cmd/go: definition of "go" directive in go.mod conflicts between //go:build proposal and other uses

[zikaeroh]

The first definition of the go directive in go.mod is that the version listed version makes a specific set of features available to the module being used. This is consistent with its use to enable //go:embed (#44557 allowed this), lazy loading (#36460), and various other language features. I saw this first well-explained via https://twitter.com/_myitcv/status/1391819772992659461.

Recent CLs have bumped all of the x/* modules up to state 1.17, e.g. CL 315570 with the statement:

Note that this does not prevent users with earlier go versions from successfully building packages from this module.

Under this definition, it's legal for a module to support versions below what is listed in go.mod, so long as the code is properly guarded by build tags or similar. The Go toolchain itself restricts a set of packages to build with 1.4+, even though go.mod always mirrors the current Go version. Other external packages like golang-migrate can safely support io/fs and embed thanks to build tags.

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The second definition of the go directive in go.mod comes from the new //go:build changes (#41184), specifically in the transition section of the proposal for "Go 1.(N+1)" (i.e. Go 1.18)

A new fix in go fix will remove // +build stanzas, making sure to leave behind equivalent //go:build lines. The removal only happens when go fix is being run in a module with a go 1.N (or later) line, which is taken as an explicit signal that the developer no longer needs compatibility with Go 1.(N−1).

This definition explicitly conflicts with the first, as it exactly says that the go directive indicates the minimum version supported by a module, and therefore it's safe to start removing things that would be required for older versions to be supported. This proposal is especially interesting, as it is changing build tags themselves, so therefore cannot be protected by build tags.

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While at the moment, I'm not sure that the //go:build changes will cause issues, if "N" in the proposal had instead meant that Go 1.17 was the version at which // +build lines started to be removed, then the two would have clashed, and any preemptive go directive bumping would have caused compatibility issues; future changes may not be as lucky with this as precedent. And, as modules move their go version up per definition 1) to obtain new features, trusting that they can still run with older versions of Go, I can see this becoming more of a problem.

Can there be some agreement on what the go directive actually implies, such that there isn't a conflict here? Or, is the definition used in //go:build spec an oversight?

There might be an issue for this ("what does the go directive mean") already, but I couldn't find it.

[dmitshur]

There might be an issue for this ("what does the go directive mean") already, but I couldn't find it.

That might be #30791.

CC @bcmills, @matloob, @jayconrod, @ianlancetaylor.