How does `Base.@locals` work?

General Usage
1

Base.@locals has this very curious implementation:

macro locals()
    return Expr(:locals)
end

How does this work? Is locals some sort of special node type in the AST? I can’t find it documented.

I don’t need to know this for anything, I’m just curious.

2

You can see what’s happening with Meta.lower, which applies syntax transformations.

julia> Meta.lower(Main, @macroexpand Base.@locals)
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─ %1 = Core.apply_type(Base.Dict, Core.Symbol, Core.Any)
│   %2 = (%1)()
└──      return %2
))))

julia> Meta.lower(Main, @macroexpand f() = Base.@locals)
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      $(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─     return $(Expr(:method, :f))
)))
│        $(Expr(:method, :f))
│   %3 = f
│   %4 = Core.Typeof(%3)
│   %5 = Core.svec(%4)
│   %6 = Core.svec()
│   %7 = Core.svec(%5, %6, $(QuoteNode(:(#= REPL[9]:1 =#))))
│        $(Expr(:method, :f, :(%7), CodeInfo(
    @ REPL[9]:1 within `none`
1 ─ %1 = Core.apply_type(Base.Dict, Core.Symbol, Core.Any)
│   %2 = (%1)()
└──      return %2
)))
│   %9 = f
└──      return %9
))))

julia> Meta.lower(Main, @macroexpand f(x) = Base.@locals)
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      $(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─     return $(Expr(:method, :f))
)))
│        $(Expr(:method, :f))
│   %3 = f
│   %4 = Core.Typeof(%3)
│   %5 = Core.svec(%4, Core.Any)
│   %6 = Core.svec()
│   %7 = Core.svec(%5, %6, $(QuoteNode(:(#= REPL[8]:1 =#))))
│        $(Expr(:method, :f, :(%7), CodeInfo(
    @ REPL[8]:1 within `none`
1 ─ %1 = Core.apply_type(Base.Dict, Core.Symbol, Core.Any)
│   %2 = (%1)()
│   %3 = $(Expr(:isdefined, :(x)))
└──      goto #3 if not %3
2 ─      Base.setindex!(%2, x, :x)
3 ┄      return %2
)))
│   %9 = f
└──      return %9
))))

In other words, yes. Expr(:locals) is a special node type that tells the syntax expansion stage to write out the code to generate the locals it knows about.

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