TIL: impl Trait vs dyn Trait in Rust

impl Trait and dyn Trait are not interchangeable. They compile to fundamentally different code.

impl Trait — static dispatch

fn process(handler: impl Handler) {
    handler.handle();
}

The compiler monomorphizes this: it generates a concrete copy of process for every type that implements Handler. Fast at runtime (no vtable lookup), larger binary, requires the type to be known at compile time.

dyn Trait — dynamic dispatch

fn process(handler: &dyn Handler) {
    handler.handle();
}

This uses a vtable — a pointer to a table of function pointers resolved at runtime. Slower (one pointer dereference per method call), smaller binary, works with heterogeneous collections (Vec<Box<dyn Handler>>).

When it matters

Use impl Trait when:

• You're writing hot-path code where the type is always known at compile time
• You want the compiler to optimize across the function boundary

Use dyn Trait when:

• You need a collection of mixed types that all implement the same trait
• The type isn't known until runtime (plugin systems, registries)

In DocSee, the CLI variant used impl Trait for its command handlers (always statically known), and the plugin loader used dyn Trait (loaded at runtime from config). Two interfaces, two right answers.