Description
What version of Go are you using (go version)?
$ go version go version go1.15.6 darwin/amd64
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/Users/davidben/Library/Caches/go-build" GOENV="/Users/davidben/Library/Application Support/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOINSECURE="" GOMODCACHE="/Users/davidben/go/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="darwin" GOPATH="/Users/davidben/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/local/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64" GCCGO="gccgo" AR="ar" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="/Users/davidben/boringssl/go.mod" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qc/[...]/T/go-build280151090=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
https://play.golang.org/p/9ZkvjGuE1so
What did you expect to see?
unicode.Is and related functions should return false on negative values, as they do for other invalid runes. bytes.Buffer.WriteRune should write a replacement character, as it does for other invalid runes. In particular, a UTF-32 decoder could easily construct a negative rune before checking. (Looks like x/text/encoding/unicode/utf32 does that and then relies on RuneLen noticing.)
What did you see instead?
unicode.Is thinks some negative values are printable, and bytes.Buffer.WriteRune accidentally runs a single-byte fast path.
I wasn't sure at first whether this was a bug, but most functions seem to check for negative values or cast to uint32, so I think these should as well. (If they expect the rune be a real code point, that should probably be documented.)