Resolve #146: prealloc linter support

Author: jirfag

Diff for: .golangci.example.yml

@@ -128,6 +128,15 @@ linters-settings:
   nakedret:
     # make an issue if func has more lines of code than this setting and it has naked returns; default is 30
     max-func-lines: 30
+  prealloc:
+    # XXX: we don't recommend using this linter before doing performance profiling.
+    # For most programs usage of prealloc will be a premature optimization.
+
+    # Report preallocation suggestions only on simple loops that have no returns/breaks/continues/gotos in them.
+    # True by default.
+    simple: true
+    range-loops: true # Report preallocation suggestions on range loops, true by default
+    for-loops: false # Report preallocation suggestions on for loops, false by default
 
 
 linters:
@@ -136,7 +145,8 @@ linters:
     - govet
   enable-all: false
   disable:
-    maligned
+    - maligned
+    - prealloc
   disable-all: false
   presets:
     - bugs

Diff for: .golangci.yml

@@ -24,4 +24,5 @@ linters-settings:
 linters:
   enable-all: true
   disable:
-    - maligned
+    - maligned
+    - prealloc

Diff for: Gopkg.lock

@@ -127,6 +127,7 @@ misspell: Finds commonly misspelled English words in comments [fast: true]
 lll: Reports long lines [fast: true]
 unparam: Reports unused function parameters [fast: false]
 nakedret: Finds naked returns in functions greater than a specified function length [fast: true]
+prealloc: Finds slice declarations that could potentially be preallocated [fast: true]
 ```
 
 Pass `-E/--enable` to enable linter and `-D/--disable` to disable:
@@ -313,6 +314,7 @@ golangci-lint linters
 - [lll](https://github.com/walle/lll) - Reports long lines
 - [unparam](https://github.com/mvdan/unparam) - Reports unused function parameters
 - [nakedret](https://github.com/alexkohler/nakedret) - Finds naked returns in functions greater than a specified function length
+- [prealloc](https://github.com/alexkohler/prealloc) - Finds slice declarations that could potentially be preallocated
 
 # Configuration
 The config file has lower priority than command-line options. If the same bool/string/int option is provided on the command-line
@@ -539,6 +541,15 @@ linters-settings:
   nakedret:
     # make an issue if func has more lines of code than this setting and it has naked returns; default is 30
     max-func-lines: 30
+  prealloc:
+    # XXX: we don't recommend using this linter before doing performance profiling.
+    # For most programs usage of prealloc will be a premature optimization.
+
+    # Report preallocation suggestions only on simple loops that have no returns/breaks/continues/gotos in them.
+    # True by default.
+    simple: true
+    range-loops: true # Report preallocation suggestions on range loops, true by default
+    for-loops: false # Report preallocation suggestions on for loops, false by default
 
 
 linters:
@@ -547,7 +558,8 @@ linters:
     - govet
   enable-all: false
   disable:
-    maligned
+    - maligned
+    - prealloc
   disable-all: false
   presets:
     - bugs
@@ -620,6 +632,7 @@ linters:
   enable-all: true
   disable:
     - maligned
+    - prealloc
 ```
 
 # False Positives
@@ -726,6 +739,7 @@ There is the most valuable changes log:
 1. Add support of the next linters:
    * unparam
    * misspell
+   * prealloc
    * nakedret
    * lll
    * depguard

Diff for: README.md

@@ -388,6 +388,7 @@ There is the most valuable changes log:
 1. Add support of the next linters:
    * unparam
    * misspell
+   * prealloc
    * nakedret
    * lll
    * depguard

Diff for: README.tmpl.md

@@ -163,19 +163,26 @@ type LintersSettings struct {
 	Lll      LllSettings
 	Unparam  UnparamSettings
 	Nakedret NakedretSettings
+	Prealloc PreallocSettings
 }
 
 type LllSettings struct {
 	LineLength int `mapstructure:"line-length"`
 }
 
+type UnparamSettings struct {
+	CheckExported bool `mapstructure:"check-exported"`
+	Algo          string
+}
+
 type NakedretSettings struct {
 	MaxFuncLines int `mapstructure:"max-func-lines"`
 }
 
-type UnparamSettings struct {
-	CheckExported bool `mapstructure:"check-exported"`
-	Algo          string
+type PreallocSettings struct {
+	Simple     bool
+	RangeLoops bool `mapstructure:"range-loops"`
+	ForLoops   bool `mapstructure:"for-loops"`
 }
 
 var defaultLintersSettings = LintersSettings{
@@ -188,6 +195,11 @@ var defaultLintersSettings = LintersSettings{
 	Nakedret: NakedretSettings{
 		MaxFuncLines: 30,
 	},
+	Prealloc: PreallocSettings{
+		Simple:     true,
+		RangeLoops: true,
+		ForLoops:   false,
+	},
 }
 
 type Linters struct {

Diff for: pkg/config/config.go

@@ -0,0 +1,39 @@
+package golinters
+
+import (
+	"context"
+	"fmt"
+	"go/ast"
+
+	"github.com/golangci/golangci-lint/pkg/lint/linter"
+	"github.com/golangci/golangci-lint/pkg/result"
+	"github.com/golangci/prealloc"
+)
+
+type Prealloc struct{}
+
+func (Prealloc) Name() string {
+	return "prealloc"
+}
+
+func (Prealloc) Desc() string {
+	return "Finds slice declarations that could potentially be preallocated"
+}
+
+func (lint Prealloc) Run(ctx context.Context, lintCtx *linter.Context) ([]result.Issue, error) {
+	var res []result.Issue
+
+	s := &lintCtx.Settings().Prealloc
+	for _, f := range lintCtx.ASTCache.GetAllValidFiles() {
+		hints := prealloc.Check([]*ast.File{f.F}, s.Simple, s.RangeLoops, s.ForLoops)
+		for _, hint := range hints {
+			res = append(res, result.Issue{
+				Pos:        f.Fset.Position(hint.Pos),
+				Text:       fmt.Sprintf("Consider preallocating %s", formatCode(hint.DeclaredSliceName, lintCtx.Cfg)),
+				FromLinter: lint.Name(),
+			})
+		}
+	}
+
+	return res, nil
+}

Diff for: pkg/golinters/prealloc.go

@@ -181,6 +181,10 @@ func GetAllSupportedLinterConfigs() []linter.Config {
 			WithPresets(linter.PresetComplexity).
 			WithSpeed(10).
 			WithURL("https://github.com/alexkohler/nakedret"),
+		linter.NewConfig(golinters.Prealloc{}).
+			WithPresets(linter.PresetPerformance).
+			WithSpeed(8).
+			WithURL("https://github.com/alexkohler/prealloc"),
 	}
 
 	if os.Getenv("GOLANGCI_COM_RUN") == "1" {
@@ -193,6 +197,7 @@ func GetAllSupportedLinterConfigs() []linter.Config {
 			golinters.Lll{}.Name():       true, // annoying
 			golinters.Unparam{}.Name():   true, // need to check it first
 			golinters.Nakedret{}.Name():  true, // annoying
+			golinters.Prealloc{}.Name():  true, // annoying
 		}
 		return enableLinterConfigs(lcs, func(lc *linter.Config) bool {
 			return !disabled[lc.Linter.Name()]

Diff for: pkg/lint/lintersdb/lintersdb.go

@@ -0,0 +1,11 @@
+// args: -Eprealloc
+package testdata
+
+func Prealloc(source []int) []int {
+	var dest []int // ERROR "Consider preallocating `dest`"
+	for _, v := range source {
+		dest = append(dest, v)
+	}
+
+	return dest
+}