internal/lsp/cache: preallocate internal maps when cloning snapshots

For large codebases, the cost of copying these maps can be fairly high,
especially when it needs to repeatedly grow the map's underlying storage.
Preallocate these to the size of the original snapshot maps to prevent
the need to grow the storage during the clone.

Updates golang/go#45686

Change-Id: I4cfcd5b7cba8110e4f7e706fd9ea968aaeb6ff0c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/312689
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

internal/lsp/cache/snapshot.go

@@ -1342,18 +1342,18 @@ func (s *snapshot) clone(ctx, bgCtx context.Context, changes map[span.URI]*fileC
 		builtin:           s.builtin,
 		initializeOnce:    s.initializeOnce,
 		initializedErr:    s.initializedErr,
-		ids:               make(map[span.URI][]packageID),
-		importedBy:        make(map[packageID][]packageID),
-		metadata:          make(map[packageID]*metadata),
-		packages:          make(map[packageKey]*packageHandle),
-		actions:           make(map[actionKey]*actionHandle),
-		files:             make(map[span.URI]source.VersionedFileHandle),
-		goFiles:           make(map[parseKey]*parseGoHandle),
-		workspacePackages: make(map[packageID]packagePath),
-		unloadableFiles:   make(map[span.URI]struct{}),
-		parseModHandles:   make(map[span.URI]*parseModHandle),
-		modTidyHandles:    make(map[span.URI]*modTidyHandle),
-		modWhyHandles:     make(map[span.URI]*modWhyHandle),
+		ids:               make(map[span.URI][]packageID, len(s.ids)),
+		importedBy:        make(map[packageID][]packageID, len(s.importedBy)),
+		metadata:          make(map[packageID]*metadata, len(s.metadata)),
+		packages:          make(map[packageKey]*packageHandle, len(s.packages)),
+		actions:           make(map[actionKey]*actionHandle, len(s.actions)),
+		files:             make(map[span.URI]source.VersionedFileHandle, len(s.files)),
+		goFiles:           make(map[parseKey]*parseGoHandle, len(s.goFiles)),
+		workspacePackages: make(map[packageID]packagePath, len(s.workspacePackages)),
+		unloadableFiles:   make(map[span.URI]struct{}, len(s.unloadableFiles)),
+		parseModHandles:   make(map[span.URI]*parseModHandle, len(s.parseModHandles)),
+		modTidyHandles:    make(map[span.URI]*modTidyHandle, len(s.modTidyHandles)),
+		modWhyHandles:     make(map[span.URI]*modWhyHandle, len(s.modWhyHandles)),
 		workspace:         newWorkspace,
 	}