cmd/go: ignore implicit imports when the -find flag is set

The documentation of the go list -find flag says that the Deps list will
be empty. However the current implementation adds implicit imports when
supporting Cgo or SWIG and when linking a main package.

Update the documentation of PackageOpts.IgnoreImport to clarify that
both explicit and implicit imports are ignored.

Add a regression test.

Fixes #46092

Change-Id: I37847528d84adb7a18eb6ff29e4af4b4318a66fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/318770
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -1797,35 +1797,37 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 		}
 	}
 
-	// Cgo translation adds imports of "unsafe", "runtime/cgo" and "syscall",
-	// except for certain packages, to avoid circular dependencies.
-	if p.UsesCgo() {
-		addImport("unsafe", true)
-	}
-	if p.UsesCgo() && (!p.Standard || !cgoExclude[p.ImportPath]) && cfg.BuildContext.Compiler != "gccgo" {
-		addImport("runtime/cgo", true)
-	}
-	if p.UsesCgo() && (!p.Standard || !cgoSyscallExclude[p.ImportPath]) {
-		addImport("syscall", true)
-	}
-
-	// SWIG adds imports of some standard packages.
-	if p.UsesSwig() {
-		addImport("unsafe", true)
-		if cfg.BuildContext.Compiler != "gccgo" {
+	if !opts.IgnoreImports {
+		// Cgo translation adds imports of "unsafe", "runtime/cgo" and "syscall",
+		// except for certain packages, to avoid circular dependencies.
+		if p.UsesCgo() {
+			addImport("unsafe", true)
+		}
+		if p.UsesCgo() && (!p.Standard || !cgoExclude[p.ImportPath]) && cfg.BuildContext.Compiler != "gccgo" {
 			addImport("runtime/cgo", true)
 		}
-		addImport("syscall", true)
-		addImport("sync", true)
+		if p.UsesCgo() && (!p.Standard || !cgoSyscallExclude[p.ImportPath]) {
+			addImport("syscall", true)
+		}
 
-		// TODO: The .swig and .swigcxx files can use
-		// %go_import directives to import other packages.
-	}
+		// SWIG adds imports of some standard packages.
+		if p.UsesSwig() {
+			addImport("unsafe", true)
+			if cfg.BuildContext.Compiler != "gccgo" {
+				addImport("runtime/cgo", true)
+			}
+			addImport("syscall", true)
+			addImport("sync", true)
 
-	// The linker loads implicit dependencies.
-	if p.Name == "main" && !p.Internal.ForceLibrary {
-		for _, dep := range LinkerDeps(p) {
-			addImport(dep, false)
+			// TODO: The .swig and .swigcxx files can use
+			// %go_import directives to import other packages.
+		}
+
+		// The linker loads implicit dependencies.
+		if p.Name == "main" && !p.Internal.ForceLibrary {
+			for _, dep := range LinkerDeps(p) {
+				addImport(dep, false)
+			}
 		}
 	}
 
@@ -2387,7 +2389,9 @@ func LoadImportWithFlags(path, srcDir string, parent *Package, stk *ImportStack,
 // PackageOpts control the behavior of PackagesAndErrors and other package
 // loading functions.
 type PackageOpts struct {
-	// IgnoreImports controls whether we ignore imports when loading packages.
+	// IgnoreImports controls whether we ignore explicit and implicit imports
+	// when loading packages.  Implicit imports are added when supporting Cgo
+	// or SWIG and when linking main packages.
 	IgnoreImports bool
 
 	// ModResolveTests indicates whether calls to the module loader should also

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

@@ -0,0 +1,39 @@
+# Issue #46092
+# go list -find should always return a package with an empty Deps list
+
+# The linker loads implicit dependencies
+go list -find -f {{.Deps}} ./cmd
+stdout '\[\]'
+
+# Cgo translation may add imports of "unsafe", "runtime/cgo" and "syscall"
+go list -find -f {{.Deps}} ./cgo
+stdout '\[\]'
+
+# SWIG adds imports of some standard packages
+go list -find -f {{.Deps}} ./swig
+stdout '\[\]'
+
+-- go.mod --
+module listfind
+
+-- cmd/main.go --
+package main
+
+func main() {}
+
+-- cgo/pkg.go --
+package cgopkg
+
+/*
+#include <limits.h>
+*/
+import "C"
+
+func F() {
+    println(C.INT_MAX)
+}
+
+-- swig/pkg.go --
+package swigpkg
+
+-- swig/a.swigcxx --