Do not panic in ty::consts::Const::try_to_target_usize() in case of size mismatch

Fixes an ICE when we try to relate an array size of type u8 to usize

Author: gurry

compiler/rustc_middle/src/ty/consts.rs

@@ -444,7 +444,11 @@ impl<'tcx> Const<'tcx> {
 
     #[inline]
     pub fn try_to_target_usize(self, tcx: TyCtxt<'tcx>) -> Option<u64> {
-        self.try_to_valtree()?.try_to_target_usize(tcx)
+        let scalar = self.try_to_valtree()?.try_to_scalar_int()?;
+        if scalar.size() != tcx.data_layout.pointer_size {
+            return None;
+        }
+        Some(scalar.to_target_usize(tcx))
     }
 
     pub fn is_ct_infer(self) -> bool {

tests/ui/consts/ice-const-size-relate-126359.rs

@@ -0,0 +1,14 @@
+// Regression test for ICE #126359
+
+// Tests that there is no ICE when the generic const
+// specifying the size of an array is of a non-usize type
+
+struct OppOrder<const N: u8 = 3, T = u32> {
+    arr: [T; N], // Array size is u8 instead of usize
+    //~^ ERROR mismatched types
+}
+
+fn main() {
+    let _ = OppOrder::<3, u32> { arr: [0, 0, 0] };
+    //~^ ERROR mismatched types
+}

tests/ui/consts/ice-const-size-relate-126359.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/ice-const-size-relate-126359.rs:12:39
+   |
+LL |     let _ = OppOrder::<3, u32> { arr: [0, 0, 0] };
+   |                                       ^^^^^^^^^ expected `3`, found `3`
+   |
+   = note: expected array `[u32; 3]`
+              found array `[u32; 3]`
+
+error[E0308]: mismatched types
+  --> $DIR/ice-const-size-relate-126359.rs:7:14
+   |
+LL |     arr: [T; N], // Array size is u8 instead of usize
+   |              ^ expected `usize`, found `u8`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.