cmd/go: add release note for rejection of non-ASCII import paths in module mode

[bcmills]

In CL 251878 (for #29101), we tightened up the import-path check in module mode so that it now disallows all non-ASCII package import paths, not just non-ASCII module paths (see #29101 (comment)).

Although we believe the number of affected packages to be very small, we should add a release note for this change so that users can easily verify that the change was intentional (see #43035).

CC @jayconrod @matloob @maruel

[jamie-digital]

This appears to disagree with the current language spec:

ImportPath       = string_lit .

Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to Unicode's L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters !"#$%&'()*,:;<=>?[]^`{|} and the Unicode replacement character U+FFFD.

I have a main package etld+1 that falls foul of this, but testing this in the playground indicates that + meets the requirements in the spec. Please could either the spec be updated to agree with the change, or the change adjusted to meet the spec? Many thanks.

[matloob]

@jamie-digital You bring up a subtle issue: the changes that we made don't affect the compiler, but just the go command and its build system. And there are other restrictions that the build system places above those in the compiler. But I agree that it may be confusing that there are two different sets of restrictions.

[gopherbot]

Change https://golang.org/cl/282194 mentions this issue: doc/go1.16: add release note for disallowing non-ASCII import paths

[jamie-digital]

@matloob Thanks for the clarification, that makes sense. It would be really helpful to have a clear set of rules the go command will accept.

[jayconrod]

@jamie-digital Module paths and versions is a list of restrictions on module paths. File path and size constraints is a list of restrictions on file and directory names within a module.