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

This reverts CL 148758 (commit 5e17ce22ec)

Reason for revert: breaks the build.

Change-Id: I6ed15b7b8f6b74d84edab9402ddf7ae87a0d0387
Reviewed-on: https://go-review.googlesource.com/c/148817
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/lostcancel.go

@@ -104,11 +104,6 @@ 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 main
+package testdata
 
 import (
 	"context"
@@ -33,12 +33,6 @@ 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