rustc::middle::const_eval : add overflow-checking for {+, -, *}.

The overflow-checking attempts to accommodate early evaluation where
we do not have type information yet.

Also, add fixme note about something that has been bothering me.

1 files changed, 297 insertions, 81 deletions

diff --git a/src/librustc/middle/const_eval.rs b/src/librustc/middle/const_eval.rs
index 173358d336b..e3e5efc53c7 100644
--- a/src/librustc/middle/const_eval.rs
+++ b/src/librustc/middle/const_eval.rs
@@ -13,6 +13,8 @@
 
 pub use self::const_val::*;
 
+use self::ErrKind::*;
+
 use metadata::csearch;
 use middle::{astencode, def};
 use middle::pat_util::def_to_path;
@@ -27,6 +29,7 @@ use syntax::{ast_map, ast_util, codemap};
 
 use std::borrow::{Cow, IntoCow};
 use std::num::wrapping::OverflowingOps;
+use std::num::ToPrimitive;
 use std::cmp::Ordering;
 use std::collections::hash_map::Entry::Vacant;
 use std::{i8, i16, i32, i64};
@@ -234,6 +237,7 @@ pub enum ErrKind {
     NotOnStruct,
     NotOnTuple,
 
+    NegateWithOverflow(i64),
     AddiWithOverflow(i64, i64),
     SubiWithOverflow(i64, i64),
     MuliWithOverflow(i64, i64),
@@ -244,6 +248,8 @@ pub enum ErrKind {
     DivideWithOverflow,
     ModuloByZero,
     ModuloWithOverflow,
+    ShiftLeftWithOverflow,
+    ShiftRightWithOverflow,
     MissingStructField,
     NonConstPath,
     ExpectedConstTuple,
@@ -257,6 +263,7 @@ pub enum ErrKind {
 impl ConstEvalErr {
     pub fn description(&self) -> Cow<str> {
         use self::ErrKind::*;
+
         match self.kind {
             CannotCast => "can't cast this type".into_cow(),
             CannotCastTo(s) => format!("can't cast this type to {}", s).into_cow(),
@@ -275,6 +282,7 @@ impl ConstEvalErr {
             NotOnStruct => "not on struct".into_cow(),
             NotOnTuple => "not on tuple".into_cow(),
 
+            NegateWithOverflow(..) => "attempted to negate with overflow".into_cow(),
             AddiWithOverflow(..) => "attempted to add with overflow".into_cow(),
             SubiWithOverflow(..) => "attempted to sub with overflow".into_cow(),
             MuliWithOverflow(..) => "attempted to mul with overflow".into_cow(),
@@ -285,6 +293,8 @@ impl ConstEvalErr {
             DivideWithOverflow   => "attempted to divide with overflow".into_cow(),
             ModuloByZero         => "attempted remainder with a divisor of zero".into_cow(),
             ModuloWithOverflow   => "attempted remainder with overflow".into_cow(),
+            ShiftLeftWithOverflow => "attempted left shift with overflow".into_cow(),
+            ShiftRightWithOverflow => "attempted right shift with overflow".into_cow(),
             MissingStructField  => "nonexistent struct field".into_cow(),
             NonConstPath        => "non-constant path in constant expr".into_cow(),
             ExpectedConstTuple => "expected constant tuple".into_cow(),
@@ -297,57 +307,294 @@ impl ConstEvalErr {
     }
 }
 
+pub type EvalResult = Result<const_val, ConstEvalErr>;
+pub type CastResult = Result<const_val, ErrKind>;
+
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub enum IntTy { I8, I16, I32, I64 }
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub enum UintTy { U8, U16, U32, U64 }
+
+impl IntTy {
+    pub fn from(tcx: &ty::ctxt, t: ast::IntTy) -> IntTy {
+        let t = if let ast::TyIs = t {
+            tcx.sess.target.int_type
+        } else {
+            t
+        };
+        match t {
+            ast::TyIs => unreachable!(),
+            ast::TyI8  => IntTy::I8,
+            ast::TyI16 => IntTy::I16,
+            ast::TyI32 => IntTy::I32,
+            ast::TyI64 => IntTy::I64,
+        }
+    }
+}
+
+impl UintTy {
+    pub fn from(tcx: &ty::ctxt, t: ast::UintTy) -> UintTy {
+        let t = if let ast::TyUs = t {
+            tcx.sess.target.uint_type
+        } else {
+            t
+        };
+        match t {
+            ast::TyUs => unreachable!(),
+            ast::TyU8  => UintTy::U8,
+            ast::TyU16 => UintTy::U16,
+            ast::TyU32 => UintTy::U32,
+            ast::TyU64 => UintTy::U64,
+        }
+    }
+}
+
 macro_rules! signal {
-    ($e:expr, $ctor:ident) => {
-        return Err(ConstEvalErr { span: $e.span, kind: ErrKind::$ctor })
+    ($e:expr, $exn:expr) => {
+        return Err(ConstEvalErr { span: $e.span, kind: $exn })
+    }
+}
+
+// The const_{int,uint}_checked_{neg,add,sub,mul,div,shl,shr} family
+// of functions catch and signal overflow errors during constant
+// evaluation.
+//
+// They all take the operator's arguments (`a` and `b` if binary), the
+// overall expression (`e`) and, if available, whole expression's
+// concrete type (`opt_ety`).
+//
+// If the whole expression's concrete type is None, then this is a
+// constant evaluation happening before type check (e.g. in the check
+// to confirm that a pattern range's left-side is not greater than its
+// right-side). We do not do arithmetic modulo the type's bitwidth in
+// such a case; we just do 64-bit arithmetic and assume that later
+// passes will do it again with the type information, and thus do the
+// overflow checks then.
+
+pub fn const_int_checked_neg<'a>(
+    a: i64, e: &'a Expr, opt_ety: Option<IntTy>) -> EvalResult {
+
+    let (min,max) = match opt_ety {
+        // (-i8::MIN is itself not an i8, etc, but this is an easy way
+        // to allow literals to pass the check. Of course that does
+        // not work for i64::MIN.)
+        Some(IntTy::I8) =>  (-(i8::MAX as i64), -(i8::MIN as i64)),
+        Some(IntTy::I16) => (-(i16::MAX as i64), -(i16::MIN as i64)),
+        Some(IntTy::I32) => (-(i32::MAX as i64), -(i32::MIN as i64)),
+        None | Some(IntTy::I64) => (-i64::MAX, -(i64::MIN+1)),
     };
 
-    ($e:expr, $ctor:ident($($arg:expr),*)) => {
-        return Err(ConstEvalErr { span: $e.span, kind: ErrKind::$ctor($($arg),*) })
+    let oflo = a < min || a > max;
+    if oflo {
+        signal!(e, NegateWithOverflow(a));
+    } else {
+        Ok(const_int(-a))
+    }
+}
+
+pub fn const_uint_checked_neg<'a>(
+    a: u64, _e: &'a Expr, _opt_ety: Option<UintTy>) -> EvalResult {
+    // This always succeeds, and by definition, returns `(!a)+1`.
+    Ok(const_uint(-a))
+}
+
+macro_rules! overflow_checking_body {
+    ($a:ident, $b:ident, $ety:ident, $overflowing_op:ident,
+     lhs: $to_8_lhs:ident $to_16_lhs:ident $to_32_lhs:ident,
+     rhs: $to_8_rhs:ident $to_16_rhs:ident $to_32_rhs:ident $to_64_rhs:ident,
+     $EnumTy:ident $T8: ident $T16: ident $T32: ident $T64: ident,
+     $result_type: ident) => { {
+        let (a,b,opt_ety) = ($a,$b,$ety);
+        match opt_ety {
+            Some($EnumTy::$T8) => match (a.$to_8_lhs(), b.$to_8_rhs()) {
+                (Some(a), Some(b)) => {
+                    let (a, oflo) = a.$overflowing_op(b);
+                    (a as $result_type, oflo)
+                }
+                (None, _) | (_, None) => (0, true)
+            },
+            Some($EnumTy::$T16) => match (a.$to_16_lhs(), b.$to_16_rhs()) {
+                (Some(a), Some(b)) => {
+                    let (a, oflo) = a.$overflowing_op(b);
+                    (a as $result_type, oflo)
+                }
+                (None, _) | (_, None) => (0, true)
+            },
+            Some($EnumTy::$T32) => match (a.$to_32_lhs(), b.$to_32_rhs()) {
+                (Some(a), Some(b)) => {
+                    let (a, oflo) = a.$overflowing_op(b);
+                    (a as $result_type, oflo)
+                }
+                (None, _) | (_, None) => (0, true)
+            },
+            None | Some($EnumTy::$T64) => match b.$to_64_rhs() {
+                Some(b) => a.$overflowing_op(b),
+                None => (0, true),
+            }
+        }
+    } }
+}
+
+macro_rules! int_arith_body {
+    ($a:ident, $b:ident, $ety:ident, $overflowing_op:ident) => {
+        overflow_checking_body!(
+            $a, $b, $ety, $overflowing_op,
+            lhs: to_i8 to_i16 to_i32,
+            rhs: to_i8 to_i16 to_i32 to_i64, IntTy I8 I16 I32 I64, i64)
     }
 }
 
-fn checked_add_int(e: &Expr, a: i64, b: i64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_add(b);
-    if !oflo { Ok(const_int(ret)) } else { signal!(e, AddiWithOverflow(a, b)) }
+macro_rules! uint_arith_body {
+    ($a:ident, $b:ident, $ety:ident, $overflowing_op:ident) => {
+        overflow_checking_body!(
+            $a, $b, $ety, $overflowing_op,
+            lhs: to_u8 to_u16 to_u32,
+            rhs: to_u8 to_u16 to_u32 to_u64, UintTy U8 U16 U32 U64, u64)
+    }
 }
-fn checked_sub_int(e: &Expr, a: i64, b: i64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_sub(b);
-    if !oflo { Ok(const_int(ret)) } else { signal!(e, SubiWithOverflow(a, b)) }
+
+macro_rules! int_shift_body {
+    ($a:ident, $b:ident, $ety:ident, $overflowing_op:ident) => {
+        overflow_checking_body!(
+            $a, $b, $ety, $overflowing_op,
+            lhs: to_i8 to_i16 to_i32,
+            rhs: to_u32 to_u32 to_u32 to_u32, IntTy I8 I16 I32 I64, i64)
+    }
 }
-fn checked_mul_int(e: &Expr, a: i64, b: i64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_mul(b);
-    if !oflo { Ok(const_int(ret)) } else { signal!(e, MuliWithOverflow(a, b)) }
+
+macro_rules! uint_shift_body {
+    ($a:ident, $b:ident, $ety:ident, $overflowing_op:ident) => {
+        overflow_checking_body!(
+            $a, $b, $ety, $overflowing_op,
+            lhs: to_u8 to_u16 to_u32,
+            rhs: to_u32 to_u32 to_u32 to_u32, UintTy U8 U16 U32 U64, u64)
+    }
 }
 
-fn checked_add_uint(e: &Expr, a: u64, b: u64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_add(b);
-    if !oflo { Ok(const_uint(ret)) } else { signal!(e, AdduWithOverflow(a, b)) }
+macro_rules! pub_fn_checked_op {
+    {$fn_name:ident ($a:ident : $a_ty:ty, $b:ident : $b_ty:ty,.. $WhichTy:ident) {
+        $ret_oflo_body:ident $overflowing_op:ident
+            $const_ty:ident $signal_exn:expr
+    }} => {
+        pub fn $fn_name<'a>($a: $a_ty,
+                            $b: $b_ty,
+                            e: &'a Expr,
+                            opt_ety: Option<$WhichTy>) -> EvalResult {
+            let (ret, oflo) = $ret_oflo_body!($a, $b, opt_ety, $overflowing_op);
+            if !oflo { Ok($const_ty(ret)) } else { signal!(e, $signal_exn) }
+        }
+    }
 }
-fn checked_sub_uint(e: &Expr, a: u64, b: u64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_sub(b);
-    if !oflo { Ok(const_uint(ret)) } else { signal!(e, SubuWithOverflow(a, b)) }
+
+pub_fn_checked_op!{ const_int_checked_add(a: i64, b: i64,.. IntTy) {
+           int_arith_body overflowing_add const_int AddiWithOverflow(a, b)
+}}
+
+pub_fn_checked_op!{ const_int_checked_sub(a: i64, b: i64,.. IntTy) {
+           int_arith_body overflowing_sub const_int SubiWithOverflow(a, b)
+}}
+
+pub_fn_checked_op!{ const_int_checked_mul(a: i64, b: i64,.. IntTy) {
+           int_arith_body overflowing_mul const_int MuliWithOverflow(a, b)
+}}
+
+pub fn const_int_checked_div<'a>(
+    a: i64, b: i64, e: &'a Expr, opt_ety: Option<IntTy>) -> EvalResult {
+    if b == 0 { signal!(e, DivideByZero); }
+    let (ret, oflo) = int_arith_body!(a, b, opt_ety, overflowing_div);
+    if !oflo { Ok(const_int(ret)) } else { signal!(e, DivideWithOverflow) }
 }
-fn checked_mul_uint(e: &Expr, a: u64, b: u64) -> Result<const_val, ConstEvalErr> {
-    let (ret, oflo) = a.overflowing_mul(b);
-    if !oflo { Ok(const_uint(ret)) } else { signal!(e, MuluWithOverflow(a, b)) }
+
+pub fn const_int_checked_rem<'a>(
+    a: i64, b: i64, e: &'a Expr, opt_ety: Option<IntTy>) -> EvalResult {
+    if b == 0 { signal!(e, ModuloByZero); }
+    let (ret, oflo) = int_arith_body!(a, b, opt_ety, overflowing_rem);
+    if !oflo { Ok(const_int(ret)) } else { signal!(e, ModuloWithOverflow) }
 }
 
+pub_fn_checked_op!{ const_int_checked_shl(a: i64, b: i64,.. IntTy) {
+           int_shift_body overflowing_shl const_int ShiftLeftWithOverflow
+}}
+
+pub_fn_checked_op!{ const_int_checked_shl_via_uint(a: i64, b: u64,.. IntTy) {
+           int_shift_body overflowing_shl const_int ShiftLeftWithOverflow
+}}
+
+pub_fn_checked_op!{ const_int_checked_shr(a: i64, b: i64,.. IntTy) {
+           int_shift_body overflowing_shr const_int ShiftRightWithOverflow
+}}
+
+pub_fn_checked_op!{ const_int_checked_shr_via_uint(a: i64, b: u64,.. IntTy) {
+           int_shift_body overflowing_shr const_int ShiftRightWithOverflow
+}}
+
+pub_fn_checked_op!{ const_uint_checked_add(a: u64, b: u64,.. UintTy) {
+           uint_arith_body overflowing_add const_uint AdduWithOverflow(a, b)
+}}
+
+pub_fn_checked_op!{ const_uint_checked_sub(a: u64, b: u64,.. UintTy) {
+           uint_arith_body overflowing_sub const_uint SubuWithOverflow(a, b)
+}}
+
+pub_fn_checked_op!{ const_uint_checked_mul(a: u64, b: u64,.. UintTy) {
+           uint_arith_body overflowing_mul const_uint MuluWithOverflow(a, b)
+}}
+
+pub fn const_uint_checked_div<'a>(
+    a: u64, b: u64, e: &'a Expr, opt_ety: Option<UintTy>) -> EvalResult {
+    if b == 0 { signal!(e, DivideByZero); }
+    let (ret, oflo) = uint_arith_body!(a, b, opt_ety, overflowing_div);
+    if !oflo { Ok(const_uint(ret)) } else { signal!(e, DivideWithOverflow) }
+}
+
+pub fn const_uint_checked_rem<'a>(
+    a: u64, b: u64, e: &'a Expr, opt_ety: Option<UintTy>) -> EvalResult {
+    if b == 0 { signal!(e, ModuloByZero); }
+    let (ret, oflo) = uint_arith_body!(a, b, opt_ety, overflowing_rem);
+    if !oflo { Ok(const_uint(ret)) } else { signal!(e, ModuloWithOverflow) }
+}
+
+pub_fn_checked_op!{ const_uint_checked_shl(a: u64, b: u64,.. UintTy) {
+           uint_shift_body overflowing_shl const_uint ShiftLeftWithOverflow
+}}
+
+pub_fn_checked_op!{ const_uint_checked_shl_via_int(a: u64, b: i64,.. UintTy) {
+           uint_shift_body overflowing_shl const_uint ShiftLeftWithOverflow
+}}
+
+pub_fn_checked_op!{ const_uint_checked_shr(a: u64, b: u64,.. UintTy) {
+           uint_shift_body overflowing_shr const_uint ShiftRightWithOverflow
+}}
+
+pub_fn_checked_op!{ const_uint_checked_shr_via_int(a: u64, b: i64,.. UintTy) {
+           uint_shift_body overflowing_shr const_uint ShiftRightWithOverflow
+}}
 
 pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
                                      e: &Expr,
-                                     ty_hint: Option<Ty<'tcx>>)
-                                     -> Result<const_val, ConstEvalErr> {
+                                     ty_hint: Option<Ty<'tcx>>) -> EvalResult {
     fn fromb(b: bool) -> const_val { const_int(b as i64) }
 
     let ety = ty_hint.or_else(|| ty::expr_ty_opt(tcx, e));
 
+    // If type of expression itself is int or uint, normalize in these
+    // bindings so that isize/usize is mapped to a type with an
+    // inherently known bitwidth.
+    let expr_int_type = ety.and_then(|ty| {
+        if let ty::ty_int(t) = ty.sty {
+            Some(IntTy::from(tcx, t)) } else { None }
+    });
+    let expr_uint_type = ety.and_then(|ty| {
+        if let ty::ty_uint(t) = ty.sty {
+            Some(UintTy::from(tcx, t)) } else { None }
+    });
+
     let result = match e.node {
       ast::ExprUnary(ast::UnNeg, ref inner) => {
         match try!(eval_const_expr_partial(tcx, &**inner, ety)) {
           const_float(f) => const_float(-f),
-          const_int(i) => const_int(-i),
-          const_uint(i) => const_uint(-i),
+          const_int(n) =>  try!(const_int_checked_neg(n, e, expr_int_type)),
+          const_uint(n) => try!(const_uint_checked_neg(n, e, expr_uint_type)),
           const_str(_) => signal!(e, NegateOnString),
           const_bool(_) => signal!(e, NegateOnBoolean),
           const_binary(_) => signal!(e, NegateOnBinary),
@@ -391,51 +638,17 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
             }
           }
           (const_int(a), const_int(b)) => {
-            let is_a_min_value = || {
-                let int_ty = match ty::expr_ty_opt(tcx, e).map(|ty| &ty.sty) {
-                    Some(&ty::ty_int(int_ty)) => int_ty,
-                    _ => return false
-                };
-                let int_ty = if let ast::TyIs = int_ty {
-                    tcx.sess.target.int_type
-                } else {
-                    int_ty
-                };
-                match int_ty {
-                    ast::TyI8 => (a as i8) == i8::MIN,
-                    ast::TyI16 =>  (a as i16) == i16::MIN,
-                    ast::TyI32 =>  (a as i32) == i32::MIN,
-                    ast::TyI64 =>  (a as i64) == i64::MIN,
-                    ast::TyIs => unreachable!()
-                }
-            };
             match op.node {
-              ast::BiAdd => try!(checked_add_int(e, a, b)),
-              ast::BiSub => try!(checked_sub_int(e, a, b)),
-              ast::BiMul => try!(checked_mul_int(e, a, b)),
-              ast::BiDiv => {
-                  if b == 0 {
-                      signal!(e, DivideByZero);
-                  } else if b == -1 && is_a_min_value() {
-                      signal!(e, DivideWithOverflow);
-                  } else {
-                      const_int(a / b)
-                  }
-              }
-              ast::BiRem => {
-                  if b == 0 {
-                      signal!(e, ModuloByZero)
-                  } else if b == -1 && is_a_min_value() {
-                      signal!(e, ModuloWithOverflow)
-                  } else {
-                      const_int(a % b)
-                  }
-              }
+              ast::BiAdd => try!(const_int_checked_add(a,b,e,expr_int_type)),
+              ast::BiSub => try!(const_int_checked_sub(a,b,e,expr_int_type)),
+              ast::BiMul => try!(const_int_checked_mul(a,b,e,expr_int_type)),
+              ast::BiDiv => try!(const_int_checked_div(a,b,e,expr_int_type)),
+              ast::BiRem => try!(const_int_checked_rem(a,b,e,expr_int_type)),
               ast::BiAnd | ast::BiBitAnd => const_int(a & b),
               ast::BiOr | ast::BiBitOr => const_int(a | b),
               ast::BiBitXor => const_int(a ^ b),
-              ast::BiShl => const_int(a << b as usize),
-              ast::BiShr => const_int(a >> b as usize),
+              ast::BiShl => try!(const_int_checked_shl(a,b,e,expr_int_type)),
+              ast::BiShr => try!(const_int_checked_shr(a,b,e,expr_int_type)),
               ast::BiEq => fromb(a == b),
               ast::BiLt => fromb(a < b),
               ast::BiLe => fromb(a <= b),
@@ -446,18 +659,16 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
           }
           (const_uint(a), const_uint(b)) => {
             match op.node {
-              ast::BiAdd => try!(checked_add_uint(e, a, b)),
-              ast::BiSub => try!(checked_sub_uint(e, a, b)),
-              ast::BiMul => try!(checked_mul_uint(e, a, b)),
-              ast::BiDiv if b == 0 => signal!(e, DivideByZero),
-              ast::BiDiv => const_uint(a / b),
-              ast::BiRem if b == 0 => signal!(e, ModuloByZero),
-              ast::BiRem => const_uint(a % b),
+              ast::BiAdd => try!(const_uint_checked_add(a,b,e,expr_uint_type)),
+              ast::BiSub => try!(const_uint_checked_sub(a,b,e,expr_uint_type)),
+              ast::BiMul => try!(const_uint_checked_mul(a,b,e,expr_uint_type)),
+              ast::BiDiv => try!(const_uint_checked_div(a,b,e,expr_uint_type)),
+              ast::BiRem => try!(const_uint_checked_rem(a,b,e,expr_uint_type)),
               ast::BiAnd | ast::BiBitAnd => const_uint(a & b),
               ast::BiOr | ast::BiBitOr => const_uint(a | b),
               ast::BiBitXor => const_uint(a ^ b),
-              ast::BiShl => const_uint(a << b as usize),
-              ast::BiShr => const_uint(a >> b as usize),
+              ast::BiShl => try!(const_uint_checked_shl(a,b,e,expr_uint_type)),
+              ast::BiShr => try!(const_uint_checked_shr(a,b,e,expr_uint_type)),
               ast::BiEq => fromb(a == b),
               ast::BiLt => fromb(a < b),
               ast::BiLe => fromb(a <= b),
@@ -469,15 +680,15 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
           // shifts can have any integral type as their rhs
           (const_int(a), const_uint(b)) => {
             match op.node {
-              ast::BiShl => const_int(a << b as usize),
-              ast::BiShr => const_int(a >> b as usize),
+              ast::BiShl => try!(const_int_checked_shl_via_uint(a,b,e,expr_int_type)),
+              ast::BiShr => try!(const_int_checked_shr_via_uint(a,b,e,expr_int_type)),
               _ => signal!(e, InvalidOpForIntUint(op.node)),
             }
           }
           (const_uint(a), const_int(b)) => {
             match op.node {
-              ast::BiShl => const_uint(a << b as usize),
-              ast::BiShr => const_uint(a >> b as usize),
+              ast::BiShl => try!(const_uint_checked_shl_via_int(a,b,e,expr_uint_type)),
+              ast::BiShr => try!(const_uint_checked_shr_via_int(a,b,e,expr_uint_type)),
               _ => signal!(e, InvalidOpForUintInt(op.node)),
             }
           }
@@ -506,7 +717,12 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
                     tcx.sess.span_fatal(target_ty.span,
                                         "target type not found for const cast")
                 });
+
         // Prefer known type to noop, but always have a type hint.
+        //
+        // FIXME (#23833): the type-hint can cause problems,
+        // e.g. `(i8::MAX + 1_i8) as u32` feeds in `u32` as result
+        // type to the sum, and thus no overflow is signaled.
         let base_hint = ty::expr_ty_opt(tcx, &**base).unwrap_or(ety);
         let val = try!(eval_const_expr_partial(tcx, &**base, Some(base_hint)));
         match cast_const(tcx, val, ety) {
@@ -607,7 +823,7 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
     Ok(result)
 }
 
-fn cast_const<'tcx>(tcx: &ty::ctxt<'tcx>, val: const_val, ty: Ty) -> Result<const_val, ErrKind> {
+fn cast_const<'tcx>(tcx: &ty::ctxt<'tcx>, val: const_val, ty: Ty) -> CastResult {
     macro_rules! convert_val {
         ($intermediate_ty:ty, $const_type:ident, $target_ty:ty) => {
             match val {