Preheat oven to 180 C / Gas 4. Grease and flour the bottom and 2cm up the sides of a 23cm x 13cm loaf tin. Lining the tin with baking parchment (with the ends 2cm over the tin's long sides) makes it easier to lift the cake out.
Stir together 75g sugar, roasted pecans and cinnamon in a small bowl.
Stir together flour, baking powder, salt and remaining 200g caster sugar in a large bowl. In another bowl, beat egg with a fork; stir in milk and oil. Add egg mixture to flour mixture. Stir until just moistened; the mixture will be lumpy. Fold in rhubarb.
Spoon 1/2 of the mixture into prepared tin, then sprinkle with 1/2 of the cinnamon mixture. Repeat with remaining cake mixture and cinnamon mixture. Using a thin metal spatula, cut down through mixture and pull up in a circular motion to marble the cinnamon mixture.
Bake until a skewer inserted in centre comes out clean, 55 to 60 minutes. Transfer loaf tin to a wire rack and let cool for 10 minutes. Remove cake and let cool completely on rack, at least 30 minutes.
Wrap loaf cake with cling film and let stand at room temperature, 8 hours to overnight, before slicing. You don't have to wait overnight, but the cake is much easier to slice and has a better texture if you do.