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Spread list tests can never touch exact-sized bounds tests

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When compiling a pattern match like

```
[] -> ..
[_] -> ..
[_, ..] -> ..
```

to a decision tree, we must make sure that the last test (len >= 1)
does not touch the branch reached by the second test (len == 1). It is
enough to ban (len >=) tests from ever touching exact-sized list
patterns, because a spread test (len >=) can never reach an exact-sized
test.

On the other hand, an exact-sized test can reach a spread pattern,
because in

```
[_, _] -> ..
[..] -> ..
```

the last branch generates tests for patterns `[]` and `[_]`, and we would
like those patterns to be covered by the spread test (len >= 0)!

Closes #4685
This commit is contained in:
Ayaz Hafiz 2022-12-05 13:45:35 -06:00
parent b6a96ebb85
commit 759127660d
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3 changed files with 62 additions and 62 deletions
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18
crates/compiler/mono/src/decision_tree.rs
View file

@ -425,6 +425,7 @@ fn flatten<'a>(
/// variables to "how to get their value". So a pattern like (Just (x,_)) will give
/// us something like ("x" => value.0.0)
#[derive(Debug)]
enum Match {
Exact(Label),
GuardOnly,
@ -795,7 +796,22 @@ fn to_relevant_branch_help<'a>(
elements,
element_layout: _,
} => match test {
IsListLen { bound: _, len } if my_arity.covers_length(*len as _) => {
IsListLen {
bound: test_bound,
len,
} if my_arity.covers_length(*len as _)
// Spread tests [_, ..] can only match spread tests, not exact-sized bounds [_].
//
// On the other hand, exact-sized tests [_] can match spread bounds [_, ..],
// because each spread bound generates 0 or more exact-sized tests.
//
// See exhaustiveness checking of lists for more details on the tests generated
// for spread bounds.
&& !matches!(
(test_bound, my_arity),
(ListLenBound::AtLeast, ListArity::Exact(..))
) =>
{
let sub_positions = elements.into_iter().enumerate().map(|(index, elem_pat)| {
let mut new_path = path.to_vec();