cmd/go/internal/modload: avoid loading the module graph to list only …

…the name of the main module

For #36460
For #29666

Change-Id: I9e46f7054d52c053be80c483757cdd34b22822d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/309190
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

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

@@ -78,6 +78,10 @@ func ListModules(ctx context.Context, args []string, mode ListMode) ([]*modinfo.
 }
 
 func listModules(ctx context.Context, rs *Requirements, args []string, mode ListMode) (_ *Requirements, mods []*modinfo.ModulePublic, mgErr error) {
+	if len(args) == 0 {
+		return rs, []*modinfo.ModulePublic{moduleInfo(ctx, rs, Target, mode)}, nil
+	}
+
 	var mg *ModuleGraph
 	if go117LazyTODO {
 		// Pull the args-loop below into another (new) loop.
@@ -90,10 +94,6 @@ func listModules(ctx context.Context, rs *Requirements, args []string, mode List
 		rs, mg, mgErr = expandGraph(ctx, rs)
 	}
 
-	if len(args) == 0 {
-		return rs, []*modinfo.ModulePublic{moduleInfo(ctx, rs, Target, mode)}, mgErr
-	}
-
 	matchedModule := map[module.Version]bool{}
 	for _, arg := range args {
 		if strings.Contains(arg, `\`) {
@@ -149,10 +149,6 @@ func listModules(ctx context.Context, rs *Requirements, args []string, mode List
 			continue
 		}
 
-		if go117LazyTODO {
-			ModRoot() // Unversioned paths require that we be inside a module.
-		}
-
 		// Module path or pattern.
 		var match func(string) bool
 		if arg == "all" {

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

@@ -22,7 +22,7 @@ cmp go.mod go.mod.orig
 
 # Update manually. Listing modules should produce an error.
 go mod edit -require=example.com/badchain/a@v1.1.0
-! go list -m
+! go list -m all
 cmp stderr list-expected
 
 # Try listing a package that imports a package