support passing unsized fn arguments

Author: RalfJung

compiler/rustc_const_eval/src/interpret/place.rs

@@ -183,6 +183,18 @@ impl<Tag: Provenance> MemPlace<Tag> {
     }
 }
 
+impl<Tag: Provenance> Place<Tag> {
+    /// Asserts that this points to some local variable.
+    /// Returns the frame idx and the variable idx.
+    #[inline]
+    pub fn assert_local(&self) -> (usize, mir::Local) {
+        match self {
+            Place::Local { frame, local } => (*frame, *local),
+            _ => bug!("assert_local: expected Place::Local, got {:?}", self),
+        }
+    }
+}
+
 impl<'tcx, Tag: Provenance> MPlaceTy<'tcx, Tag> {
     /// Produces a MemPlace that works for ZST but nothing else
     #[inline]
@@ -286,7 +298,7 @@ impl<'tcx, Tag: Provenance> PlaceTy<'tcx, Tag> {
     }
 
     #[inline]
-    pub fn assert_mem_place(self) -> MPlaceTy<'tcx, Tag> {
+    pub fn assert_mem_place(&self) -> MPlaceTy<'tcx, Tag> {
         self.try_as_mplace().unwrap()
     }
 }
@@ -899,16 +911,16 @@ where
         trace!("copy_op: {:?} <- {:?}: {}", *dest, src, dest.layout.ty);
 
         let dest = self.force_allocation(dest)?;
-        assert!(!(src.layout.is_unsized() || dest.layout.is_unsized()), "cannot copy unsized data");
-        assert_eq!(src.layout.size, dest.layout.size, "Cannot copy differently-sized data");
+        let Some((dest_size, _)) = self.size_and_align_of_mplace(&dest)? else {
+            span_bug!(self.cur_span(), "copy_op needs (dynamically) sized values")
+        };
+        if cfg!(debug_assertions) {
+            let src_size = self.size_and_align_of_mplace(&src)?.unwrap().0;
+            assert_eq!(src_size, dest_size, "Cannot copy differently-sized data");
+        }
 
         self.mem_copy(
-            src.ptr,
-            src.align,
-            dest.ptr,
-            dest.align,
-            dest.layout.size,
-            /*nonoverlapping*/ false,
+            src.ptr, src.align, dest.ptr, dest.align, dest_size, /*nonoverlapping*/ false,
         )
     }
 

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -12,8 +12,8 @@ use rustc_target::abi::call::{ArgAbi, ArgAttribute, ArgAttributes, FnAbi, PassMo
 use rustc_target::spec::abi::Abi;
 
 use super::{
-    FnVal, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, PlaceTy, Scalar,
-    StackPopCleanup, StackPopUnwind,
+    FnVal, ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemoryKind, OpTy, Operand,
+    PlaceTy, Scalar, StackPopCleanup, StackPopUnwind,
 };
 
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
@@ -185,11 +185,16 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // No question
                 return true;
             }
+            if caller_abi.layout.is_unsized() || callee_abi.layout.is_unsized() {
+                // No, no, no. We require the types to *exactly* match for unsized arguments. If
+                // these are somehow unsized "in a different way" (say, `dyn Trait` vs `[i32]`),
+                // then who knows what happens.
+                return false;
+            }
             if caller_abi.layout.size != callee_abi.layout.size
                 || caller_abi.layout.align.abi != callee_abi.layout.align.abi
             {
                 // This cannot go well...
-                // FIXME: What about unsized types?
                 return false;
             }
             // The rest *should* be okay, but we are extra conservative.
@@ -287,6 +292,31 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 caller_arg.layout.ty
             )
         }
+        // Special handling for unsized parameters.
+        if caller_arg.layout.is_unsized() {
+            // `check_argument_compat` ensures that both have the same type, so we know they will use the metadata the same way.
+            assert_eq!(caller_arg.layout.ty, callee_arg.layout.ty);
+            // We have to properly pre-allocate the memory for the callee.
+            // So let's tear down some wrappers.
+            // This all has to be in memory, there are no immediate unsized values.
+            let src = caller_arg.assert_mem_place();
+            // The destination cannot be one of these "spread args".
+            let (dest_frame, dest_local) = callee_arg.assert_local();
+            // We are just initializing things, so there can't be anything here yet.
+            assert!(matches!(
+                *self.local_to_op(&self.stack()[dest_frame], dest_local, None)?,
+                Operand::Immediate(Immediate::Uninit)
+            ));
+            // Allocate enough memory to hold `src`.
+            let Some((size, align)) = self.size_and_align_of_mplace(&src)? else {
+                span_bug!(self.cur_span(), "unsized fn arg with `extern` type tail should not be allowed")
+            };
+            let ptr = self.allocate_ptr(size, align, MemoryKind::Stack)?;
+            let dest_place =
+                MPlaceTy::from_aligned_ptr_with_meta(ptr.into(), callee_arg.layout, src.meta);
+            // Update the local to be that new place.
+            *M::access_local_mut(self, dest_frame, dest_local)? = Operand::Indirect(*dest_place);
+        }
         // We allow some transmutes here.
         // FIXME: Depending on the PassMode, this should reset some padding to uninitialized. (This
         // is true for all `copy_op`, but there are a lot of special cases for argument passing