try to make things nice without regressing perf

Author: RalfJung

compiler/rustc_middle/src/mir/consts.rs

@@ -244,6 +244,8 @@ impl<'tcx> Const<'tcx> {
             Const::Ty(c) => match c.kind() {
                 ty::ConstKind::Value(valtree) if c.ty().is_primitive() => {
                     // A valtree of a type where leaves directly represent the scalar const value.
+                    // Just checking whether it is a leaf is insufficient as e.g. references are leafs
+                    // but the leaf value is the value they point to, not the reference itself!
                     Some(valtree.unwrap_leaf().into())
                 }
                 _ => None,
@@ -255,7 +257,17 @@ impl<'tcx> Const<'tcx> {
 
     #[inline]
     pub fn try_to_scalar_int(self) -> Option<ScalarInt> {
-        self.try_to_scalar()?.try_to_int().ok()
+        // This is equivalent to `self.try_to_scalar()?.try_to_int().ok()`, but measurably faster.
+        match self {
+            Const::Val(ConstValue::Scalar(Scalar::Int(x)), _) => Some(x),
+            Const::Ty(c) => match c.kind() {
+                ty::ConstKind::Value(valtree) if c.ty().is_primitive() => {
+                    Some(valtree.unwrap_leaf())
+                }
+                _ => None,
+            },
+            _ => None,
+        }
     }
 
     #[inline]

compiler/rustc_middle/src/thir.rs

@@ -1013,12 +1013,12 @@ impl<'tcx> PatRangeBoundary<'tcx> {
             // many ranges such as '\u{037A}'..='\u{037F}', and chars can be compared
             // in this way.
             (Finite(a), Finite(b)) if matches!(ty.kind(), ty::Uint(_) | ty::Char) => {
-                let to_scalar_int = |x| match x {
+                /*let to_scalar_int = |x| match x {
                     mir::Const::Val(mir::ConstValue::Scalar(Scalar::Int(x)), _) => Some(x),
                     mir::Const::Ty(x) => Some(x.to_valtree().unwrap_leaf()),
                     _ => None,
-                };
-                if let (Some(a), Some(b)) = (to_scalar_int(a), to_scalar_int(b)) {
+                };*/
+                if let (Some(a), Some(b)) = (a.try_to_scalar_int(), b.try_to_scalar_int()) {
                     let sz = ty.primitive_size(tcx);
                     let a = a.assert_uint(sz);
                     let b = b.assert_uint(sz);