internal/span: compute utf16 length directly

computing the length by hand avoids an allocation.

Change-Id: Idc15f9347628b3b68cb3722d03c2c706a4f60a68
Reviewed-on: https://go-review.googlesource.com/c/tools/+/321469
Run-TryBot: Peter Weinberger <pjw@google.com>
Trust: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/span/utf16.go

@@ -6,7 +6,6 @@ package span
 
 import (
 	"fmt"
-	"unicode/utf16"
 	"unicode/utf8"
 )
 
@@ -41,9 +40,14 @@ func ToUTF16Column(p Point, content []byte) (int, error) {
 	// Now, truncate down to the supplied column.
 	start = start[:colZero]
 
-	// and count the number of utf16 characters
-	// in theory we could do this by hand more efficiently...
-	return len(utf16.Encode([]rune(string(start)))) + 1, nil
+	cnt := 0
+	for _, r := range string(start) {
+		cnt++
+		if r > 0xffff {
+			cnt++
+		}
+	}
+	return cnt + 1, nil // the +1 is for 1-based columns
 }
 
 // FromUTF16Column advances the point by the utf16 character offset given the