Merge pull request #18302 from DougGregor/stdlib-type-checking-perf

 [Standard library] Ascribe types to some literals to improve typecheck time

Author: DougGregor

stdlib/public/core/Hasher.swift

@@ -90,10 +90,11 @@ internal struct _HasherTailBuffer {
   internal init(tail: UInt64, byteCount: UInt64) {
     // byteCount can be any value, but we only keep the lower 8 bits.  (The
     // lower three bits specify the count of bytes stored in this buffer.)
-    // FIXME: The "as UInt64" annotations here eliminate some exponential
+    // FIXME: This should be a single expression, but it causes exponential
     // behavior in the expression type checker <rdar://problem/42672946>.
-    _sanityCheck(tail & ~((1 as UInt64) << ((byteCount & (7 as UInt64))
-          << (3 as UInt64)) - (1 as UInt64)) == (0 as UInt64))
+    let shiftedByteCount: UInt64 = ((byteCount & 7) << 3)
+    let mask: UInt64 = (1 << shiftedByteCount - 1)
+    _sanityCheck(tail & ~mask == 0)
     self.value = (byteCount &<< 56 | tail)
   }
 

stdlib/public/core/UTF16.swift

@@ -60,7 +60,9 @@ extension Unicode.UTF16 : Unicode.Encoding {
       return Unicode.Scalar(_unchecked: bits & 0xffff)
     }
     _sanityCheck(source._bitCount == 32)
-    let value = 0x10000 + (bits >> 16 & 0x03ff | (bits & 0x03ff) << 10)
+    let lower: UInt32 = bits >> 16 & 0x03ff
+    let upper: UInt32 = (bits & 0x03ff) << 10
+    let value = 0x10000 + (lower | upper)
     return Unicode.Scalar(_unchecked: value)
   }