When preparing any change that is permitted by Go 1 compatibility
but may nonetheless break some existing programs,
we first engineer the change to keep as many existing programs working as possible.
For the remaining programs,
we define a new GODEBUG setting that
allows individual programs to opt back in to the old behavior.
A GODEBUG setting may not be added if doing so is infeasible,
but that should be extremely rare.

### Iterators

The new [`iter` package](/pkg/iter/) provides the basic definitions for

Vendoring in std and cmd
========================

The Go command maintains copies of external packages needed by the
standard library in the src/vendor and src/cmd/vendor directories.

There are two modules, std and cmd, defined in src/go.mod and
src/cmd/go.mod. When a package outside std or cmd is imported
by a package inside std or cmd, the import path is interpreted
as if it had a "vendor/" prefix. For example, within "crypto/tls",

pkg go/token, const CONST Token
pkg go/token, const CONTINUE Token
pkg go/token, const DEC Token
pkg go/token, const DEFAULT Token
pkg go/token, const DEFER Token
pkg go/token, const DEFINE Token
pkg go/token, const ELLIPSIS Token
pkg go/token, const ELSE Token
pkg go/token, const EOF Token
pkg go/token, const EQL Token
pkg go/token, const FALLTHROUGH Token
pkg go/token, const FLOAT Token

The syscall package now defines [WSAENOPROTOOPT] on Windows....

The `debug/elf` package now defines [PT_OPENBSD_NOBTCFI]. This [ProgType] is
used to disable Branch Tracking Control Flow Integrity (BTCFI) enforcement

pkg go/token, const COMMENT = 2
pkg go/token, const CONST = 64
pkg go/token, const CONTINUE = 65
pkg go/token, const DEC = 38
pkg go/token, const DEFAULT = 66
pkg go/token, const DEFER = 67
pkg go/token, const DEFINE = 47
pkg go/token, const ELLIPSIS = 48
pkg go/token, const ELSE = 68
pkg go/token, const EOF = 1
pkg go/token, const EQL = 39
pkg go/token, const FALLTHROUGH = 69
pkg go/token, const FLOAT = 6

Now defines the symbol type constants [STT_RELC], [STT_SRELC], and