Remove one allocation by foregoing context.WithValue

[bouk]

Instead of we can make Context implement the context.Context interface directly through a private type, which just implements the Value method.

If you want to remove the last allocation you could also do *r = *r.WithContext((*directContext)(rctx)), although that would be modifying something you don't own.

Benchmark

Before

~/s/g/g/chi (master) go test -bench B
goos: darwin
goarch: amd64
BenchmarkMux/route:/-12                  4089687               291 ns/op             448 B/op          3 allocs/op
BenchmarkMux/route:/hi-12                3795570               306 ns/op             448 B/op          3 allocs/op
BenchmarkMux/route:/sup/123/and/this-12                  2908492               412 ns/op             448 B/op          3 allocs/op
BenchmarkMux/route:/sup/123/foo/this-12                  2328763               519 ns/op             448 B/op          3 allocs/op
BenchmarkMux/route:/sharing/z/aBc-12                     2355368               509 ns/op             448 B/op          3 allocs/op
BenchmarkMux/route:/sharing/z/aBc/twitter-12             1925193               618 ns/op             456 B/op          4 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct-12              1654678               726 ns/op             456 B/op          4 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct/download-12             1445162               827 ns/op             480 B/op          5 allocs/op
BenchmarkTreeGet-12                                              8847862               133 ns/op               0 B/op          0 allocs/op
PASS
ok      _/Users/bouke/src/github.com/go-chi/chi 16.505s

After

~/s/g/g/chi (master) go test -bench B
goos: darwin
goarch: amd64
BenchmarkMux/route:/-12                  4975675               238 ns/op             400 B/op          2 allocs/op
BenchmarkMux/route:/hi-12                4772744               255 ns/op             400 B/op          2 allocs/op
BenchmarkMux/route:/sup/123/and/this-12                  3353338               360 ns/op             400 B/op          2 allocs/op
BenchmarkMux/route:/sup/123/foo/this-12                  2503300               500 ns/op             400 B/op          2 allocs/op
BenchmarkMux/route:/sharing/z/aBc-12                     2226513               477 ns/op             400 B/op          2 allocs/op
BenchmarkMux/route:/sharing/z/aBc/twitter-12             2153860               553 ns/op             408 B/op          3 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct-12              1837567               653 ns/op             408 B/op          3 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct/download-12             1609606               748 ns/op             432 B/op          4 allocs/op
BenchmarkTreeGet-12                                              8756982               136 ns/op               0 B/op          0 allocs/op
PASS
ok      _/Users/bouke/src/github.com/go-chi/chi 16.282s

With unsafe *r = *r.WithContext() (not in this PR)

goos: darwin
goarch: amd64
BenchmarkMux/route:/-12                  9876838               113 ns/op              83 B/op          0 allocs/op
BenchmarkMux/route:/hi-12                9744604               110 ns/op              84 B/op          0 allocs/op
BenchmarkMux/route:/sup/123/and/this-12                  3793071               331 ns/op             438 B/op          0 allocs/op
BenchmarkMux/route:/sup/123/foo/this-12                  2710080               423 ns/op             568 B/op          0 allocs/op
BenchmarkMux/route:/sharing/z/aBc-12                    11624925               102 ns/op              88 B/op          0 allocs/op
BenchmarkMux/route:/sharing/z/aBc/twitter-12             3851288               306 ns/op              93 B/op          3 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct-12             11640464               112 ns/op              88 B/op          0 allocs/op
BenchmarkMux/route:/sharing/z/aBc/direct/download-12             3862566               307 ns/op              93 B/op          3 allocs/op
BenchmarkTreeGet-12                                              8824483               145 ns/op               0 B/op          0 allocs/op
PASS
ok      _/Users/bouke/src/github.com/go-chi/chi 14.098s

[pkieltyka]

This is cool, thanks for the PR. It’s tempting but I want to make it a bit more elegant

[bouk]

@pkieltyka sure, what do you have in mind?

[pkieltyka]

@bouk just minor things with comments and placement. I've pushed a follow-up commit: 364c564

[bouk]

Cool, thanks for merging 🙂

[pkieltyka]

FYI, I've had to revert this change, #581