fix: diagnostic for casting reference to slice

fixes: #118790

compiler/rustc_hir_typeck/src/cast.rs

@@ -576,7 +576,6 @@ fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) -> ErrorGuaranteed
         match self.expr_ty.kind() {
             ty::Ref(_, _, mt) => {
                 let mtstr = mt.prefix_str();
-                if self.cast_ty.is_trait() {
                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
                     Ok(s) => {
                         err.span_suggestion(
@@ -591,11 +590,6 @@ fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) -> ErrorGuaranteed
                         err.span_help(self.cast_span, msg);
                     }
                 }
-                } else {
-                    let msg =
-                        format!("consider using an implicit coercion to `&{mtstr}{tstr}` instead");
-                    err.span_help(self.span, msg);
-                }
             }
             ty::Adt(def, ..) if def.is_box() => {
                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {

tests/ui/cast/cast-to-slice.rs

@@ -0,0 +1,8 @@
+fn main() {
+    "example".as_bytes() as [char];
+    //~^ ERROR cast to unsized type
+
+    let arr: &[u8] = &[0, 2, 3];
+    arr as [char];
+    //~^ ERROR cast to unsized type
+}

tests/ui/cast/cast-to-slice.stderr

@@ -0,0 +1,19 @@
+error[E0620]: cast to unsized type: `&[u8]` as `[char]`
+  --> $DIR/cast-to-slice.rs:2:5
+   |
+LL |     "example".as_bytes() as [char]; 
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^------
+   |                             |
+   |                             help: try casting to a reference instead: `&[char]`
+
+error[E0620]: cast to unsized type: `&[u8]` as `[char]`
+  --> $DIR/cast-to-slice.rs:6:5
+   |
+LL |     arr as [char];
+   |     ^^^^^^^------
+   |            |
+   |            help: try casting to a reference instead: `&[char]`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0620`.