go_binary: Add support to set a default GODEBUG per binary

[zecke]

Go supports setting default GODEBUG values in the go.mod file or per directive in the main package at the right location. This currently does not work with rules_go/gazelle.

Introduce an explicit godebug_default attribute on go_binary so that gazelle can easily manage it.

Supports bazel-contrib/bazel-gazelle!1945

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[fmeum]

Thanks for working on this, but I think that an attribute on go_binary is the wrong location for this feature. Having potentially different semantics for the same line of code depending on the downstream consumer it is built for seems like too much of a foot gun.

Go language version and GODEBUG could be made configurable on a Go SDK and, with Bzlmod, directly via go.mod.

[zecke]

Thanks for working on this, but I think that an attribute on go_binary is the wrong location for this feature. Having potentially different semantics for the same line of code depending on the downstream consumer it is built for seems like too much of a foot gun.

My idea was to handle this like //go:embed. Have gazelle parse the .go files files and generate the go_binary based on the directives observed. Do you think something like rules_go's builder should scan the .go file to extract the directives from the main package?

//go:debug http2server=0
package main

const foo = 123