Prevent eval_const_expr_partial hides prior error in expr_cast arm.

This will help not to meet confusing errors.
In issue rust-lang#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(_) => {