add documentation to functions that call do_check and add a test against lint ordering changing

src/tools/clippy/clippy_lints/src/transmute.rs

@@ -694,7 +694,10 @@ fn is_layout_incompatible<'tcx>(cx: &LateContext<'tcx>, from: Ty<'tcx>, to: Ty<'
     }
 }
 
-/// Check if the the type conversion can be expressed as a pointer cast, instead of a transmute.
+/// Check if the the type conversion can be expressed as a pointer cast, instead of
+/// a transmute. In certain cases, including some invalid casts from array
+/// references to pointers, this may cause additional errors to be emitted and/or
+/// ICE error messages.
 fn can_be_expressed_as_pointer_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> bool {
     use CastKind::*;
     matches!(
@@ -710,7 +713,10 @@ fn can_be_expressed_as_pointer_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<
     )
 }
 
-/// If a cast from from_ty to to_ty is valid, returns an Ok containing the kind of the cast.
+/// If a cast from from_ty to to_ty is valid, returns an Ok containing the kind of
+/// the cast. In certain cases, including some invalid casts from array references
+/// to pointers, this may cause additional errors to be emitted and/or ICE error
+/// messages.
 fn check_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> Option<CastKind> {
     let hir_id = e.hir_id;
     let local_def_id = hir_id.owner;

src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.rs

@@ -63,3 +63,14 @@ fn foo(_: usize) -> u8 { 42 }
     };
     let usize_from_fn_ptr = foo as *const usize;
 }
+
+// If a ref-to-ptr cast of this form where the pointer type points to a type other
+// than the referenced type, calling `CastCheck::do_check` has been observed to
+// cause an ICE error message. `do_check` is currently called inside the
+// `transmutes_expressible_as_ptr_casts` check, but other, more specific lints
+// currently prevent it from being called in these cases. This test is meant to
+// fail if the ordering of the checks ever changes enough to cause these cases to
+// fall through into `do_check`.
+fn trigger_do_check_to_emit_error(in_param: &[i32; 1]) -> *const u8 {
+    unsafe { transmute::<&[i32; 1], *const u8>(in_param) }
+}