code review 6614058: cmd/gc: replace "typechecking loop" by nicer errors in ...

cmd/gc: replace "typechecking loop" by nicer errors in some cases.

For issue 3757:
BEFORE:  test/fixedbugs/bug463.go:12: typechecking loop involving a
             test/fixedbugs/bug463.go:12 a
             test/fixedbugs/bug463.go:12 <node DCLCONST>
AFTER:   test/fixedbugs/bug463.go:12: constant definition loop
             test/fixedbugs/bug463.go:12: a uses a

For issue 3937:
BEFORE: test/fixedbugs/bug464.go:12: typechecking loop involving foo
            test/fixedbugs/bug464.go:12 <T>
            test/fixedbugs/bug464.go:12 foo
            test/fixedbugs/bug464.go:12 <node DCLFUNC>
AFTER:  test/fixedbugs/bug464.go:12: foo is not a type

Fixes issue 3757.
Fixes issue 3937.

[remyoudompheng, 2012-10-07 08:10:21]

Hello golang-dev@googlegroups.com (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[remyoudompheng, 2012-10-07 08:11:17]

For longer loops it will print:

./titi2.go:5: constant a refers to itself: used by c (/tmp/titi2.go:7) which is
used by b (/tmp/titi2.go:6) which is used by a (/tmp/titi2.go:5)

Feel free to suggest removing the loop details, change formatting or wording.

[rsc, 2012-10-07 14:09:49]

I like the new messages, especially for the non-loops. For the actual
loops, please format the list vertically, like the existing
type-checking loop:

titi2.go:5: constant definition loop: a uses c
    titi2.go:7: c uses b
    titi2.go:6: b uses a

This is easier to scan and avoids unfortunate line wraps.

Russ

[remyoudompheng, 2012-10-07 14:50:57]

Hello golang-dev@googlegroups.com, rsc@golang.org (cc:
golang-dev@googlegroups.com),

Please take another look.

[remyoudompheng, 2012-10-07 14:52:21]

New error format: i leave the "refers to itself" in the test case because that's
how gccgo says it.

% bin/go tool 6g -S test/fixedbugs/bug463.go
test/fixedbugs/bug463.go:12: constant definition loop
	test/fixedbugs/bug463.go:12: a uses a
test/fixedbugs/bug463.go:16: constant definition loop
	test/fixedbugs/bug463.go:16: A uses B
	test/fixedbugs/bug463.go:17: B uses D
	test/fixedbugs/bug463.go:18: D uses A

[rsc, 2012-10-07 15:22:14]

LGTM

[remyoudompheng, 2012-10-07 15:35:27]

*** Submitted as http://code.google.com/p/go/source/detail?r=986908b3e4ae ***

cmd/gc: replace "typechecking loop" by nicer errors in some cases.

For issue 3757:
BEFORE:  test/fixedbugs/bug463.go:12: typechecking loop involving a
             test/fixedbugs/bug463.go:12 a
             test/fixedbugs/bug463.go:12 <node DCLCONST>
AFTER:   test/fixedbugs/bug463.go:12: constant definition loop
             test/fixedbugs/bug463.go:12: a uses a

For issue 3937:
BEFORE: test/fixedbugs/bug464.go:12: typechecking loop involving foo
            test/fixedbugs/bug464.go:12 <T>
            test/fixedbugs/bug464.go:12 foo
            test/fixedbugs/bug464.go:12 <node DCLFUNC>
AFTER:  test/fixedbugs/bug464.go:12: foo is not a type

Fixes issue 3757.
Fixes issue 3937.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6614058