Rollup merge of #55785 - stjepang:unsized-drop-forget, r=alexcrichton

Add mem::forget_unsized() for forgetting unsized values

~~Allows passing values of `T: ?Sized` types to `mem::drop` and `mem::forget`.~~

Adds `mem::forget_unsized()` that accepts `T: ?Sized`.

I had to revert the PR that removed the `forget` intrinsic and replaced it with `ManuallyDrop`: #40559
We can't use `ManuallyDrop::new()` here because it needs `T: Sized` and we don't have support for unsized return values yet (will we ever?).

r? @eddyb

Author: pietroalbini

src/libcore/intrinsics.rs

@@ -717,6 +717,10 @@ extern "rust-intrinsic" {
     /// initialize memory previous set to the result of `uninit`.
     pub fn uninit<T>() -> T;
 
+    /// Moves a value out of scope without running drop glue.
+    #[cfg(not(stage0))]
+    pub fn forget<T: ?Sized>(_: T);
+
     /// Reinterprets the bits of a value of one type as another type.
     ///
     /// Both types must have the same size. Neither the original, nor the result,

src/libcore/lib.rs

@@ -106,6 +106,7 @@
 #![feature(staged_api)]
 #![feature(stmt_expr_attributes)]
 #![feature(unboxed_closures)]
+#![feature(unsized_locals)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
 #![feature(doc_alias)]

src/libcore/mem.rs

@@ -143,6 +143,19 @@ pub fn forget<T>(t: T) {
     ManuallyDrop::new(t);
 }
 
+/// Like [`forget`], but also accepts unsized values.
+///
+/// This function is just a shim intended to be removed when the `unsized_locals` feature gets
+/// stabilized.
+///
+/// [`forget`]: fn.forget.html
+#[inline]
+#[cfg(not(stage0))]
+#[unstable(feature = "forget_unsized", issue = "0")]
+pub fn forget_unsized<T: ?Sized>(t: T) {
+    unsafe { intrinsics::forget(t) }
+}
+
 /// Returns the size of a type in bytes.
 ///
 /// More specifically, this is the offset in bytes between successive elements

src/librustc_codegen_llvm/intrinsic.rs

@@ -194,7 +194,7 @@ pub fn codegen_intrinsic_call(
             return;
         }
         // Effectively no-ops
-        "uninit" => {
+        "uninit" | "forget" => {
             return;
         }
         "needs_drop" => {

src/librustc_typeck/check/intrinsic.rs

@@ -134,6 +134,7 @@ pub fn check_intrinsic_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             "rustc_peek" => (1, vec![param(0)], param(0)),
             "init" => (1, Vec::new(), param(0)),
             "uninit" => (1, Vec::new(), param(0)),
+            "forget" => (1, vec![param(0)], tcx.mk_unit()),
             "transmute" => (2, vec![ param(0) ], param(1)),
             "move_val_init" => {
                 (1,