Unnecessary Slice-to-Slice Conversion analyzer (#7321)

* analyzer with tests
* fix bazel file
* modify analyzer to fix build issues
* add analyzer to tool chain
* remove arrays from inspections
* fix redundant [:] operator
* Merge branch 'master' into use-slice-directly
* Merge branch 'master' into use-slice-directly
* fix another inspection
* add package-level comment

tools/analyzers/slicedirect/BUILD.bazel

@@ -0,0 +1,28 @@
+load("@prysm//tools/go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_tool_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["analyzer.go"],
+    importpath = "github.com/prysmaticlabs/prysm/tools/analyzers/slicedirect",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@org_golang_x_tools//go/analysis:go_default_library",
+        "@org_golang_x_tools//go/analysis/passes/inspect:go_default_library",
+        "@org_golang_x_tools//go/ast/inspector:go_default_library",
+    ],
+)
+
+go_tool_library(
+    name = "go_tool_library",
+    srcs = ["analyzer.go"],
+    importpath = "github.com/prysmaticlabs/prysm/tools/analyzers/slicedirect",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@org_golang_x_tools//go/analysis:go_tool_library",
+        "@org_golang_x_tools//go/analysis/passes/inspect:go_tool_library",
+        "@org_golang_x_tools//go/ast/inspector:go_tool_library",
+    ],
+)
+
+# gazelle:exclude analyzer_test.go

tools/analyzers/slicedirect/analyzer.go

@@ -0,0 +1,63 @@
+// Package slicedirect implements a static analyzer to ensure that code does not contain
+// applications of [:] on expressions which are already slices.
+package slicedirect
+
+import (
+	"errors"
+	"go/ast"
+	"go/types"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+)
+
+// Doc explaining the tool.
+const Doc = "Tool to detect unnecessary slice-to-slice conversion by applying [:] to a slice expression."
+
+// Analyzer runs static analysis.
+var Analyzer = &analysis.Analyzer{
+	Name:     "slicedirect",
+	Doc:      Doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect, ok := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	if !ok {
+		return nil, errors.New("analyzer is not type *inspector.Inspector")
+	}
+
+	nodeFilter := []ast.Node{
+		(*ast.SliceExpr)(nil),
+	}
+
+	typeInfo := types.Info{Types: make(map[ast.Expr]types.TypeAndValue)}
+
+	inspect.Preorder(nodeFilter, func(node ast.Node) {
+		sliceExpr, ok := node.(*ast.SliceExpr)
+		if !ok {
+			return
+		}
+
+		if err := types.CheckExpr(pass.Fset, pass.Pkg, sliceExpr.X.Pos(), sliceExpr.X, &typeInfo); err != nil {
+			return
+		}
+
+		if sliceExpr.Low != nil || sliceExpr.High != nil {
+			return
+		}
+
+		switch x := typeInfo.Types[sliceExpr.X].Type.(type) {
+		case *types.Slice:
+			pass.Reportf(sliceExpr.Pos(), "Expression is already a slice.")
+		case *types.Basic:
+			if x.String() == "string" {
+				pass.Reportf(sliceExpr.Pos(), "Expression is already a slice.")
+			}
+		}
+	})
+
+	return nil, nil
+}

tools/analyzers/slicedirect/analyzer_test.go

@@ -0,0 +1,11 @@
+package slicedirect
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+)
+
+func TestAnalyzer(t *testing.T) {
+	analysistest.Run(t, analysistest.TestData(), Analyzer)
+}

tools/analyzers/slicedirect/testdata/BUILD.bazel

@@ -0,0 +1,8 @@
+load("@prysm//tools/go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["slice.go"],
+    importpath = "github.com/prysmaticlabs/prysm/tools/analyzers/slicedirect/testdata",
+    visibility = ["//visibility:public"],
+)

tools/analyzers/slicedirect/testdata/slice.go

@@ -0,0 +1,46 @@
+package testdata
+
+func NoIndexProvided() {
+	x := []byte{'f', 'o', 'o'}
+	y := x[:] // want "Expression is already a slice."
+	if len(y) == 3 {
+	}
+}
+
+func StartIndexProvided_NoDiagnostic() {
+	x := []byte{'f', 'o', 'o'}
+	y := x[1:]
+	if len(y) == 3 {
+	}
+}
+
+func EndIndexProvided_NoDiagnostic() {
+	x := []byte{'f', 'o', 'o'}
+	y := x[:2]
+	if len(y) == 3 {
+	}
+}
+
+func BothIndicesProvided_NoDiagnostic() {
+	x := []byte{'f', 'o', 'o'}
+	y := x[1:2]
+	if len(y) == 3 {
+	}
+}
+
+func StringSlice() {
+	x := "foo"
+	y := x[:] // want "Expression is already a slice."
+	if len(y) == 3 {
+	}
+}
+
+func SliceFromFunction() {
+	x := getSlice()[:] // want "Expression is already a slice."
+	if len(x) == 3 {
+	}
+}
+
+func getSlice() []string {
+	return []string{"bar"}
+}