Unnecessary C = in in-place expression is incompatible with immutable struct field

[guiburon]

Hi,

The macro @! always returns an expression with a leading :($(esc(C)) =.
Using the README multiplication example, what are the use-cases for returning C = mul!(C,A,B) instead of only mul!(C,A,B)?

It renders the macro incompatible with a struct field result.

struct myStruct
    C
end

S = myStruct(rand(10))
A = rand(10,10)
B = rand(10)

@! S.C = A * B   # the macro would return S.C = mul!(S.C, A, B) but throws an error

As is, the macro throws an error because :(S.C) is not a Symbol. Without this test, it later throws the error immutable struct of type myStruct cannot be changed because of the leading S.C = that would mutate the field. However, mul!(S.C, A, B) works fine because it modifies S.C in-place.

It is common to use functors for in-place operations in linear algebra (see ArnoldiMethod.jl example) but @! is incompatible as is.

I forked this repo and implemented a @! compatible with in-place operations on struct fields. I relaxed the test on :C to Union{Symbol, Expr} and removed the leading :($(esc(C)) = that the macro returns.

I am curious to hear your opinion. Thanks.