[Refactor] Pattern matching #1799

This commit is a preliminary work for the upcoming ADTs (and more generally the introduction of pattern matching, instead of just having destructuring). The refactoring aims at installing a consistent naming, simplify the representation of patterns and their associated method, and pave the way for other patterns that just records. Indeed, the previous implementation often made the implicit assumptions that patterns were only record patterns.

Record pattern was previously using a tuple `(open: bool, rest: Option<LocIdent>)` to represent the presence of either no tail, an non-capturing tail `..`, or a capturing tail `..rest`. This representation allows the illegal state `(false, Some("x"))`: indeed, if the tail is capturing, the record contract is necessarily open. This commit flattens this representation to a single enum that correctly represents those three different cases, instead of the 4 allowed by the previous representation.

The AST of patterns had a special node for an aliased pattern, which was a variant containing the alias and a potentital nested pattern. However, this doesn't model correctly patterns: usually, it doesn't make sense to stack aliases (and the parser won't accept it), but the previous representation accepted ASTs for things like `x @ y @ z @ <pat>`, which incurs additional burden to handle, although it can actually never happen. Additionally, the alias of the top pattern was duplicated as an optional field in the `LetPattern` and `FunPattern` nodes of the `Term` AST. This commit makes things simpler by storing `alias` as an optional field directly in the `Pattern` struct, which makes it accessible without having to pattern match on an enum variant, and forbids nested aliases. Doing so, we remove the duplication from the `LetPattern` and `FunPattern`, which now only takes a pattern instead of an optional identifier and a pattern, leading to code simplification.

The refactoring of patterns has introduced a slightly different algorithm for typechecking patterns, which isn't entirely backward-compatible, although it's more consistent. We'll probably rule out (i.e. depreacte) the offending special cases, but until then, this commit restores the previous behavior, which fixes a previously failing test.

This commit only applies pure renaming of several symbols of the destructuring module for improved clarity. The whole module is also moved to `term::pattern`, as patterns are just syntactic component of the term AST.

Co-authored-by: Viktor Kleen <[email protected]>

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[Refactor] Pattern matching #1799

[Refactor] Pattern matching #1799

Commits on Feb 9, 2024