needing to process arbitrary .o files. It only needs to process the .o
files generated from C files that cgo writes, and those are much more
limited in the ELF or other features that they use.

In essence, the _cgo_import.o file includes the extra linking
directives that cmd/link is not sophisticated enough to derive from _all.o
on its own. Similarly, the _all.o uses dynamic references to real

)

// basicKeys is a set of the PAX keys for which we have built-in support.
// This does not contain "charset" or "comment", which are both PAX-specific,
// so adding them as first-class features of Header is unlikely.
// Users can use the PAXRecords field to set it themselves.
var basicKeys = map[string]bool{
	paxPath: true, paxLinkpath: true, paxSize: true, paxUid: true, paxGid: true,

// the integer type used to store each numeric field
// (where timestamps are stored as the number of seconds since the Unix epoch).
//
// The table's lower portion shows specialized features of each format,
// such as supported string encodings, support for sub-second timestamps,
// or support for sparse files.
//
// The Writer currently provides no support for sparse files.
type Format int

There are, however, times when we cannot maintain complete compatibility.
If code depends on buggy (including insecure) behavior,
then fixing the bug will break that code.
New features can also have similar impacts:
enabling the HTTP/2 use by the HTTP client broke programs
connecting to servers with buggy HTTP/2 implementations.
These kinds of changes are unavoidable and

	sort.Strings(required)

	take := func(sl *[]string) string {
		s := (*sl)[0]
		*sl = (*sl)[1:]
		return s
	}

	for len(features) > 0 || len(required) > 0 {
		switch {
		case len(features) == 0 || (len(required) > 0 && required[0] < features[0]):
			feature := take(&required)
			if exceptionSet[feature] {
				// An "unfortunate" case: the feature was once
				// included in the API (e.g. go1.txt), but was