all: avoid using too many packages.Need bits

go/packages.LoadMode exists so that it only asks `go list` for the
information you need, and no more.
Some of those bits are actually pretty expensive to use.
For instance, the Syntax ones will use go/parser to load go/ast files,
and NeedDeps will mean loading info for all dependencies recursively.

We really don't need everything; in fact, our needs are rather simple.
Reduce the LoadMode sets we use, driven by skimming the code and
re-running the tests and generation steps to ensure nothing is broken.

As a very coarse benchmark, the tests for the two packages get faster
on average, running three times before and after:

	cmd/cue/cmd     goes from ~3.2s to ~2.8s
	encoding/gocode goes from ~1.1s to ~0.2s

Overall, `cue get go` and `gocode.Generate` should be slightly faster.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I08cd45b5ecd9bdd9ce84e2ceadc70880f82293c2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/538063
Unity-Result: CUEcueckoo <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

cmd/cue/cmd/get_go.go

@@ -334,10 +334,8 @@ func initInterfaces() (err error) {
 	}
 
 	cfg := &packages.Config{
-		Mode: packages.NeedName | packages.NeedFiles | packages.NeedCompiledGoFiles |
-			packages.NeedImports | packages.NeedTypes | packages.NeedTypesSizes |
-			packages.NeedSyntax | packages.NeedTypesInfo | packages.NeedDeps,
-		Dir: filepath.Join(tmpDir),
+		Mode: packages.NeedTypes | packages.NeedTypesInfo,
+		Dir:  filepath.Join(tmpDir),
 	}
 
 	p, err := packages.Load(cfg, fullPkgPath)
@@ -404,9 +402,9 @@ func extract(cmd *Command, args []string) error {
 	root := binst.Root
 
 	cfg := &packages.Config{
-		Mode: packages.NeedName | packages.NeedFiles | packages.NeedCompiledGoFiles |
-			packages.NeedImports | packages.NeedTypes | packages.NeedTypesSizes |
-			packages.NeedSyntax | packages.NeedTypesInfo | packages.NeedDeps |
+		Mode: packages.NeedName | packages.NeedCompiledGoFiles |
+			packages.NeedImports | packages.NeedTypes |
+			packages.NeedSyntax | packages.NeedTypesInfo |
 			packages.NeedModule,
 	}
 	pkgs, err := packages.Load(cfg, args...)

cue/testdata/gen.go

@@ -49,7 +49,7 @@ func main() {
 	log.SetFlags(log.Lshortfile)
 
 	cfg := &packages.Config{
-		Mode: packages.NeedSyntax | packages.NeedTypes | packages.NeedTypesInfo,
+		Mode: packages.NeedSyntax | packages.NeedTypesInfo,
 
 		Tests: true,
 	}

encoding/gocode/generator.go

@@ -115,7 +115,7 @@ func Generate(pkgPath string, inst *cue.Instance, c *Config) (b []byte, err erro
 
 	if pkgPath != "" {
 		loadCfg := &packages.Config{
-			Mode: packages.NeedName | packages.NeedFiles | packages.NeedCompiledGoFiles | packages.NeedImports | packages.NeedTypes | packages.NeedTypesSizes | packages.NeedSyntax | packages.NeedTypesInfo | packages.NeedDeps,
+			Mode: packages.NeedName | packages.NeedTypes | packages.NeedTypesInfo,
 		}
 		pkgs, err := packages.Load(loadCfg, pkgPath)
 		if err != nil {