Ideas for an analyser for returning errors from defer

Still working out how to traverse the AST to be able to see the objects
I need.

5 files changed, 110 insertions, 0 deletions

diff --git a/defererr.go b/defererr.go
new file mode 100644
index 0000000..d512cbf
--- /dev/null
+++ b/defererr.go
@@ -0,0 +1,61 @@
+// TODO: doc
+package defererr
+
+import (
+	"fmt"
+	"go/ast"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+var Analyzer = &analysis.Analyzer{
+	Name: "defererr",
+	Doc:  "reports issues returning errors from defer",
+	Run:  run,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	// TODO: Find defer closure
+	// Does it set error defined in outer scope?
+	// Does outer scope declare error variable in signature?
+	// Is err variable returned after closure?
+
+	for _, file := range pass.Files {
+		ast.Inspect(
+			file,
+			func(node ast.Node) bool {
+				deferStmt, ok := node.(*ast.DeferStmt)
+				if !ok {
+					return true
+				}
+
+				// Look for a function literal after the `defer` statement.
+				funcLit, ok := deferStmt.Call.Fun.(*ast.FuncLit)
+				if !ok {
+					return true
+				}
+
+				funcScope := pass.TypesInfo.Scopes[funcLit.Type]
+
+				// Try to find the function where the defer is defined. Note, defer can be defined in an inner block.
+				funcType, ok := funcScope.Parent().(*ast.FuncType)
+				if !ok {
+					return true
+				}
+				fmt.Printf("func: %#v\n", funcType)
+
+				if funcLit.Type.Results == nil {
+					return true
+				}
+
+				for _, returnVal := range funcLit.Type.Results.List {
+					fmt.Printf("returnVal: %#v\n", returnVal)
+				}
+
+				return true
+			},
+		)
+	}
+
+	return nil, nil
+}
diff --git a/defererr_test.go b/defererr_test.go
new file mode 100644
index 0000000..a94b3b9
--- /dev/null
+++ b/defererr_test.go
@@ -0,0 +1,14 @@
+package defererr_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+	"gopkg.teddywing.com/defererr"
+)
+
+func Test(t *testing.T) {
+	testdata := analysistest.TestData()
+
+	analysistest.Run(t, testdata, defererr.Analyzer, ".")
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..61d0b34
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,10 @@
+module gopkg.teddywing.com/defererr
+
+go 1.20
+
+require golang.org/x/tools v0.9.1
+
+require (
+	golang.org/x/mod v0.10.0 // indirect
+	golang.org/x/sys v0.8.0 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..cf10794
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,7 @@
+golang.org/x/mod v0.10.0 h1:lFO9qtOdlre5W1jxS3r/4szv2/6iXxScdzjoBMXNhYk=
+golang.org/x/mod v0.10.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
+golang.org/x/sync v0.2.0 h1:PUR+T4wwASmuSTYdKjYHI5TD22Wy5ogLU5qZCOLxBrI=
+golang.org/x/sys v0.8.0 h1:EBmGv8NaZBZTWvrbjNoL6HVt+IVy3QDQpJs7VRIw3tU=
+golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/tools v0.9.1 h1:8WMNJAz3zrtPmnYC7ISf5dEn3MT0gY7jBJfw27yrrLo=
+golang.org/x/tools v0.9.1/go.mod h1:owI94Op576fPu3cIGQeHs3joujW/2Oc6MtlxbF5dfNc=
diff --git a/testdata/signature.go b/testdata/signature.go
new file mode 100644
index 0000000..17cfd0b
--- /dev/null
+++ b/testdata/signature.go
@@ -0,0 +1,18 @@
+package main
+
+import "errors"
+
+func declareErrInSignature() error { // want "return signature should be '(err error)'"
+	var err error // Should use variable declared in signature
+
+	err = nil
+	if err != nil {
+		return err
+	}
+
+	defer func() {
+		err = errors.New("defer error")
+	}()
+
+	return nil // want "does not return 'err'"
+}