cmd/vet: support importing from source

Add a -source flag to cmd/vet that instructs
it to typecheck purely from source code.

Updates #16086
Fixes #19332

Change-Id: Ic83d0f14d5bb837a329d539b2873aeccdf7bf669
Reviewed-on: https://go-review.googlesource.com/37690Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/doc.go

@@ -34,6 +34,9 @@ If any flags are explicitly set to true, only those tests are run. Conversely, i
 any flag is explicitly set to false, only those tests are disabled.  Thus -printf=true
 runs the printf check, -printf=false runs all checks except the printf check.
 
+By default vet uses the object files generated by 'go install some/pkg' to typecheck the code.
+If the -source flag is provided, vet uses only source code.
+
 Available checks:
 
 Assembly declarations

src/cmd/vet/main.go

@@ -25,6 +25,7 @@ import (
 
 var (
 	verbose = flag.Bool("v", false, "verbose")
+	source  = flag.Bool("source", false, "import from source instead of compiled object files")
 	tags    = flag.String("tags", "", "space-separated list of build tags to apply when parsing")
 	tagList = []string{} // exploded version of tags flag; set in main
 )

src/cmd/vet/types.go

@@ -61,7 +61,11 @@ func importType(path, name string) types.Type {
 
 func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) error {
 	if stdImporter == nil {
-		stdImporter = importer.Default()
+		if *source {
+			stdImporter = importer.For("source", nil)
+		} else {
+			stdImporter = importer.Default()
+		}
 		inittypes()
 	}
 	pkg.defs = make(map[*ast.Ident]types.Object)