gopls/internal/golang: Rename imports: be defensive

This CL makes the map lookup/type assertion defensive.
I have no hypothesis for why it fails (it should be sound
given the postcondition of the "pass 1" loop), but
this will at least stop the tool from crashing.

Fixes golang/go#71656

Change-Id: I32576689e4a2e2fcf7f2640f877a8580107cc387
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667195
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/golang/rename.go

@@ -1152,7 +1152,12 @@ func renameImports(ctx context.Context, snapshot *cache.Snapshot, mp *metadata.P
 					continue // not the import we're looking for
 				}
 
-				pkgname := pkg.TypesInfo().Implicits[imp].(*types.PkgName)
+				pkgname, ok := pkg.TypesInfo().Implicits[imp].(*types.PkgName)
+				if !ok {
+					// "can't happen", but be defensive (#71656)
+					return fmt.Errorf("internal error: missing type information for %s import at %s",
+						imp.Path.Value, safetoken.StartPosition(pkg.FileSet(), imp.Pos()))
+				}
 
 				pkgScope := pkg.Types().Scope()
 				fileScope := pkg.TypesInfo().Scopes[f.File]