cmd/go: don't compute Deps fields if they're not needed

If the user provides the -json flag to explicitly specify fields, but
doesn't specify the Deps or DepsErrors fields, skip computing the deps
fields.

For #29666

Change-Id: I15596c374aba1af13bdf5808d11d54abdc838667
Reviewed-on: https://go-review.googlesource.com/c/go/+/392495
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/list/list.go

@@ -568,6 +568,13 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 		IgnoreImports:   *listFind,
 		ModResolveTests: *listTest,
 		LoadVCS:         true,
+		// SuppressDeps is set if the user opts to explicitly ask for the json fields they
+		// need, don't ask for Deps or DepsErrors. It's not set when using a template string,
+		// even if *listFmt doesn't contain .Deps because Deps are used to find import cycles
+		// for test variants of packages and users who have been providing format strings
+		// might not expect those errors to stop showing up.
+		// See issue #52443.
+		SuppressDeps: !listJsonFields.needAny("Deps", "DepsErrors"),
 	}
 	pkgs := load.PackagesAndErrors(ctx, pkgOpts, args)
 	if !*listE {

src/cmd/go/internal/load/pkg.go

@@ -1938,7 +1938,9 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 		}
 	}
 	p.Internal.Imports = imports
+	if !opts.SuppressDeps {
 		p.collectDeps()
+	}
 	if p.Error == nil && p.Name == "main" && !p.Internal.ForceLibrary && len(p.DepsErrors) == 0 {
 		// TODO(bcmills): loading VCS metadata can be fairly slow.
 		// Consider starting this as a background goroutine and retrieving the result
@@ -2679,6 +2681,12 @@ type PackageOpts struct {
 
 	// LoadVCS controls whether we also load version-control metadata for main packages.
 	LoadVCS bool
+
+	// NeedDepsFields is true if the caller does not need Deps and DepsErrors to be populated
+	// on the package. TestPackagesAndErrors examines the  Deps field to determine if the test
+	// variant has an import cycle, so SuppressDeps should not be set if TestPackagesAndErrors
+	// will be called on the package.
+	SuppressDeps bool
 }
 
 // PackagesAndErrors returns the packages named by the command line arguments

src/cmd/go/testdata/script/list_json_fields.txt

@@ -21,6 +21,11 @@ cmp stdout want-json-name.txt
 go list -json=ImportPath,Name,GoFiles,Imports
 cmp stdout want-json-multiple.txt
 
+# Test -json=<field> with Deps outputs the Deps field.
+go list -json=Deps
+stdout '"Deps": \['
+stdout '"errors",'
+
 -- go.mod --
 module example.com/a