Implement ProjectionElem::Subslice

example/mini_core_hello_world.rs

@@ -1,6 +1,6 @@
 // Adapted from https://github.com/sunfishcode/mir2cranelift/blob/master/rust-examples/nocore-hello-world.rs
 
-#![feature(no_core, unboxed_closures, start, lang_items, box_syntax)]
+#![feature(no_core, unboxed_closures, start, lang_items, box_syntax, slice_patterns)]
 #![no_core]
 #![allow(dead_code)]
 
@@ -184,4 +184,10 @@ fn main() {
     }
 
     [NoisyDropInner, NoisyDropInner];
+
+    let x = &[0u32, 42u32] as &[u32];
+    match x {
+        [] => assert_eq!(0u32, 1),
+        [_, ref y..] => assert_eq!(&x[1] as *const u32 as usize, &y[0] as *const u32 as usize),
+    }
 }

src/base.rs

@@ -1117,12 +1117,34 @@ pub fn trans_place<'a, 'tcx: 'a>(
                     };
                     base.place_index(fx, index)
                 }
-                ProjectionElem::Subslice { from, to } => unimpl!(
-                    "projection subslice {:?} from {} to {}",
-                    projection.base,
-                    from,
-                    to
-                ),
+                ProjectionElem::Subslice { from, to } => {
+                    // These indices are generated by slice patterns.
+                    // slice[from:-to] in Python terms.
+
+                    match base.layout().ty.sty {
+                        ty::Array(elem_ty, len) => {
+                            let elem_layout = fx.layout_of(elem_ty);
+                            let ptr = base.to_addr(fx);
+                            let len = crate::constant::force_eval_const(fx, len).unwrap_usize(fx.tcx);
+                            CPlace::Addr(
+                                fx.bcx.ins().iadd_imm(ptr, elem_layout.size.bytes() as i64 * from as i64),
+                                None,
+                                fx.layout_of(fx.tcx.mk_array(elem_ty, len - from as u64 - to as u64)),
+                            )
+                        }
+                        ty::Slice(elem_ty) => {
+                            let elem_layout = fx.layout_of(elem_ty);
+                            let (ptr, len) = base.to_addr_maybe_unsized(fx);
+                            let len = len.unwrap();
+                            CPlace::Addr(
+                                fx.bcx.ins().iadd_imm(ptr, elem_layout.size.bytes() as i64 * from as i64),
+                                Some(fx.bcx.ins().iadd_imm(len, -(from as i64 + to as i64))),
+                                base.layout(),
+                            )
+                        }
+                        _ => unreachable!(),
+                    }
+                }
                 ProjectionElem::Downcast(_adt_def, variant) => base.downcast_variant(fx, variant),
             }
         }