Handle .= in @!

src/base.jl

@@ -250,6 +250,7 @@ possible(::typeof(_setproperty!), x, ::Any, ::Any) = ismutablestruct(x)
     materialize!!(dest, x)
 """
 materialize!!(dest, x) = may(materialize!, dest, x)
+# TODO: maybe instantiate `x` and be aware of `x`'s style
 
 pure(::typeof(materialize!)) = NoBang.materialize
 possible(::typeof(materialize!), x, ::Any) = ismutable(x)

src/core.jl

@@ -44,3 +44,6 @@ ismutablestruct(::Type{<:NamedTuple}) = false
 
 # trymutate(::typeof(push!)) = push!!
 # trymutate(::typeof(append!)) = append!!
+
+struct Undefined end
+ismutable(::Undefined) = false

src/macro.jl

@@ -14,7 +14,15 @@ julia> @! push!(empty!((0, 1)), 2, 3)
 macro !(expr)
     foldexpr(expr) do x
         if Meta.isexpr(x, :call)
+            # TODO: handle `x .* y`
             return Expr(:call, Expr(:call, _maybb, x.args[1]), x.args[2:end]...)
+        elseif Meta.isexpr(x, :.=) && x.args[1] isa Symbol
+            @assert length(x.args) == 2
+            lhs, rhs = x.args
+            return :($lhs = $materialize!!(
+                $Base.@isdefined($lhs) ? $lhs : $(Undefined()),
+                $rhs,
+            ))
         end
         return x
     end |> esc

test/test_macro.jl

@@ -7,4 +7,29 @@ include("preamble.jl")
     @test (@! push!(empty!((0, 1)), 2, 3)) == (2, 3)
 end
 
+@testset ".=" begin
+    @test !@isdefined y
+    @test begin
+        x = SVector(1, 2)
+        @! y .= (*).(x, 2)
+    end === SVector(2, 4)
+
+    @test !@isdefined y
+    @test begin
+        x = SVector(1, 2)
+        y = SVector(0, 0)
+        @! y .= (*).(x, 2)
+    end === SVector(2, 4)
+
+    let x = [1, 2]
+        y = [0, 0]
+        @test (@! y .= (*).(x, 2))::Vector{Int} == [2, 4]
+    end
+
+    let x = [1, 2]
+        y = SVector(0, 0)
+        @test (@! y .= (*).(x, 2))::Vector{Int} == [2, 4]
+    end
+end
+
 end  # module