cmd/vet: lostcancel: suppress the check in the main.main function

When main.main returns, the process exits, so there's no need to cancel contexts.

This change was initially reviewed as
https://go-review.googlesource.com/c/go/+/106915/4
but somehow I messed up and committed patchset 5, which was
effectively empty.

Change-Id: Ic4250eb6563af9bc734e429aafc7081ca7d0e012
Reviewed-on: https://go-review.googlesource.com/c/148758
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/vet/lostcancel.go

@@ -104,6 +104,11 @@ func checkLostCancel(f *File, node ast.Node) {
 	var sig *types.Signature
 	switch node := node.(type) {
 	case *ast.FuncDecl:
+		if node.Name.Name == "main" && node.Recv == nil && f.file.Name.Name == "main" {
+			// Returning from main.main terminates the process,
+			// so there's no need to cancel contexts.
+			return
+		}
 		obj := f.pkg.defs[node.Name]
 		if obj == nil {
 			return // type error (e.g. duplicate function declaration)

src/cmd/vet/testdata/lostcancel.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package testdata
+package main
 
 import (
 	"context"
@@ -33,6 +33,12 @@ func _() {
 	ctx, _ = context.WithDeadline() // ERROR "the cancel function returned by context.WithDeadline should be called, not discarded, to avoid a context leak"
 }
 
+// Return from main is handled specially.
+// Since the program exits, there's no need to call cancel.
+func main() {
+	var ctx, cancel = context.WithCancel()
+}
+
 func _() {
 	ctx, cancel := context.WithCancel()
 	defer cancel() // ok