go-marshal: Improve collision detection of import statments.

Previously, the import statement collision detection mechanism aborted
go-marshal whenever it detected two imports in any package that has
the same local name. Consider this trivial package, defined by the the
following two source files:

file1.go:

package example
import (
        path/a/to/foo
)
...

file2.go:

package example
import (
       another/package/with/final/component/foo
)
...

Go-marshal previously couldn't handle generating code for the the
above package, even if none of the types marked for marshalling used
either of the imported foo packages. This turns out to be too
restrictive as we run into this a lot in practice. Examples include
"encoding/binary" vs "gvisor/pkg/binary/binary", and "sync" vs
"gvisor/pkg/sync/sync".

This change allows go-marshal to proceed with marshalling, and only
abort if the code generated by go-marshal references any such
ambiguous import names.

PiperOrigin-RevId: 304131190

1 files changed, 2 insertions, 2 deletions

diff --git a/tools/go_marshal/gomarshal/generator.go b/tools/go_marshal/gomarshal/generator.go
index 935a36b25..43e668b63 100644
--- a/tools/go_marshal/gomarshal/generator.go
+++ b/tools/go_marshal/gomarshal/generator.go
@@ -326,7 +326,7 @@ func (g *Generator) collectImports(a *ast.File, f *token.FileSet) map[string]imp
 
 			// Make sure we have an import that doesn't use any local names that
 			// would conflict with identifiers in the generated code.
-			if len(i.name) == 1 {
+			if len(i.name) == 1 && i.name != "_" {
 				abortAt(f.Position(spec.Pos()), fmt.Sprintf("Import has a single character local name '%s'; this may conflict with code generated by go_marshal, use a multi-character import alias", i.name))
 			}
 			if _, ok := badIdentsMap[i.name]; ok {
@@ -421,7 +421,7 @@ func (g *Generator) Run() error {
 			// the list of imports we need to copy to the generated code.
 			for name, _ := range impl.is {
 				if !g.imports.markUsed(name) {
-					panic(fmt.Sprintf("Generated code for '%s' referenced a non-existent import with local name '%s'", impl.typeName(), name))
+					panic(fmt.Sprintf("Generated code for '%s' referenced a non-existent import with local name '%s'. Either go-marshal needs to add an import to the generated file, or a package in an input source file has a package name differ from the final component of its path, which go-marshal doesn't know how to detect; use an import alias to work around this limitation.", impl.typeName(), name))
 				}
 			}
 			ts = append(ts, g.generateOneTestSuite(t))