internal/gcimporter: avoid a crash when exporting a struct with no pkg

This is an arbitrary fix to avoid a crash while exporting an empty
struct with no ambient package. Unfortunately, the struct encoding
assumes that there is an implied package for structs, which at least in
invalid code may not be the case.

Fixes golang/go#60891

Change-Id: I8acd591ac15cdc1770d615fdca57dcb7bb9b7651
Reviewed-on: https://go-review.googlesource.com/c/tools/+/504556
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

internal/gcimporter/gcimporter_test.go

@@ -809,6 +809,9 @@ func TestExportInvalid(t *testing.T) {
 		// It must be possible to export a constant with unknown kind, even if its
 		// type is known.
 		{"issue 60605", `package p; const EPSILON float64 = 1e-`, "EPSILON"},
+
+		// We must not crash when exporting a struct with unknown package.
+		{"issue 60891", `package p; type I[P any] int; const C I[struct{}] = 42`, "C"},
 	}
 
 	for _, test := range tests {

internal/gcimporter/iexport.go

@@ -673,6 +673,9 @@ func (w *exportWriter) qualifiedType(obj *types.TypeName) {
 	w.pkg(obj.Pkg())
 }
 
+// TODO(rfindley): what does 'pkg' even mean here? It would be better to pass
+// it in explicitly into signatures and structs that may use it for
+// constructing fields.
 func (w *exportWriter) typ(t types.Type, pkg *types.Package) {
 	w.data.uint64(w.p.typOff(t, pkg))
 }
@@ -773,7 +776,21 @@ func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 		if n > 0 {
 			w.setPkg(t.Field(0).Pkg(), true) // qualifying package for field objects
 		} else {
-			w.setPkg(pkg, true)
+			// TODO(rfindley): improve this very hacky logic.
+			//
+			// The importer expects a package to be set for all struct types, even
+			// those with no fields. A better encoding might be to set NumFields
+			// before pkg. setPkg panics with a nil package, which may be possible
+			// to reach with invalid packages (and perhaps valid packages, too?), so
+			// (arbitrarily) set the localpkg if available.
+			switch {
+			case pkg != nil:
+				w.setPkg(pkg, true)
+			case w.p.shallow:
+				w.setPkg(w.p.localpkg, true)
+			default:
+				panic(internalErrorf("no package to set for empty struct"))
+			}
 		}
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {