1 files changed, 108 insertions, 0 deletions
diff --git a/compiler/rustc_hir_typeck/src/intrinsicck.rs b/compiler/rustc_hir_typeck/src/intrinsicck.rs
new file mode 100644
index 000000000..9812d96fc
--- /dev/null
+++ b/compiler/rustc_hir_typeck/src/intrinsicck.rs
@@ -0,0 +1,108 @@
+use hir::HirId;
+use rustc_errors::struct_span_err;
+use rustc_hir as hir;
+use rustc_index::vec::Idx;
+use rustc_middle::ty::layout::{LayoutError, SizeSkeleton};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_target::abi::{Pointer, VariantIdx};
+
+use super::FnCtxt;
+
+/// If the type is `Option<T>`, it will return `T`, otherwise
+/// the type itself. Works on most `Option`-like types.
+fn unpack_option_like<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Ty<'tcx> {
+    let ty::Adt(def, substs) = *ty.kind() else { return ty };
+
+    if def.variants().len() == 2 && !def.repr().c() && def.repr().int.is_none() {
+        let data_idx;
+
+        let one = VariantIdx::new(1);
+        let zero = VariantIdx::new(0);
+
+        if def.variant(zero).fields.is_empty() {
+            data_idx = one;
+        } else if def.variant(one).fields.is_empty() {
+            data_idx = zero;
+        } else {
+            return ty;
+        }
+
+        if def.variant(data_idx).fields.len() == 1 {
+            return def.variant(data_idx).fields[0].ty(tcx, substs);
+        }
+    }
+
+    ty
+}
+
+impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
+    pub fn check_transmute(&self, from: Ty<'tcx>, to: Ty<'tcx>, hir_id: HirId) {
+        let tcx = self.tcx;
+        let span = tcx.hir().span(hir_id);
+        let normalize = |ty| {
+            let ty = self.resolve_vars_if_possible(ty);
+            self.tcx.normalize_erasing_regions(self.param_env, ty)
+        };
+        let from = normalize(from);
+        let to = normalize(to);
+        trace!(?from, ?to);
+
+        // Transmutes that are only changing lifetimes are always ok.
+        if from == to {
+            return;
+        }
+
+        let skel = |ty| SizeSkeleton::compute(ty, tcx, self.param_env);
+        let sk_from = skel(from);
+        let sk_to = skel(to);
+        trace!(?sk_from, ?sk_to);
+
+        // Check for same size using the skeletons.
+        if let (Ok(sk_from), Ok(sk_to)) = (sk_from, sk_to) {
+            if sk_from.same_size(sk_to) {
+                return;
+            }
+
+            // Special-case transmuting from `typeof(function)` and
+            // `Option<typeof(function)>` to present a clearer error.
+            let from = unpack_option_like(tcx, from);
+            if let (&ty::FnDef(..), SizeSkeleton::Known(size_to)) = (from.kind(), sk_to) && size_to == Pointer.size(&tcx) {
+                struct_span_err!(tcx.sess, span, E0591, "can't transmute zero-sized type")
+                    .note(&format!("source type: {from}"))
+                    .note(&format!("target type: {to}"))
+                    .help("cast with `as` to a pointer instead")
+                    .emit();
+                return;
+            }
+        }
+
+        // Try to display a sensible error with as much information as possible.
+        let skeleton_string = |ty: Ty<'tcx>, sk| match sk {
+            Ok(SizeSkeleton::Known(size)) => format!("{} bits", size.bits()),
+            Ok(SizeSkeleton::Pointer { tail, .. }) => format!("pointer to `{tail}`"),
+            Err(LayoutError::Unknown(bad)) => {
+                if bad == ty {
+                    "this type does not have a fixed size".to_owned()
+                } else {
+                    format!("size can vary because of {bad}")
+                }
+            }
+            Err(err) => err.to_string(),
+        };
+
+        let mut err = struct_span_err!(
+            tcx.sess,
+            span,
+            E0512,
+            "cannot transmute between types of different sizes, \
+                                        or dependently-sized types"
+        );
+        if from == to {
+            err.note(&format!("`{from}` does not have a fixed size"));
+        } else {
+            err.note(&format!("source type: `{}` ({})", from, skeleton_string(from, sk_from)))
+                .note(&format!("target type: `{}` ({})", to, skeleton_string(to, sk_to)));
+        }
+        err.emit();
+    }
+}