Rollup merge of rust-lang#156104 - aobatact:try-as-dyn-unsized, r=oli-obk

Relax `T: Sized` bound on `try_as_dyn` / `try_as_dyn_mut`

`trait_info_of` already returns `None` for unsized types, so allowing `T: ?Sized` is sound and lets callers in generic contexts use these functions without a separate `Sized` bound. For unsized `T`, the function always returns `None`.

Tracking issue: rust-lang#144361

## Motivation

Currently, it is not possible to use `try_as_dyn` as "specialization-like" dispatch in
blanket impls with `?Sized` type.

```rust
// Cannot add ?Sized now.
impl<T: 'static /* + ?Sized */, S: Serializer + 'static> Serialize<S> for T {
    fn serialize(&self, serializer: &mut S) -> Result<S::Ok, S::Error> {
        if let Some(spec) = try_as_dyn::<_, dyn SpecializedSer<S>>(self) {
            spec.specialized_serialize(serializer)
        } else {
            // fall back to compile-time reflection via TypeId
            ...
        }
    }
}
```

Author: JonathanBrouwer

library/core/src/any.rs

@@ -1007,18 +1007,23 @@ pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
 #[must_use]
 #[unstable(feature = "try_as_dyn", issue = "144361")]
 pub const fn try_as_dyn<
-    T: Any + 'static,
+    T: Any + ?Sized + 'static,
     U: ptr::Pointee<Metadata = ptr::DynMetadata<U>> + ?Sized + 'static,
 >(
     t: &T,
 ) -> Option<&U> {
+    // For unsized `T`, `trait_info_of` always returns `None` (vtable lookup is
+    // only supported for sized types). The function therefore unconditionally
+    // returns `None` in that case.
     let vtable: Option<ptr::DynMetadata<U>> =
         const { TypeId::of::<T>().trait_info_of::<U>().as_ref().map(TraitImpl::get_vtable) };
     match vtable {
         Some(dyn_metadata) => {
-            let pointer = ptr::from_raw_parts(t, dyn_metadata);
+            let pointer = ptr::from_raw_parts(t as *const T as *const (), dyn_metadata);
             // SAFETY: `t` is a reference to a type, so we know it is valid.
             // `dyn_metadata` is a vtable for T, implementing the trait of `U`.
+            // `T` is sized here because `trait_info_of` only returns `Some` for sized types,
+            // so the thin data pointer fully describes the value.
             Some(unsafe { &*pointer })
         }
         None => None,
@@ -1061,18 +1066,23 @@ pub const fn try_as_dyn<
 #[must_use]
 #[unstable(feature = "try_as_dyn", issue = "144361")]
 pub const fn try_as_dyn_mut<
-    T: Any + 'static,
+    T: Any + ?Sized + 'static,
     U: ptr::Pointee<Metadata = ptr::DynMetadata<U>> + ?Sized + 'static,
 >(
     t: &mut T,
 ) -> Option<&mut U> {
+    // For unsized `T`, `trait_info_of` always returns `None` (vtable lookup is
+    // only supported for sized types). The function therefore unconditionally
+    // returns `None` in that case.
     let vtable: Option<ptr::DynMetadata<U>> =
         const { TypeId::of::<T>().trait_info_of::<U>().as_ref().map(TraitImpl::get_vtable) };
     match vtable {
         Some(dyn_metadata) => {
-            let pointer = ptr::from_raw_parts_mut(t, dyn_metadata);
+            let pointer = ptr::from_raw_parts_mut(t as *mut T as *mut (), dyn_metadata);
             // SAFETY: `t` is a reference to a type, so we know it is valid.
             // `dyn_metadata` is a vtable for T, implementing the trait of `U`.
+            // `T` is sized here because `trait_info_of` only returns `Some` for sized types,
+            // so the thin data pointer fully describes the value.
             Some(unsafe { &mut *pointer })
         }
         None => None,

tests/ui/any/try_as_dyn_unsized.rs

@@ -0,0 +1,34 @@
+//@ run-pass
+#![feature(try_as_dyn)]
+
+use std::fmt::Debug;
+
+// Generic over `?Sized` T: relies on the relaxed bound on `try_as_dyn`.
+fn try_debug<T: ?Sized + 'static>(t: &T) -> Option<String> {
+    std::any::try_as_dyn::<T, dyn Debug>(t).map(|d| format!("{d:?}"))
+}
+
+fn try_debug_mut<T: ?Sized + 'static>(t: &mut T) -> Option<String> {
+    std::any::try_as_dyn_mut::<T, dyn Debug>(t).map(|d| format!("{d:?}"))
+}
+
+fn main() {
+    // Sized case still works through a `?Sized` generic context.
+    let x: i32 = 7;
+    assert_eq!(try_debug(&x).as_deref(), Some("7"));
+
+    let mut y: i32 = 8;
+    assert_eq!(try_debug_mut(&mut y).as_deref(), Some("8"));
+
+    // Unsized `T` always returns `None`, even though `str: Debug` and
+    // `[T]: Debug` hold — vtable lookup for unsized impl types is not
+    // currently supported by `TypeId::trait_info_of`.
+    let s: &str = "hello";
+    assert!(try_debug::<str>(s).is_none());
+
+    let slice: &[i32] = &[1, 2, 3];
+    assert!(try_debug::<[i32]>(slice).is_none());
+
+    let dyn_any: &dyn std::any::Any = &0i32;
+    assert!(try_debug::<dyn std::any::Any>(dyn_any).is_none());
+}