Implement json.encode_indent

[tetromino]

See https://bazel.build/rules/lib/core/json#encode_indent

Fixes #614

[adonovan]

LGTM, thanks.

[adonovan]

strings.Builder

will save an allocation.

[tetromino]

json.Encode is built around growing a bytes.Buffer; I think making a version that appends to a strings.Builder is a bit out of scope of this pr (although potentially useful).

But you are right that allocating buf via new is a waste - fixed.

[adonovan]

What I meant was that bytes.Buffer is more efficient when you want the final result as a []byte slice (buf.Bytes()), and strings.Builder is more efficient when you want the result as a string (buf.String()). (The difference is one allocation of a copy in each case.)

Since here we want String, we should use strings.Builder; no other change is needed.

BTW, allocating buf via new(T) or var x T in fact makes no difference: all that matters is whether the address of the variable escapes, which is independent of how you create the variable (unlike C++, where the syntax determines heap vs. stack allocation).

[tetromino]

Currently starlark-go relies on json.Indent for indenting, which writes its output into a *bytes.Buffer: https://pkg.go.dev/encoding/json#Indent

Maybe I'm missing something obvious, but I don't see how it can be efficiently made to write into a strings.Builder.

BTW, allocating buf via new(T) or var x T in fact makes no difference

Thank you for correcting my wrong mental model of Go's allocator!

[adonovan]

Oh, good point, the standard encoding/json package is indeed a constraint. This is fine.