Reintroduce TypeError::FixedArraySize

Author: varkor

src/librustc/ty/error.rs

@@ -23,6 +23,7 @@ pub enum TypeError<'tcx> {
     AbiMismatch(ExpectedFound<abi::Abi>),
     Mutability,
     TupleSize(ExpectedFound<usize>),
+    FixedArraySize(ExpectedFound<u64>),
     ArgCount,
 
     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
@@ -99,6 +100,12 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
                        values.expected,
                        values.found)
             }
+            FixedArraySize(values) => {
+                write!(f, "expected an array with a fixed size of {} elements, \
+                           found one with {} elements",
+                       values.expected,
+                       values.found)
+            }
             ArgCount => {
                 write!(f, "incorrect number of function parameters")
             }

src/librustc/ty/relate.rs

@@ -472,8 +472,22 @@ pub fn super_relate_tys<'a, 'gcx, 'tcx, R>(relation: &mut R,
         (&ty::Array(a_t, sz_a), &ty::Array(b_t, sz_b)) =>
         {
             let t = relation.relate(&a_t, &b_t)?;
-            let sz = relation.relate(&sz_a, &sz_b)?;
-            Ok(tcx.mk_ty(ty::Array(t, sz)))
+            match relation.relate(&sz_a, &sz_b) {
+                Ok(sz) => Ok(tcx.mk_ty(ty::Array(t, sz))),
+                Err(err) => {
+                    // Check whether the lengths are both `usize`s,
+                    // but differ in value, for better diagnostics.
+                    // FIXME(eddyb) get the right param_env.
+                    match (sz_a.assert_usize(tcx), sz_b.assert_usize(tcx)) {
+                        (Some(sz_a_val), Some(sz_b_val)) => {
+                            Err(TypeError::FixedArraySize(
+                                expected_found(relation, &sz_a_val, &sz_b_val)
+                            ))
+                        }
+                        _ => return Err(err),
+                    }
+                }
+            }
         }
 
         (&ty::Slice(a_t), &ty::Slice(b_t)) =>

src/librustc/ty/structural_impls.rs

@@ -716,6 +716,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
             AbiMismatch(x) => AbiMismatch(x),
             Mutability => Mutability,
             TupleSize(x) => TupleSize(x),
+            FixedArraySize(x) => FixedArraySize(x),
             ArgCount => ArgCount,
             RegionsDoesNotOutlive(a, b) => {
                 return tcx.lift(&(a, b)).map(|(a, b)| RegionsDoesNotOutlive(a, b))
@@ -1294,6 +1295,7 @@ EnumTypeFoldableImpl! {
         (ty::error::TypeError::AbiMismatch)(x),
         (ty::error::TypeError::Mutability),
         (ty::error::TypeError::TupleSize)(x),
+        (ty::error::TypeError::FixedArraySize)(x),
         (ty::error::TypeError::ArgCount),
         (ty::error::TypeError::RegionsDoesNotOutlive)(a, b),
         (ty::error::TypeError::RegionsInsufficientlyPolymorphic)(a, b),

src/test/ui/consts/const-array-oob-arith.stderr

@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/const-array-oob-arith.rs:7:45
    |
 LL | const BLUB: [i32; (ARR[0] - 40) as usize] = [5];
-   |                                             ^^^ expected `2usize`, found `1usize`
+   |                                             ^^^ expected an array with a fixed size of 2 elements, found one with 1 elements
    |
    = note: expected type `[i32; 2]`
               found type `[i32; 1]`
@@ -11,7 +11,7 @@ error[E0308]: mismatched types
   --> $DIR/const-array-oob-arith.rs:8:44
    |
 LL | const BOO: [i32; (ARR[0] - 41) as usize] = [5, 99];
-   |                                            ^^^^^^^ expected `1usize`, found `2usize`
+   |                                            ^^^^^^^ expected an array with a fixed size of 1 elements, found one with 2 elements
    |
    = note: expected type `[i32; 1]`
               found type `[i32; 2]`