The fiximports command fixes import declarations to use the canonical import path for packages that have an "import comment" as defined by https://golang.org/s/go14customimport.
The Go 1 custom import path mechanism lets the maintainer of a package give it a stable name by which clients may import and "go get" it, independent of the underlying version control system (such as Git) or server (such as github.com) that hosts it. Requests for the custom name are redirected to the underlying name. This allows packages to be migrated from one underlying server or system to another without breaking existing clients.
Because this redirect mechanism creates aliases for existing packages, it's possible for a single program to import the same package by its canonical name and by an alias. The resulting executable will contain two copies of the package, which is wasteful at best and incorrect at worst.
To avoid this, "go build" reports an error if it encounters a special comment like the one below, and if the import path in the comment does not match the path of the enclosing package relative to GOPATH/src:
$ grep ^package $GOPATH/src/github.com/bob/vanity/foo/foo.go package foo // import "vanity.com/foo"
The error from "go build" indicates that the package canonically known as "vanity.com/foo" is locally installed under the non-canonical name "github.com/bob/vanity/foo".
When a package that you depend on introduces a custom import comment, and your workspace imports it by the non-canonical name, your build will stop working as soon as you update your copy of that package using "go get -u".
The purpose of the fiximports tool is to fix up all imports of the non-canonical path within a Go workspace, replacing them with imports of the canonical path. Following a run of fiximports, the workspace will no longer depend on the non-canonical copy of the package, so it should be safe to delete. It may be necessary to run "go get -u" again to ensure that the package is locally installed under its canonical path, if it was not already.
The fiximports tool operates locally; it does not make HTTP requests and does not discover new custom import comments. It only operates on non-canonical packages present in your workspace.
The -baddomains flag is a list of domain names that should always be considered non-canonical. You can use this if you wish to make sure that you no longer have any dependencies on packages from that domain, even those that do not yet provide a canonical import path comment. For example, the default value of -baddomains includes the moribund code hosting site code.google.com, so fiximports will report an error for each import of a package from this domain remaining after canonicalization.
To see the changes fiximports would make without applying them, use the -n flag.