x/tools/imports: add FormatOnly option

[moorereason]

Proposal

Add a FormatOnly field to the imports.Options type. Setting FormatOnly to true will bypass the "fix" functionality so that no imports will be inserted or deleted. Default would be false and backward compatible.

Thanks for your consideration.

Rationale

I'm working on a Go code generator and would like for the generated import stanza to be formatted according to the standard goimports style. Some projects enforce goimports formatting such that improperly formatted files will cause a CI build failure.

However, we don't want the import-insertion logic of the imports package to execute. If our generator creates code with missing imports, we consider that a bug in the generator and don't want to mask the bugs by using imports.Process. We've also seen goimports insert the wrong package and want to avoid that scenario, too.

This proposal would also disable the deletion of unused imports, but that functionality is easily replicated with astutil.UsesImport. In our project, we already delete unused imports manually and would continue doing so prior to using imports.Process.

Alternatives

Add an option to go/format

An option could to be added to go/format to enable goimports formatting, but I'm doubtful that the Go team would care to alter go/format for this use. go/format does one thing (with no options): canonical gofmt formatting. Unless we're willing to change the canon (blasphemy!), this is a dead end.

Copy code

We looked at copying the necessary code out of x/tools/imports, but we didn't like that option. It ended up being more code than we cared to copy and added a lot of complexity that we felt was unnecessary.

Proof of Concept

diff --git a/imports/imports.go b/imports/imports.go
index e30946b..5342801 100644
--- a/imports/imports.go
+++ b/imports/imports.go
@@ -31,6 +31,8 @@ type Options struct {
        Comments  bool // Print comments (true if nil *Options provided)
        TabIndent bool // Use tabs for indent (true if nil *Options provided)
        TabWidth  int  // Tab width (8 if nil *Options provided)
+
+       FormatOnly bool // Disable the insertion and deletion of imports
 }

 // Process formats and adjusts imports for the provided file.
@@ -46,9 +48,11 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
                return nil, err
        }

-       _, err = fixImports(fileSet, file)
-       if err != nil {
-               return nil, err
+       if !opt.FormatOnly {
+               _, err = fixImports(fileSet, file)
+               if err != nil {
+                       return nil, err
+               }
        }

        sortImports(fileSet, file)

[bradfitz]

I created this mess (goimports having an opinion on import blocks, where gofmt doesn't) so I'll take this bug. No promises, but I promise to at least think about it.

[bketelsen]

Yay!