Fix whitespace around :: when LHS is missing

`::` can be used without a LHS in e.g. function definitions like
`f(::Int) = ...`.

src/chisels.jl

@@ -82,3 +82,10 @@ end
 function is_operator_leaf(node::JuliaSyntax.GreenNode)
     return is_leaf(node) && JuliaSyntax.is_operator(node)
 end
+
+function first_non_whitespace_child(node::JuliaSyntax.GreenNode)
+    @assert !is_leaf(node)
+    children = JuliaSyntax.children(node)::AbstractVector
+    idx = findfirst(!JuliaSyntax.is_whitespace, children)::Int
+    return children[idx]
+end

src/runestone.jl

@@ -308,6 +308,12 @@ function no_spaces_around_x(ctx::Context, node::JuliaSyntax.GreenNode, is_x::F)
 
     looking_for_x = false
 
+    # K"::" is a special case here since it can be used without an LHS in e.g. function
+    # definitions like `f(::Int) = ...`.
+    if JuliaSyntax.kind(node) === K"::"
+        looking_for_x = is_x(first_non_whitespace_child(node))::Bool
+    end
+
     for (i, child) in pairs(children)
         span_sum += JuliaSyntax.span(child)
         if (i == 1 || i == length(children)) && JuliaSyntax.kind(child) === K"Whitespace"

test/runtests.jl

@@ -171,6 +171,8 @@ end
     @test format_string("a::T") == "a::T"
     @test format_string("a::T::S") == "a::T::S"
     @test format_string("a  ::  T") == "a::T"
+    @test format_string("f(::T)::T = 1") == "f(::T)::T = 1"
+    @test format_string("f(:: T) :: T = 1") == "f(::T)::T = 1"
     # K"<:" and K">:"
     @test format_string("a<:T") == "a <: T"
     @test format_string("a>:T") == "a >: T"