feat(experimental): Issue errors for unreachable match branches #7556
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jfecher
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jfecher
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Feb 28, 2025
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| /// A Pattern is anything that can appear before the `=>` in a match rule. | ||
| #[derive(Debug, Clone)] | ||
| enum Pattern { | ||
| /// A pattern checking for a tag and possibly binding variables such as `Some(42)` | ||
| Constructor(Constructor, Vec<Pattern>), | ||
| /// An integer literal pattern such as `4`, `12345`, or `-56` | ||
| Int(SignedField), | ||
| /// A pattern binding a variable such as `a` or `_` | ||
| Binding(DefinitionId), | ||
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| /// Multiple patterns combined with `|` where we should match this pattern if any | ||
| /// constituent pattern matches. e.g. `Some(3) | None` or `Some(1) | Some(2) | None` | ||
| #[allow(unused)] | ||
| Or(Vec<Pattern>), | ||
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| /// An integer range pattern such as `1..20` which will match any integer n such that | ||
| /// 1 <= n < 20. | ||
| #[allow(unused)] | ||
| Range(SignedField, SignedField), | ||
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| /// An error occurred while translating this pattern. This Pattern kind always translates | ||
| /// to a Fail branch in the decision tree, although the compiler is expected to halt | ||
| /// with errors before execution. | ||
| Error, | ||
| } | ||
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| #[derive(Clone)] | ||
| struct Column { | ||
| variable_to_match: DefinitionId, | ||
| pattern: Pattern, | ||
| } | ||
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| impl Column { | ||
| fn new(variable_to_match: DefinitionId, pattern: Pattern) -> Self { | ||
| Column { variable_to_match, pattern } | ||
| } | ||
| } | ||
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| #[derive(Clone)] | ||
| pub(super) struct Row { | ||
| columns: Vec<Column>, | ||
| guard: Option<RowBody>, | ||
| body: RowBody, | ||
| original_body: RowBody, | ||
| location: Location, | ||
| } | ||
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| type RowBody = ExprId; | ||
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| impl Row { | ||
| fn new(columns: Vec<Column>, guard: Option<RowBody>, body: RowBody, location: Location) -> Row { | ||
| Row { columns, guard, body, original_body: body, location } | ||
| } | ||
| } | ||
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| impl Row { | ||
| fn remove_column(&mut self, variable: DefinitionId) -> Option<Column> { | ||
| self.columns | ||
| .iter() | ||
| .position(|c| c.variable_to_match == variable) | ||
| .map(|idx| self.columns.remove(idx)) | ||
| } | ||
| } | ||
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All this was just moved from the bottom of the file to the top.
The only change is the addition of original_body: RowBody and location: Location in Row
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Description
Problem*
Resolves
Summary*
Implements "unreachable case" errors for patterns in match expressions that are redundant with other patterns.
Additional Context
This took more time than expected. While the initial error was easy it was surprisingly difficult to get it to stop erroring if there were also any other errors in constructing the match rows. E.g. a program like
Will give a type error on
3 => (),but will also give a separate error saying that it is unreachable. This extra error broke existing match tests (which mostly only expect 1 error) and was difficult to remove since it is technically correct. In the end I opted to not compile the decision tree of a match at all if there were any errors in constructing the pattern rows. This is what Ante does and it seems to match Rust as well. Neither of those languages issue errors for obviously redundant cases like the secondNone => (),if there was a type error or a name resolution error in constructing the match tree.Documentation*
Check one:
PR Checklist*
cargo fmton default settings.