If It Ain't Broken, Go Fix It!

Hi ,

The new go fix command of Go 1.26 should have really been named go polish or go beef-it-up.

What go fix, you ask? Well, yeah, go fix isn't exactly the most popular tool in the Go toolchain. In the early days of Go, it served for fixing Go code when a new release changed library APIs. But Go turned out to be so backward-compatible that such fixes aren't really necessary, and go fix became as irrelevant as a tool can become.

The Go team pulled the plug on the old go fix by reinventing it from the ground up. Now it acts similar to go vet: It has a list of static analyzers that detect outdated language constructs and auto-updates them to their modern counterpart. I tried go fix on a few repos, and it reliably modernized old-fashioned code.

For example, it found an occasion for using the quite new min() function.

Old code:

upper := 3
if len(dirs) < 3 {
    upper = len(dirs)
}

New code:

upper := min(len(dirs), 3)

It turned an old-fashioned

for i := 0; i < 100; i++ {

into

for i := range 100 {

and turned a Split() into SplitSeq() and a HasPrefix()/TrimPrefix() combo into CutPrefix().

Before:

for _, line := range strings.Split(head, "\n") {
    line = strings.TrimSpace(line)
    if strings.HasPrefix(line, key+":") {
        return strings.TrimSpace(strings.Trim(strings.TrimPrefix(line, key+":"), " \""))

After:

for line := range strings.SplitSeq(head, "\n") {
    line = strings.TrimSpace(line)
    if after, ok := strings.CutPrefix(line, key+":"); ok {
        return strings.TrimSpace(strings.Trim(after, " \""))

Here is the full list of current analyzers:

(From go tool fix help.)

While go fix uses the same architecture as go vet, the analyzers are different (take a look at go tool vet help for comparison). So go fix is not a go vet plus auto-apply. Rather, go fix handles specific cases that can be "fixed" (that is, modernized) right away, while go vet only suggests possible changes but leaves it to the user to review, assess, and apply or ignore a suggestion.

If you want to learn more, Go feature: Modernized go fix showcases each of the above analyzers. (The article came out during this newsletter's winter break, but I'm happily linking to it now.) The Go team also announced upcoming blog articles about details of Go 1.26's changes and additions, and I expect one about go fix among them. Stay tuned.

–Christoph

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