Final answer:
The 2017 film with Idris Elba as the gunslinger and Matthew McConaughey as the man in black is called 'The Dark Tower'. It is based on a novel series written by Stephen King.
Step-by-step explanation:
The film featuring Idris Elba as the gunslinger and Matthew McConaughey as the man in black is called The Dark Tower. It is a 2017 movie based on the novel series written by Stephen King. The story revolves around the gunslinger's pursuit of the man in black in a post-apocalyptic world.