Fix the printnode override feature & add test for it

src/printing.jl

@@ -194,7 +194,9 @@ function print_tree(printnode::Function, io::IO, node;
                     prefix::AbstractString="",
                    )
     # Get node representation as string
-    str = repr_node(node, context=io)
+    buf = IOBuffer()
+    printnode(IOContext(buf, io), node)
+    str = String(take!(buf))
 
     # Copy buffer to output, prepending prefix to each line
     for (i, line) in enumerate(split(str, '\n'))
@@ -249,7 +251,6 @@ function print_tree(printnode::Function, io::IO, node;
 
         # Print key
         if this_printkeys
-            buf = IOBuffer()
             print_child_key(IOContext(buf, io), child_key)
             key_str = String(take!(buf))
 
@@ -271,14 +272,16 @@ print_tree(node; kw...) = print_tree(stdout, node; kw...)
 
 """
     repr_tree(tree; context=nothing, kw...)
+    repr_tree(f, tree; context=nothing, kw...)
 
 Get the string result of calling [`print_tree`](@ref) with the supplied arguments.
 
 The `context` argument works as it does in `Base.repr`.
 """
-function repr_tree(tree; context=nothing, kw...)
+repr_tree(tree; context=nothing, kw...) = repr_tree(printnode, tree; context=nothing, kw...)
+function repr_tree(f, tree; context=nothing, kw...)
     buf = IOBuffer()
     io = context === nothing ? buf : IOContext(buf, context)
-    print_tree(io, tree; kw...)
+    print_tree(f, io, tree; kw...)
     return String(take!(buf))
 end

test/printing.jl

@@ -188,4 +188,25 @@ end
                    │  └─ 5
                    └─ "baz"
                    """)
+
+    # Test printnode override
+    str = repr_tree(tree) do io, s
+        if s isa BoxNode
+            print(io, s.s)
+        else
+            AbstractTrees.printnode(io, s)
+        end
+    end
+
+    @test endswith(str, """
+                   ├─ "foo"
+                   ├─ bar
+                   │  ├─ 1
+                   │  ├─ 2:4
+                   │  │  ├─ 2
+                   │  │  ├─ 3
+                   │  │  └─ 4
+                   │  └─ 5
+                   └─ "baz"
+                   """)
 end