go/build: missing Position information for go:embed patterns

[bcmills]

Go 1.16 adds support for go:embed patterns in source files (#41191), which are parsed and reported by the go/build package in the {,Test,XTest}EmbedPatterns fields of the build.Package struct as of CL 243941.

Much like imports, embed patterns can introduce errors in the build due to missing inputs — in this case, missing embedded files. Those errors need to be attributed back to source file positions.

For imports, the build.Package struct represents the file and line information as token.Position values in the {,Test,XTest}ImportPos fields. However, the corresponding information for embed patterns seems not to be present.

This came to my attention via CL 280452.

[bcmills]

Marking as release-blocker for Go 1.16 because it impacts go:embed API changes for 1.16 (#43407; CC @golang/release, @rsc).

However, we could perhaps decide to ship the go/build API changes as-is and follow up with fields for the missing position information in a subsequent release, in which case this issue should be re-milestoned.

[gopherbot]

Change https://golang.org/cl/283638 mentions this issue: cmd/go/internal/load: report positions for embed errors

[gopherbot]

Change https://golang.org/cl/283636 mentions this issue: go/build: report positions for go:embed directives

[gopherbot]

Change https://golang.org/cl/285213 mentions this issue: doc/go1.16: mention go/build changes