Trait coercion when assigning struct field

[kylecook80]

Rust: master (0.10-pre)
OS: Mac OS X 10.9
Architecture: x86_64

There is some syntax that supposedly should work (according to both my experience and those on the rust irc channel) that results in a compilation error:

pub fn set_game_state<T: GameState>(&mut self, state: ~T) {
    self.state = state;
}

This results in a compilation error of:

error: mismatched types: expected `~game_state::GameState:Send` but found `~T` (expected trait game_state::GameState but found ~-ptr)

This code does work:

pub fn set_game_state(&mut self, state: ~GameState) {
    self.state = state;
}

[alexcrichton]

Closing in favor of #10296. It looks like self.state is a trait object of the GameState trait. By default ~TraitObject is equivalent to ~TraitObject: Send which means that the underlying type must also be Send. You can modify the first function to T: GameState + Send to get the first function to compile, but it's very similar to the second function anyway.

[nikomatsakis]

Can you please supply the definition of self here?

On Wed, Mar 05, 2014 at 09:43:42PM -0800, Kyle Cook wrote:

Rust: master (0.10-pre)
OS: Mac OS X 10.9
Architecture: x86_64

There is some syntax that supposedly should work (according to both my experience and those on the rust irc channel) that results in a compilation error:

pub fn set_game_state<T: GameState>(&mut self, state: ~T) {
    self.state = state;
}

This results in a compilation error of:

error: mismatched types: expected `~game_state::GameState:Send` but found `~T` (expected trait game_state::GameState but found ~-ptr)

This code does work:

pub fn set_game_state(&mut self, state: ~GameState) {
    self.state = state;
}

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