internal/lsp/cache: improve snapshot clone perfomance

The existing implementation uses a lot of URI.Filename() calls,
which are pretty expensive. Moreover, these calls are not necessary,
as long as all the actions could be done with the raw URI string.
This patch removes such calls and uses simple string casts.

Updates golang/go#45686

Change-Id: Ibe11735969eaf0cfe33024f08418e14bf71e7fc4
GitHub-Last-Rev: 67a3ccd
GitHub-Pull-Request: #306
Reviewed-on: https://go-review.googlesource.com/c/tools/+/312809
Reviewed-by: Rebecca Stambler <[email protected]>
Trust: Rebecca Stambler <[email protected]>
Trust: Suzy Mueller <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>

internal/lsp/cache/snapshot.go

@@ -1282,13 +1282,12 @@ func contains(views []*View, view *View) bool {
 }
 
 func inVendor(uri span.URI) bool {
-	toSlash := filepath.ToSlash(uri.Filename())
-	if !strings.Contains(toSlash, "/vendor/") {
+	if !strings.Contains(string(uri), "/vendor/") {
 		return false
 	}
 	// Only packages in _subdirectories_ of /vendor/ are considered vendored
 	// (/vendor/a/foo.go is vendored, /vendor/foo.go is not).
-	split := strings.Split(toSlash, "/vendor/")
+	split := strings.Split(string(uri), "/vendor/")
 	if len(split) < 2 {
 		return false
 	}
@@ -1551,7 +1550,7 @@ func (s *snapshot) clone(ctx, bgCtx context.Context, changes map[span.URI]*fileC
 				// For internal tests, we need _test files, not just the normal
 				// ones. External tests only have _test files, but we can check
 				// them anyway.
-				if m.forTest != "" && !strings.HasSuffix(uri.Filename(), "_test.go") {
+				if m.forTest != "" && !strings.HasSuffix(string(uri), "_test.go") {
 					continue
 				}
 				if _, ok := result.files[uri]; ok {