Module narrowing 

Which is tests an expression proves. #1654 (runtime-semantics track S5, Decision A): the one structural rule the resolver, the checker and the emitter all read, so their views of an is binding’s scope cannot drift.

An e is P test with bindings brings those bindings into scope wherever the test is known to have matched. Given a Boolean expression and whether it evaluated to true or false, matched_is_tests returns the is tests that must then have matched:

expression	when true	when false
e is P	the test	—
a && b	a when true, b when true	— (either may have failed)
a || b	— (either may have held)	a when false, b when false
!e	e when false	e when true
a implies b	— (!a || b)	a when true, b when false
(e)	e	e

The consumers apply it at these scopes:

• an if’s then-branch: its condition when true;
• an if’s else-branch: its condition when false (if !(o is Some(v)) { … } else { v }, type-system §2.3.6);
• the right operand of && and implies: the left operand when true, since the right is evaluated only then.

The right operand of || (the left operand when false) is not scoped in this slice: nothing lowers it with bindings yet, and a scope the checker accepts must be one the emitter lowers.

Functions

matched_is_tests

The is tests (ExprKind::Is nodes) that expr proves matched when it evaluates to when_true, in source order.