Why do Context::input(...) and Context::input_for(...) take a write lock on the Context?

[voidburn]

Hey folks,

is there a reason why the first two methods below are taking a write lock on the context, even though they don't expose a mutable InputState reference to the reader function? The inconsistency with the rest of the code of that implementation caught my eye, and got me curious as for the reasons why.

impl Context {
    #[inline]
    pub fn input<R>(&self, reader: impl FnOnce(&InputState) -> R) -> R {
        self.write(move |ctx| reader(&ctx.viewport().input))
    }

    /// This will create a `InputState::default()` if there is no input state for that viewport
    #[inline]
    pub fn input_for<R>(&self, id: ViewportId, reader: impl FnOnce(&InputState) -> R) -> R {
        self.write(move |ctx| reader(&ctx.viewport_for(id).input))
    }

    /// Read-write access to [`InputState`].
    #[inline]
    pub fn input_mut<R>(&self, writer: impl FnOnce(&mut InputState) -> R) -> R {
        self.input_mut_for(self.viewport_id(), writer)
    }

    /// This will create a `InputState::default()` if there is no input state for that viewport
    #[inline]
    pub fn input_mut_for<R>(&self, id: ViewportId, writer: impl FnOnce(&mut InputState) -> R) -> R {
        self.write(move |ctx| writer(&mut ctx.viewport_for(id).input))
    }
}