[24.0 backport] add //go:build directives to prevent downgrading to go1.16 language

[thaJeztah]

• backport add //go:build directives to prevent downgrading to go1.16 language #4719

This is a follow-up to 0e73168

This repository is not yet a module (i.e., does not have a go.mod). This is not problematic when building the code in GOPATH or "vendor" mode, but when using the code as a module-dependency (in module-mode), different semantics are applied since Go1.21, which switches Go language versions on a per-module, per-package, or even per-file base.

A condensed summary of that logic [is as follows][1]:

• For modules that have a go.mod containing a go version directive; that version is considered a minimum required version (starting with the go1.19.13 and go1.20.8 patch releases: before those, it was only a recommendation).
• For dependencies that don't have a go.mod (not a module), go language version go1.16 is assumed.
• Likewise, for modules that have a go.mod, but the file does not have a go version directive, go language version go1.16 is assumed.
• If a go.work file is present, but does not have a go version directive, language version go1.17 is assumed.

When switching language versions, Go downgrades the language version, which means that language features (such as generics, and any) are not available, and compilation fails. For example:

# github.com/docker/cli/cli/context/store
/go/pkg/mod/github.com/docker/[email protected]+incompatible/cli/context/store/storeconfig.go:6:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
/go/pkg/mod/github.com/docker/[email protected]+incompatible/cli/context/store/store.go:74:12: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)

Note that these fallbacks are per-module, per-package, and can even be per-file, so (indirect) dependencies can still use modern language features, as long as their respective go.mod has a version specified.

Unfortunately, these failures do not occur when building locally (using vendor / GOPATH mode), but will affect consumers of the module.

Obviously, this situation is not ideal, and the ultimate solution is to move to go modules (add a go.mod), but this comes with a non-insignificant risk in other areas (due to our complex dependency tree).

We can revert to using go1.16 language features only, but this may be limiting, and may still be problematic when (e.g.) matching signatures of dependencies.

There is an escape hatch: adding a //go:build directive to files that make use of go language features. From the [go toolchain docs][2]:

The go line for each module sets the language version the compiler enforces
when compiling packages in that module. The language version can be changed
on a per-file basis by using a build constraint.

For example, a module containing code that uses the Go 1.21 language version
should have a go.mod file with a go line such as go 1.21 or go 1.21.3.
If a specific source file should be compiled only when using a newer Go
toolchain, adding //go:build go1.22 to that source file both ensures that
only Go 1.22 and newer toolchains will compile the file and also changes
the language version in that file to Go 1.22.

This patch adds //go:build directives to those files using recent additions to the language. It's currently using go1.19 as version to match the version in our "vendor.mod", but we can consider being more permissive ("any" requires go1.18 or up), or more "optimistic" (force go1.21, which is the version we currently use to build).

For completeness sake, note that any file without a //go:build directive will continue to use go1.16 language version when used as a module.

[1]: https://github.com/golang/go/blob/58c28ba286dd0e98fe4cca80f5d64bbcb824a685/src/cmd/go/internal/gover/version.go#L9-L56 [2]; https://go.dev/doc/toolchain#:~:text=The%20go%20line%20for,file%20to%20Go%201.22

(cherry picked from commit 70216b6)

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