@u_str macro hygiene?

[goretkin]

I'm not sure if this is intentional or not, but I expected it to be sufficient to just import Unitful: @u_str in order to use e.g. u"m".

julia> import Unitful: @u_str

julia> u"m"
ERROR: LoadError: Symbol `m` was found in unit module Unitful, but was not loaded into Main. Consider `using Unitful` within `Main`?
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] replace_value(::Module, ::Symbol) at /Users/goretkin/.julia/packages/Unitful/ytsW0/src/user.jl:550
 [3] @u_str(::LineNumberNode, ::Module, ::Any) at /Users/goretkin/.julia/packages/Unitful/ytsW0/src/user.jl:505
in expression starting at REPL[4]:1

julia> import Unitful: m

julia> u"m"
ERROR: LoadError: Symbol `m` was found in unit module Unitful, but was not loaded into Main. Consider `using Unitful` within `Main`?
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] replace_value(::Module, ::Symbol) at /Users/goretkin/.julia/packages/Unitful/ytsW0/src/user.jl:550
 [3] @u_str(::LineNumberNode, ::Module, ::Any) at /Users/goretkin/.julia/packages/Unitful/ytsW0/src/user.jl:505
in expression starting at REPL[6]:1

julia> using Unitful

julia> u"m"
m

julia> m = "garbage"
ERROR: cannot assign a value to variable Unitful.m from module Main
Stacktrace:
 [1] top-level scope at REPL[9]:1

[c42f]

This is a consequence of the way that Unitful allows u_str to be dynamically aware of symbols from unit extension modules, but without breaking precompilation; the heuristic is that there must be a binding for the extension module available.

Perhaps we could do better, but for now the workaround for this is to make sure there's a binding for Unitful itself in Main:

using Unitful: Unitful, @u_str