unsafe: {String,SliceData} cannot be guarded with build tags

[twmb]

If a go.mod uses anything prior to go 1.20, a library cannot use unsafe.SliceData even if the usage is guarded in a file with Go version build tags (//go:build go1.20).

Using unsafe.SliceData or unsafe.String will print something along the lines of...

./go120.go:7:9: unsafe.SliceData requires go1.20 or later (-lang was set to go1.19; check go.mod)
./go120.go:8:9: unsafe.String requires go1.20 or later (-lang was set to go1.19; check go.mod)

This restriction did not exist for unsafe.Slice -- unsafe.Slice could be guarded with build tags. A similar thing happens when using unsafe.Slice with a go.mod that indicates 1.16.

Does this issue reproduce with the latest release?

Yes.

What did you do?

https://go.dev/play/p/fK1zbeRZrwW

What did you expect to see?

In the above playground link, no error for both Go 1.19 and Go 1.20.

What did you see instead?

In the above playground link, no error for Go 1.19 but a compile time error for Go 1.20.

[cuiweixie]

if you change go version in go.mod to go 1.16, unsafe.Slice report error too.
check this code, https://go.dev/play/p/QIszeqeVWK3.
currently, the compiler do not check the tag, it use go version in go.mod.

[twmb]

@cuiweixie thank you, something must've been wrong in my local setup last night -- I confirm this also happens for unsafe.Slice in a go1.16 module, even when unsafe.Slice is guarded by build tags.

[bcmills]

This is true of builtins in general too. It's unfortunate but hard to fix.

Note that setting go 1.20 in your go.mod file won't actually prevent previous Go versions from building your module — the go version is currently more “Go version needed for full fidelity” rather than “minimum Go version needed” (compare #57001).

[zigo101]

Looks same as #58342.

[zigo101]

Can this by solved by supporting go toolchain.version in go.mod files?

[bcmills]

#59033 is probably relevant.

[mitar]

I was hit by this as well, exactly with unsafe.SliceData. It works with Go 1.21, but not with prior Go versions.

[bcmills]

Duplicate of #59033