Summary

Embedding files into a Go executable with "embed" package is a strong feature implemented in version 1.16. In this issue, I propose some evolution of the concept to enable not only embedding files as possible now, but any type of arbitrary code execution at compile time.

Concept

Current capability to embed files is based on a compiler directive which "magically" populates a variable as below:

//go:embed sample.txt
var text string

In this proposal I suggest embedding files would be executed from within a function named func build(). A comparison with "embed" package is below

var text string

func build() {
    content, err := ioutil.ReadFile("sample.txt")
    if err != nil {
        log.Fatal(err) // This becomes a compile-time error
    }

    text = string(content)
}

The func build() would operate similarly as existing with func init(). But while init() is a run-time preliminary execution code, build() would be run at compile-time only and pre-populate global variables.

A build function would enable not only to embed files, but also to apply calculations which would then populate global variables at compile time. Example of use case for such feature would be the calculation of CRC tables, among other big datasets which should be ready by compile time.

Benefits

• Arbitrary code execution enables developer to express any time of code as build-time execution;
• Wider use-case then only embedding files;
• Consistency with other initialization function func init();
• Prevent run-time impact (delay) with init functions doing preparation work;

Risks / Impacts

This feature may conflict with existing code depending on package-level build() function, being used for different purposes.