remove use of from_u32_unchecked

Author: nikomatsakis

src/librustc/ty/sty.rs

@@ -1274,9 +1274,7 @@ impl DebruijnIndex {
     /// you would need to shift the index for `'a` into 1 new binder.
     #[must_use]
     pub fn shifted_in(self, amount: u32) -> DebruijnIndex {
-        unsafe {
-            DebruijnIndex::from_u32_unchecked(self.as_u32() + amount)
-        }
+        DebruijnIndex::from_u32(self.as_u32() + amount)
     }
 
     /// Update this index in place by shifting it "in" through
@@ -1289,9 +1287,7 @@ impl DebruijnIndex {
     /// `amount` number of new binders.
     #[must_use]
     pub fn shifted_out(self, amount: u32) -> DebruijnIndex {
-        unsafe {
-            DebruijnIndex::from_u32_unchecked(self.as_u32() - amount)
-        }
+        DebruijnIndex::from_u32(self.as_u32() - amount)
     }
 
     /// Update in place by shifting out from `amount` binders.

src/librustc_data_structures/indexed_vec.rs

@@ -104,7 +104,7 @@ macro_rules! newtype_index {
         impl $type {
             $v const MAX_AS_U32: u32 = $max;
 
-            $v const MAX: $type = unsafe { $type::from_u32_unchecked($max) };
+            $v const MAX: $type = $type::from_u32_const($max);
 
             #[inline]
             $v fn from_usize(value: usize) -> Self {
@@ -122,6 +122,24 @@ macro_rules! newtype_index {
                 }
             }
 
+            /// Hacky variant of `from_u32` for use in constants.
+            /// This version checks the "max" constraint by using an
+            /// invalid array dereference.
+            #[inline]
+            $v const fn from_u32_const(value: u32) -> Self {
+                // This will fail at const eval time unless `value <=
+                // max` is true (in which case we get the index 0).
+                // It will also fail at runtime, of course, but in a
+                // kind of wacky way.
+                let _ = ["out of range value used"][
+                    !(value <= $max) as usize
+                ];
+
+                unsafe {
+                    $type::from_u32_unchecked(value)
+                }
+            }
+
             #[inline]
             $v const unsafe fn from_u32_unchecked(value: u32) -> Self {
                 $type { private: ::std::num::NonZeroU32::new_unchecked(value + 1) }
@@ -424,7 +442,7 @@ macro_rules! newtype_index {
                    const $name:ident = $constant:expr,
                    $($tokens:tt)*) => (
         $(#[doc = $doc])*
-        pub const $name: $type = unsafe { $type::from_u32_unchecked($constant) };
+        pub const $name: $type = $type::from_u32_const($constant);
         newtype_index!(
             @derives      [$($derives,)*]
             @type         [$type]