go/analysis/passes/vet: fork cmd/vet@31d19c0

This change creates a fork of vet from the standard distribution.

It was created by this script:

 $ mkdir go/analysis/passes/vet/
 $ cd go/analysis/passes/vet/
 $ (cd $GOROOT/src/cmd/vet >/dev/null && git co 31d19c0 && tar cf - .) | tar xf -
 $ rm -fr all              # We'll deal with cmd/vet/all later.
 $ rm -fr internal/cfg     # Published as golang.org/x/tools/go/cfg.
 $ sed -i -e '1s?^?// +build ignore\n\n?' *.go

All the Go files have been tagged "ignore" for now.
A series of follow-up changes will convert each vet check
into an instance of the new go/analysis API's Analyzer.

At some point soon, cmd/vet in the standard distribution will use a
vendored copy of this code. Until then we will periodically integrate
any changes made to cmd/vet to this fork. The current version of
cmd/vet will be recorded in the REVISION file.

Change-Id: I0c63eeb17cc612b3f013679595dcbc71a90950f7
Reviewed-on: https://go-review.googlesource.com/138137
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/passes/vet/README

@@ -0,0 +1,33 @@
+Vet is a tool that checks correctness of Go programs. It runs a suite of tests,
+each tailored to check for a particular class of errors. Examples include incorrect
+Printf format verbs and malformed build tags.
+
+Over time many checks have been added to vet's suite, but many more have been
+rejected as not appropriate for the tool. The criteria applied when selecting which
+checks to add are:
+
+Correctness:
+
+Vet's checks are about correctness, not style. A vet check must identify real or
+potential bugs that could cause incorrect compilation or execution. A check that
+only identifies stylistic points or alternative correct approaches to a situation
+is not acceptable.
+
+Frequency:
+
+Vet is run every day by many programmers, often as part of every compilation or
+submission. The cost in execution time is considerable, especially in aggregate,
+so checks must be likely enough to find real problems that they are worth the
+overhead of the added check. A new check that finds only a handful of problems
+across all existing programs, even if the problem is significant, is not worth
+adding to the suite everyone runs daily.
+
+Precision:
+
+Most of vet's checks are heuristic and can generate both false positives (flagging
+correct programs) and false negatives (not flagging incorrect ones). The rate of
+both these failures must be very small. A check that is too noisy will be ignored
+by the programmer overwhelmed by the output; a check that misses too many of the
+cases it's looking for will give a false sense of security. Neither is acceptable.
+A vet check must be accurate enough that everything it reports is worth examining,
+and complete enough to encourage real confidence.

go/analysis/passes/vet/REVISION

@@ -0,0 +1 @@
+cmd/vet@31d19c0