internal/lsp/lsppos: reduce allocations in NewMapper

Before, NewMapper accounts for 2.1% of bytes allocated in the
WorkspaceSymbols benchmark.  This change causes the newline index
table to be allocated once instead of by appending.
The function now accounts for 0.55%.

Change-Id: I9172dd34ee2be9e7175e311d4a6518f1e6660a5f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/415501
Auto-Submit: Alan Donovan <[email protected]>
gopls-CI: kokoro <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

internal/lsp/lsppos/lsppos.go

@@ -17,6 +17,7 @@
 package lsppos
 
 import (
+	"bytes"
 	"errors"
 	"sort"
 	"unicode/utf8"
@@ -36,9 +37,10 @@ type Mapper struct {
 
 // NewMapper creates a new Mapper for the given content.
 func NewMapper(content []byte) *Mapper {
+	nlines := bytes.Count(content, []byte("\n"))
 	m := &Mapper{
 		content: content,
-		lines:   []int{0},
+		lines:   make([]int, 1, nlines+1), // initially []int{0}
 	}
 	for offset, b := range content {
 		if b == '\n' {