Add pattern match exhaustiveness checking
Implements the exhaustiveness algorithm to detect non-exhaustive pattern matches: - Detects missing Bool patterns (true/false) - Detects missing Option patterns (Some/None) - Detects missing Result patterns (Ok/Err) - Recognizes wildcards and variable patterns as catch-alls - Warns about redundant patterns after catch-all patterns - Integrates with the type checker to report errors Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
This commit is contained in:
408
src/exhaustiveness.rs
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408
src/exhaustiveness.rs
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//! Pattern match exhaustiveness checking
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//!
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//! Implements the "usefulness" algorithm to check if pattern matches
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//! cover all possible cases.
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use crate::ast::{Literal, LiteralKind, MatchArm, Pattern};
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use crate::types::{Type, TypeDef, TypeEnv, VariantDef};
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use std::collections::HashSet;
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/// Result of exhaustiveness checking
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#[derive(Debug, Clone)]
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pub struct ExhaustivenessResult {
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pub is_exhaustive: bool,
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pub missing_patterns: Vec<String>,
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pub redundant_arms: Vec<usize>,
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}
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/// Check if a match expression is exhaustive
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pub fn check_exhaustiveness(
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scrutinee_type: &Type,
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arms: &[MatchArm],
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env: &TypeEnv,
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) -> ExhaustivenessResult {
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let patterns: Vec<&Pattern> = arms.iter().map(|arm| &arm.pattern).collect();
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// Check for guards - patterns with guards don't guarantee coverage
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let has_guards = arms.iter().any(|arm| arm.guard.is_some());
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// Get missing patterns
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let missing = find_missing_patterns(scrutinee_type, &patterns, env);
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// Find redundant arms (patterns that can never match)
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let redundant = find_redundant_arms(&patterns);
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// If any pattern has a guard, we can't guarantee exhaustiveness
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// unless there's an unconditional wildcard at the end
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let is_exhaustive = if has_guards {
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// Check if last pattern is an unconditional catch-all
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arms.last()
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.map(|arm| arm.guard.is_none() && is_catch_all(&arm.pattern))
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.unwrap_or(false)
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} else {
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missing.is_empty()
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};
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ExhaustivenessResult {
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is_exhaustive,
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missing_patterns: missing,
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redundant_arms: redundant,
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}
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}
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/// Check if a pattern is a catch-all (matches anything)
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fn is_catch_all(pattern: &Pattern) -> bool {
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match pattern {
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Pattern::Wildcard(_) => true,
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Pattern::Var(_) => true,
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_ => false,
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}
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}
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/// Find patterns that are missing from coverage
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fn find_missing_patterns(
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scrutinee_type: &Type,
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patterns: &[&Pattern],
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env: &TypeEnv,
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) -> Vec<String> {
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// If any pattern is a catch-all, nothing is missing
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if patterns.iter().any(|p| is_catch_all(p)) {
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return Vec::new();
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}
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match scrutinee_type {
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Type::Bool => check_bool_exhaustiveness(patterns),
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Type::Option(inner) => check_option_exhaustiveness(patterns, inner, env),
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Type::Named(name) => check_named_type_exhaustiveness(patterns, name, env),
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Type::Tuple(elements) => check_tuple_exhaustiveness(patterns, elements, env),
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// For other types (Int, String, etc.), we can't enumerate all values
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// So we need a wildcard pattern
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Type::Int | Type::Float | Type::String | Type::Char => {
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vec!["_".to_string()]
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}
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// Unit type has exactly one value
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Type::Unit => Vec::new(),
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// For type variables and other complex types, assume exhaustive if there's any pattern
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_ => {
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if patterns.is_empty() {
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vec!["_".to_string()]
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} else {
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Vec::new()
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}
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}
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}
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}
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/// Check Bool exhaustiveness
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fn check_bool_exhaustiveness(patterns: &[&Pattern]) -> Vec<String> {
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let mut has_true = false;
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let mut has_false = false;
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for pattern in patterns {
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match pattern {
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Pattern::Literal(Literal {
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kind: LiteralKind::Bool(true),
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..
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}) => has_true = true,
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Pattern::Literal(Literal {
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kind: LiteralKind::Bool(false),
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..
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}) => has_false = true,
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_ => {}
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}
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}
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let mut missing = Vec::new();
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if !has_true {
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missing.push("true".to_string());
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}
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if !has_false {
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missing.push("false".to_string());
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}
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missing
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}
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/// Check Option exhaustiveness
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fn check_option_exhaustiveness(
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patterns: &[&Pattern],
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_inner: &Type,
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_env: &TypeEnv,
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) -> Vec<String> {
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let mut has_none = false;
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let mut has_some = false;
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for pattern in patterns {
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if let Pattern::Constructor { name, .. } = pattern {
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match name.name.as_str() {
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"None" => has_none = true,
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"Some" => has_some = true,
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_ => {}
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}
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}
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}
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let mut missing = Vec::new();
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if !has_none {
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missing.push("None".to_string());
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}
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if !has_some {
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missing.push("Some(_)".to_string());
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}
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missing
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}
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/// Check named type (enum) exhaustiveness
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fn check_named_type_exhaustiveness(
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patterns: &[&Pattern],
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type_name: &str,
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env: &TypeEnv,
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) -> Vec<String> {
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// Look up the type definition
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let typedef = match env.types.get(type_name) {
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Some(td) => td,
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None => return Vec::new(), // Unknown type, assume exhaustive
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};
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// Handle Result specially since it's common
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if type_name == "Result" {
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return check_result_exhaustiveness(patterns);
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}
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// Get all constructors for enum types
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let constructors: Vec<&VariantDef> = match typedef {
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TypeDef::Enum(variants) => variants.iter().collect(),
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_ => return Vec::new(), // Not an enum, assume exhaustive
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};
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// Find which constructors are covered
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let mut covered: HashSet<&str> = HashSet::new();
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for pattern in patterns {
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if let Pattern::Constructor { name, .. } = pattern {
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covered.insert(&name.name);
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}
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}
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// Find missing constructors
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constructors
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.iter()
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.filter(|v| !covered.contains(v.name.as_str()))
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.map(|v| format_constructor_pattern(v))
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.collect()
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}
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/// Check Result exhaustiveness
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fn check_result_exhaustiveness(patterns: &[&Pattern]) -> Vec<String> {
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let mut has_ok = false;
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let mut has_err = false;
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for pattern in patterns {
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if let Pattern::Constructor { name, .. } = pattern {
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match name.name.as_str() {
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"Ok" => has_ok = true,
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"Err" => has_err = true,
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_ => {}
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}
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}
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}
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let mut missing = Vec::new();
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if !has_ok {
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missing.push("Ok(_)".to_string());
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}
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if !has_err {
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missing.push("Err(_)".to_string());
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}
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missing
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}
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/// Check tuple exhaustiveness
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fn check_tuple_exhaustiveness(
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patterns: &[&Pattern],
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_elements: &[Type],
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_env: &TypeEnv,
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) -> Vec<String> {
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// Tuples need a pattern that matches the whole tuple
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// For simplicity, we just check if there's any tuple pattern
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let has_tuple_pattern = patterns.iter().any(|p| matches!(p, Pattern::Tuple { .. }));
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if has_tuple_pattern {
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Vec::new()
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} else {
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vec!["(_, ...)".to_string()]
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}
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}
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/// Format a variant as a pattern suggestion
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fn format_constructor_pattern(variant: &VariantDef) -> String {
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use crate::types::VariantFieldsDef;
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match &variant.fields {
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VariantFieldsDef::Unit => variant.name.clone(),
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VariantFieldsDef::Tuple(fields) => {
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let wildcards: Vec<&str> = fields.iter().map(|_| "_").collect();
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format!("{}({})", variant.name, wildcards.join(", "))
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}
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VariantFieldsDef::Record(fields) => {
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let wildcards: Vec<String> = fields.iter().map(|(n, _)| format!("{}: _", n)).collect();
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format!("{} {{ {} }}", variant.name, wildcards.join(", "))
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}
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}
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}
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/// Find redundant arms that can never match
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fn find_redundant_arms(patterns: &[&Pattern]) -> Vec<usize> {
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let mut redundant = Vec::new();
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let mut seen_catch_all = false;
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for (i, pattern) in patterns.iter().enumerate() {
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if seen_catch_all {
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// Any pattern after a catch-all is redundant
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redundant.push(i);
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} else if is_catch_all(pattern) {
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seen_catch_all = true;
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}
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}
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redundant
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}
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/// Generate a hint message for missing patterns
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pub fn missing_patterns_hint(missing: &[String]) -> String {
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if missing.is_empty() {
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return String::new();
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}
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if missing.len() == 1 {
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format!("Pattern '{}' is not covered.", missing[0])
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} else if missing.len() <= 3 {
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format!("Patterns {} are not covered.", missing.join(", "))
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} else {
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format!(
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"Patterns {}, and {} more are not covered.",
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missing[..2].join(", "),
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missing.len() - 2
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)
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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use crate::ast::{Ident, Span};
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fn span() -> Span {
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Span::default()
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}
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fn make_ident(name: &str) -> Ident {
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Ident {
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name: name.to_string(),
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span: span(),
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}
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}
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#[test]
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fn test_bool_exhaustive() {
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let patterns = vec![
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Pattern::Literal(Literal {
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kind: LiteralKind::Bool(true),
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span: span(),
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}),
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Pattern::Literal(Literal {
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kind: LiteralKind::Bool(false),
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span: span(),
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}),
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];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let missing = check_bool_exhaustiveness(&refs);
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assert!(missing.is_empty());
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}
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#[test]
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fn test_bool_missing_true() {
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let patterns = vec![Pattern::Literal(Literal {
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kind: LiteralKind::Bool(false),
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span: span(),
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})];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let missing = check_bool_exhaustiveness(&refs);
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assert_eq!(missing, vec!["true"]);
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}
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#[test]
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fn test_option_exhaustive() {
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let patterns = vec![
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Pattern::Constructor {
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name: make_ident("None"),
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fields: vec![],
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span: span(),
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},
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Pattern::Constructor {
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name: make_ident("Some"),
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fields: vec![Pattern::Wildcard(span())],
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span: span(),
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},
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];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let env = TypeEnv::new();
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let missing = check_option_exhaustiveness(&refs, &Type::Int, &env);
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assert!(missing.is_empty());
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}
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#[test]
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fn test_option_missing_none() {
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let patterns = vec![Pattern::Constructor {
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name: make_ident("Some"),
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fields: vec![Pattern::Wildcard(span())],
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span: span(),
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}];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let env = TypeEnv::new();
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let missing = check_option_exhaustiveness(&refs, &Type::Int, &env);
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assert!(missing.contains(&"None".to_string()));
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}
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#[test]
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fn test_wildcard_covers_all() {
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let patterns = vec![Pattern::Wildcard(span())];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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assert!(is_catch_all(&patterns[0]));
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let env = TypeEnv::new();
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let missing = find_missing_patterns(&Type::Bool, &refs, &env);
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assert!(missing.is_empty());
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}
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#[test]
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fn test_redundant_after_wildcard() {
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let patterns = vec![
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Pattern::Wildcard(span()),
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Pattern::Literal(Literal {
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kind: LiteralKind::Bool(true),
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span: span(),
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}),
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];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let redundant = find_redundant_arms(&refs);
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assert_eq!(redundant, vec![1]);
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}
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#[test]
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fn test_result_exhaustive() {
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let patterns = vec![
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Pattern::Constructor {
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name: make_ident("Ok"),
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fields: vec![Pattern::Wildcard(span())],
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span: span(),
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},
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Pattern::Constructor {
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name: make_ident("Err"),
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fields: vec![Pattern::Wildcard(span())],
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span: span(),
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},
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];
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let refs: Vec<&Pattern> = patterns.iter().collect();
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let missing = check_result_exhaustiveness(&refs);
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assert!(missing.is_empty());
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}
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}
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