Add staticcheck tool

vendor/honnef.co/go/tools/simple/s1040/s1040.go

@@ -0,0 +1,73 @@
+package s1040
+
+import (
+	"fmt"
+	"go/ast"
+	"go/types"
+
+	"honnef.co/go/tools/analysis/code"
+	"honnef.co/go/tools/analysis/facts/generated"
+	"honnef.co/go/tools/analysis/lint"
+	"honnef.co/go/tools/analysis/report"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+)
+
+var SCAnalyzer = lint.InitializeAnalyzer(&lint.Analyzer{
+	Analyzer: &analysis.Analyzer{
+		Name:     "S1040",
+		Run:      run,
+		Requires: []*analysis.Analyzer{inspect.Analyzer, generated.Analyzer},
+	},
+	Doc: &lint.RawDocumentation{
+		Title: "Type assertion to current type",
+		Text: `The type assertion \'x.(SomeInterface)\', when \'x\' already has type
+\'SomeInterface\', can only fail if \'x\' is nil. Usually, this is
+left-over code from when \'x\' had a different type and you can safely
+delete the type assertion. If you want to check that \'x\' is not nil,
+consider being explicit and using an actual \'if x == nil\' comparison
+instead of relying on the type assertion panicking.`,
+		Since: "2021.1",
+		// MergeIfAll because x might have different types under different build tags.
+		// You shouldn't write code like that…
+		MergeIf: lint.MergeIfAll,
+	},
+})
+
+var Analyzer = SCAnalyzer.Analyzer
+
+func run(pass *analysis.Pass) (any, error) {
+	fn := func(node ast.Node) {
+		expr := node.(*ast.TypeAssertExpr)
+		if expr.Type == nil {
+			// skip type switches
+			//
+			// TODO(dh): we could flag type switches, too, when a case
+			// statement has the same type as expr.X – however,
+			// depending on the location of that case, it might behave
+			// identically to a default branch. we need to think
+			// carefully about the instances we want to flag. We also
+			// have to take nil interface values into consideration.
+			//
+			// It might make more sense to extend SA4020 to handle
+			// this.
+			return
+		}
+		t1 := pass.TypesInfo.TypeOf(expr.Type)
+		t2 := pass.TypesInfo.TypeOf(expr.X)
+		if types.IsInterface(t1) && types.Identical(t1, t2) {
+			report.Report(pass, expr,
+				fmt.Sprintf("type assertion to the same type: %s already has type %s", report.Render(pass, expr.X), report.Render(pass, expr.Type)),
+				report.FilterGenerated())
+		}
+	}
+
+	// TODO(dh): add suggested fixes. we need different fixes depending on the context:
+	// - assignment with 1 or 2 lhs
+	// - assignment to blank identifiers (as the first, second or both lhs)
+	// - initializers in if statements, with the same variations as above
+
+	code.Preorder(pass, fn, (*ast.TypeAssertExpr)(nil))
+	return nil, nil
+}