uncheckedflushreturn: flag discarded errors in defer ...Flush()

pkg/linters/uncheckedflushreturn/testdata/src/uncheckedflushreturn/uncheckedflushreturn.go

@@ -21,6 +21,12 @@ func badBlankAssign() {
 	_ = bw.Flush() // want `error return from Flush\(\) is discarded`
 }
 
+func deferBad() {
+	var sb strings.Builder
+	bw := bufio.NewWriter(&sb)
+	defer bw.Flush() // want `error return from Flush\(\) is discarded`
+}
+
 func good() {
 	var sb strings.Builder
 	bw := bufio.NewWriter(&sb)
@@ -44,3 +50,9 @@ func suppressed() {
 	tw := tabwriter.NewWriter(&sb, 0, 0, 1, ' ', 0)
 	tw.Flush() //nolint:uncheckedflushreturn
 }
+
+func deferSuppressed() {
+	var sb strings.Builder
+	bw := bufio.NewWriter(&sb)
+	defer bw.Flush() //nolint:uncheckedflushreturn
+}

pkg/linters/uncheckedflushreturn/uncheckedflushreturn.go

@@ -42,7 +42,7 @@ func run(pass *analysis.Pass) (any, error) {
 		return nil, err
 	}
 
-	nodeFilter := []ast.Node{(*ast.ExprStmt)(nil), (*ast.AssignStmt)(nil)}
+	nodeFilter := []ast.Node{(*ast.ExprStmt)(nil), (*ast.AssignStmt)(nil), (*ast.DeferStmt)(nil)}
 	insp.Preorder(nodeFilter, func(n ast.Node) {
 		switch stmt := n.(type) {
 		case *ast.ExprStmt:
@@ -57,6 +57,12 @@ func run(pass *analysis.Pass) (any, error) {
 				return
 			}
 			checkDiscardedFlushAssign(pass, stmt, noLintIndex)
+		case *ast.DeferStmt:
+			position := pass.Fset.PositionFor(stmt.Pos(), false)
+			if filecheck.ShouldSkipFilename(position.Filename, generatedFiles) {
+				return
+			}
+			checkDiscardedFlushDefer(pass, stmt, noLintIndex)
 		}
 	})
 	return nil, nil
@@ -95,6 +101,18 @@ func checkDiscardedFlushAssign(pass *analysis.Pass, assign *ast.AssignStmt, noLi
 	reportUncheckedFlush(pass, call, noLintIndex)
 }
 
+// checkDiscardedFlushDefer flags defer x.Flush() statements where the error
+// return is dropped when the deferred call executes.
+func checkDiscardedFlushDefer(pass *analysis.Pass, stmt *ast.DeferStmt, noLintIndex nolint.DirectiveIndex) {
+	if stmt.Call == nil {
+		return
+	}
+	if !isFlushCallReturningError(pass, stmt.Call) {
+		return
+	}
+	reportUncheckedFlush(pass, stmt.Call, noLintIndex)
+}
+
 func reportUncheckedFlush(pass *analysis.Pass, call *ast.CallExpr, noLintIndex nolint.DirectiveIndex) {
 	position := pass.Fset.PositionFor(call.Pos(), false)
 	if nolint.HasDirectiveForLinter(position, noLintIndex, "uncheckedflushreturn") {