..489259d1a6b 100644
--- a/compiler/rustc_codegen_cranelift/example/mini_core.rs
+++ b/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);
diff --git a/compiler/rustc_codegen_cranelift/example/mini_core_hello_world.rs b/compiler/rustc_codegen_cranelift/example/mini_core_hello_world.rs
index 85ca908d0a2..0f1245c2758 100644
--- a/compiler/rustc_codegen_cranelift/example/mini_core_hello_world.rs
+++ b/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),
diff --git a/compiler/rustc_codegen_gcc/example/mini_core.rs b/compiler/rustc_codegen_gcc/example/mini_core.rs
index a8435287d9f..ddcbb0d9fc7 100644
--- a/compiler/rustc_codegen_gcc/example/mini_core.rs
+++ b/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);
diff --git a/compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs b/compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs
index 69d591565ac..14fd9eeffa6 100644
--- a/compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs
+++ b/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,
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 43ba2dc2874..ba837ea9a78 100644
--- a/library/core/src/intrinsics.rs
+++ b/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.
     ///
diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs
index 4ce7c8c2b60..71ea3b4ba85 100644
--- a/library/core/src/mem/mod.rs
+++ b/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>()
 }