Module rustc_mir_build::thir::pattern::deconstruct_pat
source · [−]Expand description
super::usefulness explains most of what is happening in this file. As explained there,
values and patterns are made from constructors applied to fields. This file defines a
Constructor enum, a Fields struct, and various operations to manipulate them and convert
them from/to patterns.
There’s one idea that is not detailed in super::usefulness because the details are not
needed there: constructor splitting.
Constructor splitting
The idea is as follows: given a constructor c and a matrix, we want to specialize in turn
with all the value constructors that are covered by c, and compute usefulness for each.
Instead of listing all those constructors (which is intractable), we group those value
constructors together as much as possible. Example:
match (0, false) {
(0 ..=100, true) => {} // `p_1`
(50..=150, false) => {} // `p_2`
(0 ..=200, _) => {} // `q`
}The naive approach would try all numbers in the range 0..=200. But we can be a lot more
clever: 0 and 1 for example will match the exact same rows, and return equivalent
witnesses. In fact all of 0..50 would. We can thus restrict our exploration to 4
constructors: 0..50, 50..=100, 101..=150 and 151..=200. That is enough and infinitely
more tractable.
We capture this idea in a function split(p_1 ... p_n, c) which returns a list of constructors
c' covered by c. Given such a c', we require that all value ctors c'' covered by c'
return an equivalent set of witnesses after specializing and computing usefulness.
In the example above, witnesses for specializing by c'' covered by 0..50 will only differ
in their first element.
We usually also ask that the c' together cover all of the original c. However we allow
skipping some constructors as long as it doesn’t change whether the resulting list of witnesses
is empty of not. We use this in the wildcard _ case.
Splitting is implemented in the Constructor::split function. We don’t do splitting for
or-patterns; instead we just try the alternatives one-by-one. For details on splitting
wildcards, see SplitWildcard; for integer ranges, see SplitIntRange; for slices, see
SplitVarLenSlice.
Structs
clone_and_forget_reachability if you’re sure.Constructor.IntRanges always store a contiguous range. This means that values are
encoded such that 0 encodes the minimum value for the integer,
regardless of the signedness.
For example, the pattern -128..=127i8 is encoded as 0..=255.
This makes comparisons and arithmetic on interval endpoints much more
straightforward. See signed_bias for details.Enums
Fields.