Fix ICE when casting `&[T]` to `*const [T]`

Closes #16947

src/librustc/middle/trans/expr.rs

@@ -1859,6 +1859,19 @@ pub fn cast_type_kind(tcx: &ty::ctxt, t: ty::t) -> cast_kind {
     }
 }
 
+fn cast_is_noop(t_in: ty::t, t_out: ty::t) -> bool {
+    if ty::type_is_boxed(t_in) || ty::type_is_boxed(t_out) {
+        return false;
+    }
+
+    match (ty::deref(t_in, true), ty::deref(t_out, true)) {
+        (Some(ty::mt{ ty: t_in, .. }), Some(ty::mt{ ty: t_out, .. })) => {
+            t_in == t_out
+        }
+        _ => false
+    }
+}
+
 fn trans_imm_cast<'a>(bcx: &'a Block<'a>,
                       expr: &ast::Expr,
                       id: ast::NodeId)
@@ -1877,7 +1890,13 @@ fn trans_imm_cast<'a>(bcx: &'a Block<'a>,
 
     // Convert the value to be cast into a ValueRef, either by-ref or
     // by-value as appropriate given its type:
-    let datum = unpack_datum!(bcx, trans(bcx, expr));
+    let mut datum = unpack_datum!(bcx, trans(bcx, expr));
+
+    if cast_is_noop(datum.ty, t_out) {
+        datum.ty = t_out;
+        return DatumBlock::new(bcx, datum);
+    }
+
     let newval = match (k_in, k_out) {
         (cast_integral, cast_integral) => {
             let llexpr = datum.to_llscalarish(bcx);

src/librustc/middle/typeck/check/mod.rs

@@ -1502,10 +1502,9 @@ fn check_cast(fcx: &FnCtxt,
     } else if ty::type_is_region_ptr(t_e) && ty::type_is_unsafe_ptr(t_1) {
         fn types_compatible(fcx: &FnCtxt, sp: Span,
                             t1: ty::t, t2: ty::t) -> bool {
-            if !ty::type_is_vec(t1) {
-                // If the type being casted from is not a vector, this special
-                // case does not apply.
-                return false
+            match ty::get(t1).sty {
+                ty::ty_vec(_, Some(_)) => {}
+                _ => return false
             }
             if ty::type_needs_infer(t2) {
                 // This prevents this special case from going off when casting
@@ -1529,7 +1528,7 @@ fn check_cast(fcx: &FnCtxt,
         // need to special-case obtaining an unsafe pointer
         // from a region pointer to a vector.
 
-        /* this cast is only allowed from &[T] to *T or
+        /* this cast is only allowed from &[T, ..n] to *T or
         &T to *T. */
         match (&ty::get(t_e).sty, &ty::get(t_1).sty) {
             (&ty::ty_rptr(_, ty::mt { ty: mt1, mutbl: ast::MutImmutable }),

src/test/run-pass/dst-raw.rs

@@ -53,6 +53,15 @@ pub fn main() {
         assert!(len == 3);
     }
 
+    // raw slice with explicit cast
+    let a = &[1i, 2, 3] as *const [_];
+    unsafe {
+        let b = (*a)[2];
+        assert!(b == 3);
+        let len = (*a).len();
+        assert!(len == 3);
+    }
+
     // raw DST struct with slice
     let c: *const Foo<[_]> = &Foo {f: [1i, 2, 3]};
     unsafe {
@@ -85,6 +94,14 @@ pub fn main() {
         assert!(len == 3);
     }
 
+    let a = &mut [1i, 2, 3] as *mut [_];
+    unsafe {
+        let b = (*a)[2];
+        assert!(b == 3);
+        let len = (*a).len();
+        assert!(len == 3);
+    }
+
     let c: *mut Foo<[_]> = &mut Foo {f: [1i, 2, 3]};
     unsafe {
         let b = (&*c).f[0];