Minimize explicit sharing of writable data

Reason

If you don't share writable data, you can't have a data race. The less sharing you do, the less chance you have to forget to synchronize access (and get data races). The less sharing you do, the less chance you have to wait on a lock (so performance can improve).

Example

bool validate(const vector<Reading>&);
Graph<Temp_node> temperature_gradients(const vector<Reading>&);
Image altitude_map(const vector<Reading>&);
// ...

void process_readings(const vector<Reading>& surface_readings)
{
    auto h1 = async([&] { if (!validate(surface_readings)) throw Invalid_data{}; });
    auto h2 = async([&] { return temperature_gradients(surface_readings); });
    auto h3 = async([&] { return altitude_map(surface_readings); });
    // ...
    h1.get();
    auto v2 = h2.get();
    auto v3 = h3.get();
    // ...
}

Without those consts, we would have to review every asynchronously invoked function for potential data races on surface_readings. Making surface_readings be const (with respect to this function) allows reasoning using only the function body.

Note

Immutable data can be safely and efficiently shared. No locking is needed: You can't have a data race on a constant. See also CP.mess: Message Passing and CP.31: prefer pass by value.

Enforcement

???