const-ify some {IndexVec, IndexSlice} methods

compiler/rustc_index/src/slice.rs

@@ -23,12 +23,12 @@ pub struct IndexSlice<I: Idx, T> {
 
 impl<I: Idx, T> IndexSlice<I, T> {
     #[inline]
-    pub fn empty() -> &'static Self {
-        Default::default()
+    pub const fn empty() -> &'static Self {
+        Self::from_raw(&[])
     }
 
     #[inline]
-    pub fn from_raw(raw: &[T]) -> &Self {
+    pub const fn from_raw(raw: &[T]) -> &Self {
         let ptr: *const [T] = raw;
         // SAFETY: `IndexSlice` is `repr(transparent)` over a normal slice
         unsafe { &*(ptr as *const Self) }
@@ -42,10 +42,15 @@ impl<I: Idx, T> IndexSlice<I, T> {
     }
 
     #[inline]
-    pub fn len(&self) -> usize {
+    pub const fn len(&self) -> usize {
         self.raw.len()
     }
 
+    #[inline]
+    pub const fn is_empty(&self) -> bool {
+        self.raw.is_empty()
+    }
+
     /// Gives the next index that will be assigned when `push` is called.
     ///
     /// Manual bounds checks can be done using `idx < slice.next_index()`
@@ -55,11 +60,6 @@ impl<I: Idx, T> IndexSlice<I, T> {
         I::new(self.len())
     }
 
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.raw.is_empty()
-    }
-
     #[inline]
     pub fn iter(&self) -> slice::Iter<'_, T> {
         self.raw.iter()

compiler/rustc_index/src/vec.rs

@@ -26,12 +26,12 @@ pub struct IndexVec<I: Idx, T> {
 
 impl<I: Idx, T> IndexVec<I, T> {
     #[inline]
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         IndexVec { raw: Vec::new(), _marker: PhantomData }
     }
 
     #[inline]
-    pub fn from_raw(raw: Vec<T>) -> Self {
+    pub const fn from_raw(raw: Vec<T>) -> Self {
         IndexVec { raw, _marker: PhantomData }
     }