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proposal: Remove redundant package aliases from import blocks #296

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proposal: Remove redundant package aliases from import blocks #296

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18 changes: 18 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -229,6 +229,24 @@ import (

</details>

**Redundant package aliases are removed**

<details><summary><i>Example</i></summary>

```go
import (
math "math"
)
```

```go
import (
"math"
)
```

</details>

**Short case clauses should take a single line**

<details><summary><i>Example</i></summary>
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31 changes: 23 additions & 8 deletions format/format.go
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@ import (
"github.com/google/go-cmp/cmp"
"golang.org/x/mod/semver"
"golang.org/x/tools/go/ast/astutil"
"golang.org/x/tools/go/packages"

"mvdan.cc/gofumpt/internal/govendor/go/format"
"mvdan.cc/gofumpt/internal/version"
Expand Down Expand Up @@ -479,7 +480,7 @@ func (f *fumpter) applyPre(c *astutil.Cursor) {

case *ast.GenDecl:
if node.Tok == token.IMPORT && node.Lparen.IsValid() {
f.joinStdImports(node)
f.tidyImports(node)
}

// Single var declarations shouldn't use parentheses, unless
Expand Down Expand Up @@ -930,9 +931,10 @@ func isCgoImport(decl *ast.GenDecl) bool {
return v == "C"
}

// joinStdImports ensures that all standard library imports are together and at
// the top of the imports list.
func (f *fumpter) joinStdImports(d *ast.GenDecl) {
// tidyImports ensures that:
// - all standard library imports are together and at the top of the imports list, and
// - any redundant package aliases are removed.
func (f *fumpter) tidyImports(d *ast.GenDecl) {
var std, other []ast.Spec
firstGroup := true
lastEnd := d.Pos()
Expand All @@ -956,6 +958,23 @@ func (f *fumpter) joinStdImports(d *ast.GenDecl) {

for i, spec := range d.Specs {
spec := spec.(*ast.ImportSpec)
path, err := strconv.Unquote(spec.Path.Value)
if err != nil {
panic(err) // should never error
}

// If we import a package with an alias (i.e., if spec.Name is non-nil), and
// if the alias is the same as the package's name, then we remove the alias.
if spec.Name != nil {
packages, err := packages.Load(&packages.Config{Mode: packages.NeedName}, path)
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Unfortunately I think this is kind of a deal breaker - it would slow down formatting significantly. Right now the formatting is syntax-based alone, it doesn't load type information or any package information. All it looks at is what the main module path is, for the sake of grouping imports. gofmt has the same limitation, also for the sake of performance.

For example, I regularly run gofumpt -l -w . on large projects, so if that has to load every package in a named import for every file, that would likely mean hundreds of calls to go list, as well as orders of magnitude more opened files and syscalls.

That said, I agree that this is a good rule to enforce. Perhaps it should go into a tool that already loads type information - the two obvious candidates that come to mind are gopls (since it already has an "organize imports" code action akin to the old goimports) and staticcheck (since it has similar rules already like https://staticcheck.dev/docs/checks#ST1019).

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You're right: it does seem to slow things down quite a lot.

The monorepo I work in has around 1.1m lines of Go. Before the change, it took ~9s to act on the whole codebase; after, it takes ~27s. It's a pity: I try to be in the business of making things faster, not slower.

I'll see about contributing a new check to staticcheck.

Thanks for the quick response!

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I should note that you could make this faster in gofumpt by e.g. caching the calls to go/packages, or calling it upfront on all packages to format to reduce the number of execs to go list. But I still think that would be a significant slow-down, and going against the gofmt philosophy of formatting based on syntax alone.

For instance, we aim to provide a Go API compatible with https://pkg.go.dev/go/format. Such an API is self-contained, it doesn't require type information nor the ability to call go list.

if err != nil || len(packages) != 1 {
panic(fmt.Sprintf("loading the package %q: %v", path, err)) // should never error
}
if packages[0].Name == spec.Name.Name {
spec.Name = nil
}
}

if coms := f.commentsBetween(lastEnd, spec.Pos()); len(coms) > 0 {
lastEnd = coms[len(coms)-1].End()
}
Expand All @@ -966,10 +985,6 @@ func (f *fumpter) joinStdImports(d *ast.GenDecl) {
lastEnd = spec.End()
}

path, err := strconv.Unquote(spec.Path.Value)
if err != nil {
panic(err) // should never error
}
periodIndex := strings.IndexByte(path, '.')
slashIndex := strings.IndexByte(path, '/')
switch {
Expand Down
16 changes: 14 additions & 2 deletions testdata/script/std-imports.txtar
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Original file line number Diff line number Diff line change
Expand Up @@ -48,7 +48,7 @@ import (
import (
"foo.local"
"foo.local/three"
math "math"
maths "math"
)

import (
Expand Down Expand Up @@ -99,6 +99,12 @@ import (
"k8s.io/apimachinery/pkg/types"
"sigs.k8s.io/yaml"
)

// Redundant package aliases are removed.
import (
filepath "path/filepath"
"time"
)
-- foo.go.golden --
package p

Expand Down Expand Up @@ -135,7 +141,7 @@ import (

// We need to split std vs non-std in this case too.
import (
math "math"
maths "math"

"foo.local"
"foo.local/three"
Expand Down Expand Up @@ -191,3 +197,9 @@ import (

"sigs.k8s.io/yaml"
)

// Redundant package aliases are removed.
import (
"path/filepath"
"time"
)
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