abseil-time-subtraction

Finds and fixes absl::Time subtraction expressions to do subtraction in the Time domain instead of the numeric domain.

There are two cases of Time subtraction in which deduce additional type information:

• When the result is an absl::Duration and the first argument is an absl::Time.
• When the second argument is a absl::Time.

In the first case, we must know the result of the operation, since without that the second operand could be either an absl::Time or an absl::Duration. In the second case, the first operand must be an absl::Time, because subtracting an absl::Time from an absl::Duration is not defined.

Examples:

int x;
absl::Time t;

// Original - absl::Duration result and first operand is a absl::Time.
absl::Duration d = absl::Seconds(absl::ToUnixSeconds(t) - x);

// Suggestion - Perform subtraction in the Time domain instead.
absl::Duration d = t - absl::FromUnixSeconds(x);


// Original - Second operand is an absl::Time.
int i = x - absl::ToUnixSeconds(t);

// Suggestion - Perform subtraction in the Time domain instead.
int i = absl::ToInt64Seconds(absl::FromUnixSeconds(x) - t);