Prevent eval_const_expr_partial hides prior error in expr_cast arm.

This will help not to meet confusing errors.
In issue #5873, the error was "expected constant expr for vector length: Can't cast str to int".
It was originally "expected constant expr for vector length: Non-constant path in constant expr"
This patch make the original error to be printed.

src/librustc/middle/const_eval.rs

@@ -371,32 +371,31 @@ pub fn eval_const_expr_partial(tcx: middle::ty::ctxt, e: @expr)
       expr_cast(base, _) => {
         let ety = ty::expr_ty(tcx, e);
         let base = eval_const_expr_partial(tcx, base);
-        match ty::get(ety).sty {
-          ty::ty_float(_) => {
-            match base {
-              Ok(const_uint(u)) => Ok(const_float(u as f64)),
-              Ok(const_int(i)) => Ok(const_float(i as f64)),
-              Ok(const_float(_)) => base,
-              _ => Err(~"Can't cast float to str")
-            }
-          }
-          ty::ty_uint(_) => {
-            match base {
-              Ok(const_uint(_)) => base,
-              Ok(const_int(i)) => Ok(const_uint(i as u64)),
-              Ok(const_float(f)) => Ok(const_uint(f as u64)),
-              _ => Err(~"Can't cast str to uint")
-            }
-          }
-          ty::ty_int(_) | ty::ty_bool => {
-            match base {
-              Ok(const_uint(u)) => Ok(const_int(u as i64)),
-              Ok(const_int(_)) => base,
-              Ok(const_float(f)) => Ok(const_int(f as i64)),
-              _ => Err(~"Can't cast str to int")
+        match /*bad*/copy base {
+            Err(_) => base,
+            Ok(val) => {
+                match ty::get(ety).sty {
+                    ty::ty_float(_) => match val {
+                        const_uint(u) => Ok(const_float(u as f64)),
+                        const_int(i) => Ok(const_float(i as f64)),
+                        const_float(_) => base,
+                        _ => Err(~"Can't cast float to str"),
+                    },
+                    ty::ty_uint(_) => match val {
+                        const_uint(_) => base,
+                        const_int(i) => Ok(const_uint(i as u64)),
+                        const_float(f) => Ok(const_uint(f as u64)),
+                        _ => Err(~"Can't cast str to uint"),
+                    },
+                    ty::ty_int(_) | ty::ty_bool => match val {
+                        const_uint(u) => Ok(const_int(u as i64)),
+                        const_int(_) => base,
+                        const_float(f) => Ok(const_int(f as i64)),
+                        _ => Err(~"Can't cast str to int"),
+                    },
+                    _ => Err(~"Can't cast this type")
+                }
             }
-          }
-          _ => Err(~"Can't cast this type")
         }
       }
       expr_path(_) => {