//! This module implements match statement exhaustiveness checking and usefulness checking //! for match arms. //! //! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which //! contains very detailed documentation about the algorithms used here. I've duplicated //! most of that documentation below. //! //! This file includes the logic for exhaustiveness and usefulness checking for //! pattern-matching. Specifically, given a list of patterns for a type, we can //! tell whether: //! - (a) the patterns cover every possible constructor for the type (exhaustiveness). //! - (b) each pattern is necessary (usefulness). //! //! The algorithm implemented here is a modified version of the one described in //! <http://moscova.inria.fr/~maranget/papers/warn/index.html>. //! However, to save future implementors from reading the original paper, we //! summarise the algorithm here to hopefully save time and be a little clearer //! (without being so rigorous). //! //! The core of the algorithm revolves about a "usefulness" check. In particular, we //! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as //! a matrix). `U(P, p)` represents whether, given an existing list of patterns //! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously- //! uncovered values of the type). //! //! If we have this predicate, then we can easily compute both exhaustiveness of an //! entire set of patterns and the individual usefulness of each one. //! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard //! match doesn't increase the number of values we're matching) //! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a //! pattern to those that have come before it doesn't increase the number of values //! we're matching). //! //! During the course of the algorithm, the rows of the matrix won't just be individual patterns, //! but rather partially-deconstructed patterns in the form of a list of patterns. The paper //! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the //! new pattern `p`. //! //! For example, say we have the following: //! //! ```ignore //! // x: (Option<bool>, Result<()>) //! match x { //! (Some(true), _) => (), //! (None, Err(())) => (), //! (None, Err(_)) => (), //! } //! ``` //! //! Here, the matrix `P` starts as: //! //! ```text //! [ //! [(Some(true), _)], //! [(None, Err(()))], //! [(None, Err(_))], //! ] //! ``` //! //! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering //! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because //! all the values it covers are already covered by row 2. //! //! A list of patterns can be thought of as a stack, because we are mainly interested in the top of //! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks. //! To match the paper, the top of the stack is at the beginning / on the left. //! //! There are two important operations on pattern-stacks necessary to understand the algorithm: //! //! 1. We can pop a given constructor off the top of a stack. This operation is called //! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or //! `None`) and `p` a pattern-stack. //! If the pattern on top of the stack can cover `c`, this removes the constructor and //! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns. //! Otherwise the pattern-stack is discarded. //! This essentially filters those pattern-stacks whose top covers the constructor `c` and //! discards the others. //! //! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we //! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the //! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get //! nothing back. //! //! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1` //! on top of the stack, and we have four cases: //! //! * 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We push onto //! the stack the arguments of this constructor, and return the result: //! //! r_1, .., r_a, p_2, .., p_n //! //! * 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and return //! nothing. //! * 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has //! arguments (its arity), and return the resulting stack: //! //! _, .., _, p_2, .., p_n //! //! * 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack: //! //! S(c, (r_1, p_2, .., p_n)) //! S(c, (r_2, p_2, .., p_n)) //! //! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is //! a pattern-stack. //! This is used when we know there are missing constructor cases, but there might be //! existing wildcard patterns, so to check the usefulness of the matrix, we have to check //! all its *other* components. //! //! It is computed as follows. We look at the pattern `p_1` on top of the stack, //! and we have three cases: //! * 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing. //! * 1.2. `p_1 = _`. We return the rest of the stack: //! //! p_2, .., p_n //! //! * 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack: //! //! D((r_1, p_2, .., p_n)) //! D((r_2, p_2, .., p_n)) //! //! Note that the OR-patterns are not always used directly in Rust, but are used to derive the //! exhaustive integer matching rules, so they're written here for posterity. //! //! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by //! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with //! the given constructor, and popping a wildcard keeps those rows that start with a wildcard. //! //! //! The algorithm for computing `U` //! ------------------------------- //! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns). //! That means we're going to check the components from left-to-right, so the algorithm //! operates principally on the first component of the matrix and new pattern-stack `p`. //! This algorithm is realised in the `is_useful` function. //! //! Base case (`n = 0`, i.e., an empty tuple pattern): //! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), then //! `U(P, p)` is false. //! - Otherwise, `P` must be empty, so `U(P, p)` is true. //! //! Inductive step (`n > 0`, i.e., whether there's at least one column [which may then be expanded //! into further columns later]). We're going to match on the top of the new pattern-stack, `p_1`: //! //! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern. //! Then, the usefulness of `p_1` can be reduced to whether it is useful when //! we ignore all the patterns in the first column of `P` that involve other constructors. //! This is where `S(c, P)` comes in: //! //! ```text //! U(P, p) := U(S(c, P), S(c, p)) //! ``` //! //! This special case is handled in `is_useful_specialized`. //! //! For example, if `P` is: //! //! ```text //! [ //! [Some(true), _], //! [None, 0], //! ] //! ``` //! //! and `p` is `[Some(false), 0]`, then we don't care about row 2 since we know `p` only //! matches values that row 2 doesn't. For row 1 however, we need to dig into the //! arguments of `Some` to know whether some new value is covered. So we compute //! `U([[true, _]], [false, 0])`. //! //! - If `p_1 == _`, then we look at the list of constructors that appear in the first component of //! the rows of `P`: //! - If there are some constructors that aren't present, then we might think that the //! wildcard `_` is useful, since it covers those constructors that weren't covered //! before. //! That's almost correct, but only works if there were no wildcards in those first //! components. So we need to check that `p` is useful with respect to the rows that //! start with a wildcard, if there are any. This is where `D` comes in: //! `U(P, p) := U(D(P), D(p))` //! //! For example, if `P` is: //! ```text //! [ //! [_, true, _], //! [None, false, 1], //! ] //! ``` //! and `p` is `[_, false, _]`, the `Some` constructor doesn't appear in `P`. So if we //! only had row 2, we'd know that `p` is useful. However row 1 starts with a //! wildcard, so we need to check whether `U([[true, _]], [false, 1])`. //! //! - Otherwise, all possible constructors (for the relevant type) are present. In this //! case we must check whether the wildcard pattern covers any unmatched value. For //! that, we can think of the `_` pattern as a big OR-pattern that covers all //! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for //! example. The wildcard pattern is useful in this case if it is useful when //! specialized to one of the possible constructors. So we compute: //! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))` //! //! For example, if `P` is: //! ```text //! [ //! [Some(true), _], //! [None, false], //! ] //! ``` //! and `p` is `[_, false]`, both `None` and `Some` constructors appear in the first //! components of `P`. We will therefore try popping both constructors in turn: we //! compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]], //! [false])` for the `None` constructor. The first case returns true, so we know that //! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched //! before. //! //! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately: //! //! ```text //! U(P, p) := U(P, (r_1, p_2, .., p_n)) //! || U(P, (r_2, p_2, .., p_n)) //! ``` use std::{iter, sync::Arc}; use arena::Idx; use hir_def::{ adt::VariantData, body::Body, expr::{Expr, Literal, Pat, PatId}, AdtId, EnumVariantId, StructId, VariantId, }; use smallvec::{smallvec, SmallVec}; use crate::{db::HirDatabase, ApplicationTy, InferenceResult, Ty, TypeCtor}; #[derive(Debug, Clone, Copy)] /// Either a pattern from the source code being analyzed, represented as /// as `PatId`, or a `Wild` pattern which is created as an intermediate /// step in the match checking algorithm and thus is not backed by a /// real `PatId`. /// /// Note that it is totally valid for the `PatId` variant to contain /// a `PatId` which resolves to a `Wild` pattern, if that wild pattern /// exists in the source code being analyzed. enum PatIdOrWild { PatId(PatId), Wild, } impl PatIdOrWild { fn as_pat(self, cx: &MatchCheckCtx) -> Pat { match self { PatIdOrWild::PatId(id) => cx.body.pats[id].clone(), PatIdOrWild::Wild => Pat::Wild, } } fn as_id(self) -> Option<PatId> { match self { PatIdOrWild::PatId(id) => Some(id), PatIdOrWild::Wild => None, } } } impl From<PatId> for PatIdOrWild { fn from(pat_id: PatId) -> Self { Self::PatId(pat_id) } } impl From<&PatId> for PatIdOrWild { fn from(pat_id: &PatId) -> Self { Self::PatId(*pat_id) } } #[derive(Debug, Clone, Copy, PartialEq)] pub(super) enum MatchCheckErr { NotImplemented, MalformedMatchArm, /// Used when type inference cannot resolve the type of /// a pattern or expression. Unknown, } /// The return type of `is_useful` is either an indication of usefulness /// of the match arm, or an error in the case the match statement /// is made up of types for which exhaustiveness checking is currently /// not completely implemented. /// /// The `std::result::Result` type is used here rather than a custom enum /// to allow the use of `?`. pub(super) type MatchCheckResult<T> = Result<T, MatchCheckErr>; #[derive(Debug)] /// A row in a Matrix. /// /// This type is modeled from the struct of the same name in `rustc`. pub(super) struct PatStack(PatStackInner); type PatStackInner = SmallVec<[PatIdOrWild; 2]>; impl PatStack { pub(super) fn from_pattern(pat_id: PatId) -> PatStack { Self(smallvec!(pat_id.into())) } pub(super) fn from_wild() -> PatStack { Self(smallvec!(PatIdOrWild::Wild)) } fn from_slice(slice: &[PatIdOrWild]) -> PatStack { Self(SmallVec::from_slice(slice)) } fn from_vec(v: PatStackInner) -> PatStack { Self(v) } fn get_head(&self) -> Option<PatIdOrWild> { self.0.first().copied() } fn tail(&self) -> &[PatIdOrWild] { self.0.get(1..).unwrap_or(&[]) } fn to_tail(&self) -> PatStack { Self::from_slice(self.tail()) } fn replace_head_with<I, T>(&self, pats: I) -> PatStack where I: Iterator<Item = T>, T: Into<PatIdOrWild>, { let mut patterns: PatStackInner = smallvec![]; for pat in pats { patterns.push(pat.into()); } for pat in &self.0[1..] { patterns.push(*pat); } PatStack::from_vec(patterns) } /// Computes `D(self)`. /// /// See the module docs and the associated documentation in rustc for details. fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> { if matches!(self.get_head()?.as_pat(cx), Pat::Wild) { Some(self.to_tail()) } else { None } } /// Computes `S(constructor, self)`. /// /// See the module docs and the associated documentation in rustc for details. fn specialize_constructor( &self, cx: &MatchCheckCtx, constructor: &Constructor, ) -> MatchCheckResult<Option<PatStack>> { let head = match self.get_head() { Some(head) => head, None => return Ok(None), }; let head_pat = head.as_pat(cx); let result = match (head_pat, constructor) { (Pat::Tuple { args: pat_ids, ellipsis }, &Constructor::Tuple { arity }) => { if let Some(ellipsis) = ellipsis { let (pre, post) = pat_ids.split_at(ellipsis); let n_wild_pats = arity.saturating_sub(pat_ids.len()); let pre_iter = pre.iter().map(Into::into); let wildcards = iter::repeat(PatIdOrWild::Wild).take(n_wild_pats); let post_iter = post.iter().map(Into::into); Some(self.replace_head_with(pre_iter.chain(wildcards).chain(post_iter))) } else { Some(self.replace_head_with(pat_ids.iter())) } } (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => { match cx.body.exprs[lit_expr] { Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => { Some(self.to_tail()) } // it was a bool but the value doesn't match Expr::Literal(Literal::Bool(_)) => None, // perhaps this is actually unreachable given we have // already checked that these match arms have the appropriate type? _ => return Err(MatchCheckErr::NotImplemented), } } (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?), (Pat::Path(_), constructor) => { // unit enum variants become `Pat::Path` let pat_id = head.as_id().expect("we know this isn't a wild"); let variant_id: VariantId = match constructor { &Constructor::Enum(e) => e.into(), &Constructor::Struct(s) => s.into(), _ => return Err(MatchCheckErr::NotImplemented), }; if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) { None } else { Some(self.to_tail()) } } (Pat::TupleStruct { args: ref pat_ids, ellipsis, .. }, constructor) => { let pat_id = head.as_id().expect("we know this isn't a wild"); let variant_id: VariantId = match constructor { &Constructor::Enum(e) => e.into(), &Constructor::Struct(s) => s.into(), _ => return Err(MatchCheckErr::MalformedMatchArm), }; if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) { None } else { let constructor_arity = constructor.arity(cx)?; if let Some(ellipsis_position) = ellipsis { // If there are ellipsis in the pattern, the ellipsis must take the place // of at least one sub-pattern, so `pat_ids` should be smaller than the // constructor arity. if pat_ids.len() < constructor_arity { let mut new_patterns: Vec<PatIdOrWild> = vec![]; for pat_id in &pat_ids[0..ellipsis_position] { new_patterns.push((*pat_id).into()); } for _ in 0..(constructor_arity - pat_ids.len()) { new_patterns.push(PatIdOrWild::Wild); } for pat_id in &pat_ids[ellipsis_position..pat_ids.len()] { new_patterns.push((*pat_id).into()); } Some(self.replace_head_with(new_patterns.into_iter())) } else { return Err(MatchCheckErr::MalformedMatchArm); } } else { // If there is no ellipsis in the tuple pattern, the number // of patterns must equal the constructor arity. if pat_ids.len() == constructor_arity { Some(self.replace_head_with(pat_ids.into_iter())) } else { return Err(MatchCheckErr::MalformedMatchArm); } } } } (Pat::Record { args: ref arg_patterns, .. }, constructor) => { let pat_id = head.as_id().expect("we know this isn't a wild"); let (variant_id, variant_data) = match constructor { &Constructor::Enum(e) => ( e.into(), cx.db.enum_data(e.parent).variants[e.local_id].variant_data.clone(), ), &Constructor::Struct(s) => { (s.into(), cx.db.struct_data(s).variant_data.clone()) } _ => return Err(MatchCheckErr::MalformedMatchArm), }; if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) { None } else { match variant_data.as_ref() { VariantData::Record(struct_field_arena) => { // Here we treat any missing fields in the record as the wild pattern, as // if the record has ellipsis. We want to do this here even if the // record does not contain ellipsis, because it allows us to continue // enforcing exhaustiveness for the rest of the match statement. // // Creating the diagnostic for the missing field in the pattern // should be done in a different diagnostic. let patterns = struct_field_arena.iter().map(|(_, struct_field)| { arg_patterns .iter() .find(|pat| pat.name == struct_field.name) .map(|pat| PatIdOrWild::from(pat.pat)) .unwrap_or(PatIdOrWild::Wild) }); Some(self.replace_head_with(patterns)) } _ => return Err(MatchCheckErr::Unknown), } } } (Pat::Or(_), _) => return Err(MatchCheckErr::NotImplemented), (_, _) => return Err(MatchCheckErr::NotImplemented), }; Ok(result) } /// A special case of `specialize_constructor` where the head of the pattern stack /// is a Wild pattern. /// /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns /// (N >= 0), where N is the arity of the given constructor. fn expand_wildcard( &self, cx: &MatchCheckCtx, constructor: &Constructor, ) -> MatchCheckResult<PatStack> { assert_eq!( Pat::Wild, self.get_head().expect("expand_wildcard called on empty PatStack").as_pat(cx), "expand_wildcard must only be called on PatStack with wild at head", ); let mut patterns: PatStackInner = smallvec![]; for _ in 0..constructor.arity(cx)? { patterns.push(PatIdOrWild::Wild); } for pat in &self.0[1..] { patterns.push(*pat); } Ok(PatStack::from_vec(patterns)) } } /// A collection of PatStack. /// /// This type is modeled from the struct of the same name in `rustc`. pub(super) struct Matrix(Vec<PatStack>); impl Matrix { pub(super) fn empty() -> Self { Self(vec![]) } pub(super) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) { if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) { // Or patterns are expanded here for pat_id in pat_ids { self.0.push(PatStack::from_pattern(pat_id)); } } else { self.0.push(row); } } fn is_empty(&self) -> bool { self.0.is_empty() } fn heads(&self) -> Vec<PatIdOrWild> { self.0.iter().flat_map(|p| p.get_head()).collect() } /// Computes `D(self)` for each contained PatStack. /// /// See the module docs and the associated documentation in rustc for details. fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self { Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx))) } /// Computes `S(constructor, self)` for each contained PatStack. /// /// See the module docs and the associated documentation in rustc for details. fn specialize_constructor( &self, cx: &MatchCheckCtx, constructor: &Constructor, ) -> MatchCheckResult<Self> { let mut new_matrix = Matrix::empty(); for pat in &self.0 { if let Some(pat) = pat.specialize_constructor(cx, constructor)? { new_matrix.push(cx, pat); } } Ok(new_matrix) } fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self { let mut matrix = Matrix::empty(); for pat in iter { // using push ensures we expand or-patterns matrix.push(cx, pat); } matrix } } #[derive(Clone, Debug, PartialEq)] /// An indication of the usefulness of a given match arm, where /// usefulness is defined as matching some patterns which were /// not matched by an prior match arms. /// /// We may eventually need an `Unknown` variant here. pub(super) enum Usefulness { Useful, NotUseful, } pub(super) struct MatchCheckCtx<'a> { pub(super) match_expr: Idx<Expr>, pub(super) body: Arc<Body>, pub(super) infer: Arc<InferenceResult>, pub(super) db: &'a dyn HirDatabase, } /// Given a set of patterns `matrix`, and pattern to consider `v`, determines /// whether `v` is useful. A pattern is useful if it covers cases which were /// not previously covered. /// /// When calling this function externally (that is, not the recursive calls) it /// expected that you have already type checked the match arms. All patterns in /// matrix should be the same type as v, as well as they should all be the same /// type as the match expression. pub(super) fn is_useful( cx: &MatchCheckCtx, matrix: &Matrix, v: &PatStack, ) -> MatchCheckResult<Usefulness> { // Handle two special cases: // - enum with no variants // - `!` type // In those cases, no match arm is useful. match cx.infer[cx.match_expr].strip_references() { Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) => { if cx.db.enum_data(*enum_id).variants.is_empty() { return Ok(Usefulness::NotUseful); } } Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) => { return Ok(Usefulness::NotUseful); } _ => (), } let head = match v.get_head() { Some(head) => head, None => { let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful }; return Ok(result); } }; if let Pat::Or(pat_ids) = head.as_pat(cx) { let mut found_unimplemented = false; let any_useful = pat_ids.iter().any(|&pat_id| { let v = PatStack::from_pattern(pat_id); match is_useful(cx, matrix, &v) { Ok(Usefulness::Useful) => true, Ok(Usefulness::NotUseful) => false, _ => { found_unimplemented = true; false } } }); return if any_useful { Ok(Usefulness::Useful) } else if found_unimplemented { Err(MatchCheckErr::NotImplemented) } else { Ok(Usefulness::NotUseful) }; } if let Some(constructor) = pat_constructor(cx, head)? { let matrix = matrix.specialize_constructor(&cx, &constructor)?; let v = v .specialize_constructor(&cx, &constructor)? .expect("we know this can't fail because we get the constructor from `v.head()` above"); is_useful(&cx, &matrix, &v) } else { // expanding wildcard let mut used_constructors: Vec<Constructor> = vec![]; for pat in matrix.heads() { if let Some(constructor) = pat_constructor(cx, pat)? { used_constructors.push(constructor); } } // We assume here that the first constructor is the "correct" type. Since we // only care about the "type" of the constructor (i.e. if it is a bool we // don't care about the value), this assumption should be valid as long as // the match statement is well formed. We currently uphold this invariant by // filtering match arms before calling `is_useful`, only passing in match arms // whose type matches the type of the match expression. match &used_constructors.first() { Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => { // If all constructors are covered, then we need to consider whether // any values are covered by this wildcard. // // For example, with matrix '[[Some(true)], [None]]', all // constructors are covered (`Some`/`None`), so we need // to perform specialization to see that our wildcard will cover // the `Some(false)` case. // // Here we create a constructor for each variant and then check // usefulness after specializing for that constructor. let mut found_unimplemented = false; for constructor in constructor.all_constructors(cx) { let matrix = matrix.specialize_constructor(&cx, &constructor)?; let v = v.expand_wildcard(&cx, &constructor)?; match is_useful(&cx, &matrix, &v) { Ok(Usefulness::Useful) => return Ok(Usefulness::Useful), Ok(Usefulness::NotUseful) => continue, _ => found_unimplemented = true, }; } if found_unimplemented { Err(MatchCheckErr::NotImplemented) } else { Ok(Usefulness::NotUseful) } } _ => { // Either not all constructors are covered, or the only other arms // are wildcards. Either way, this pattern is useful if it is useful // when compared to those arms with wildcards. let matrix = matrix.specialize_wildcard(&cx); let v = v.to_tail(); is_useful(&cx, &matrix, &v) } } } } #[derive(Debug, Clone, Copy)] /// Similar to TypeCtor, but includes additional information about the specific /// value being instantiated. For example, TypeCtor::Bool doesn't contain the /// boolean value. enum Constructor { Bool(bool), Tuple { arity: usize }, Enum(EnumVariantId), Struct(StructId), } impl Constructor { fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> { let arity = match self { Constructor::Bool(_) => 0, Constructor::Tuple { arity } => *arity, Constructor::Enum(e) => { match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() { VariantData::Tuple(struct_field_data) => struct_field_data.len(), VariantData::Record(struct_field_data) => struct_field_data.len(), VariantData::Unit => 0, } } &Constructor::Struct(s) => match cx.db.struct_data(s).variant_data.as_ref() { VariantData::Tuple(struct_field_data) => struct_field_data.len(), VariantData::Record(struct_field_data) => struct_field_data.len(), VariantData::Unit => 0, }, }; Ok(arity) } fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> { match self { Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)], Constructor::Tuple { .. } | Constructor::Struct(_) => vec![*self], Constructor::Enum(e) => cx .db .enum_data(e.parent) .variants .iter() .map(|(local_id, _)| { Constructor::Enum(EnumVariantId { parent: e.parent, local_id }) }) .collect(), } } } /// Returns the constructor for the given pattern. Should only return None /// in the case of a Wild pattern. fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> { let res = match pat.as_pat(cx) { Pat::Wild => None, Pat::Tuple { .. } => { let pat_id = pat.as_id().expect("we already know this pattern is not a wild"); Some(Constructor::Tuple { arity: cx.infer.type_of_pat[pat_id].as_tuple().ok_or(MatchCheckErr::Unknown)?.len(), }) } Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] { Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)), _ => return Err(MatchCheckErr::NotImplemented), }, Pat::TupleStruct { .. } | Pat::Path(_) | Pat::Record { .. } => { let pat_id = pat.as_id().expect("we already know this pattern is not a wild"); let variant_id = cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckErr::Unknown)?; match variant_id { VariantId::EnumVariantId(enum_variant_id) => { Some(Constructor::Enum(enum_variant_id)) } VariantId::StructId(struct_id) => Some(Constructor::Struct(struct_id)), _ => return Err(MatchCheckErr::NotImplemented), } } _ => return Err(MatchCheckErr::NotImplemented), }; Ok(res) } fn all_constructors_covered( cx: &MatchCheckCtx, constructor: &Constructor, used_constructors: &[Constructor], ) -> bool { match constructor { Constructor::Tuple { arity } => { used_constructors.iter().any(|constructor| match constructor { Constructor::Tuple { arity: used_arity } => arity == used_arity, _ => false, }) } Constructor::Bool(_) => { if used_constructors.is_empty() { return false; } let covers_true = used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true))); let covers_false = used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false))); covers_true && covers_false } Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| { for constructor in used_constructors { if let Constructor::Enum(e) = constructor { if id == e.local_id { return true; } } } false }), &Constructor::Struct(s) => used_constructors.iter().any(|constructor| match constructor { &Constructor::Struct(sid) => sid == s, _ => false, }), } } #[cfg(test)] mod tests { use crate::diagnostics::tests::check_diagnostics; #[test] fn empty_tuple() { check_diagnostics( r#" fn main() { match () { } //^^ Missing match arm match (()) { } //^^^^ Missing match arm match () { _ => (), } match () { () => (), } match (()) { (()) => (), } } "#, ); } #[test] fn tuple_of_two_empty_tuple() { check_diagnostics( r#" fn main() { match ((), ()) { } //^^^^^^^^ Missing match arm match ((), ()) { ((), ()) => (), } } "#, ); } #[test] fn boolean() { check_diagnostics( r#" fn test_main() { match false { } //^^^^^ Missing match arm match false { true => (), } //^^^^^ Missing match arm match (false, true) {} //^^^^^^^^^^^^^ Missing match arm match (false, true) { (true, true) => (), } //^^^^^^^^^^^^^ Missing match arm match (false, true) { //^^^^^^^^^^^^^ Missing match arm (false, true) => (), (false, false) => (), (true, false) => (), } match (false, true) { (true, _x) => (), } //^^^^^^^^^^^^^ Missing match arm match false { true => (), false => (), } match (false, true) { (false, _) => (), (true, false) => (), (_, true) => (), } match (false, true) { (true, true) => (), (true, false) => (), (false, true) => (), (false, false) => (), } match (false, true) { (true, _x) => (), (false, true) => (), (false, false) => (), } match (false, true, false) { (false, ..) => (), (true, ..) => (), } match (false, true, false) { (.., false) => (), (.., true) => (), } match (false, true, false) { (..) => (), } } "#, ); } #[test] fn tuple_of_tuple_and_bools() { check_diagnostics( r#" fn main() { match (false, ((), false)) {} //^^^^^^^^^^^^^^^^^^^^ Missing match arm match (false, ((), false)) { (true, ((), true)) => (), } //^^^^^^^^^^^^^^^^^^^^ Missing match arm match (false, ((), false)) { (true, _) => (), } //^^^^^^^^^^^^^^^^^^^^ Missing match arm match (false, ((), false)) { (true, ((), true)) => (), (true, ((), false)) => (), (false, ((), true)) => (), (false, ((), false)) => (), } match (false, ((), false)) { (true, ((), true)) => (), (true, ((), false)) => (), (false, _) => (), } } "#, ); } #[test] fn enums() { check_diagnostics( r#" enum Either { A, B, } fn main() { match Either::A { } //^^^^^^^^^ Missing match arm match Either::B { Either::A => (), } //^^^^^^^^^ Missing match arm match &Either::B { //^^^^^^^^^^ Missing match arm Either::A => (), } match Either::B { Either::A => (), Either::B => (), } match &Either::B { Either::A => (), Either::B => (), } } "#, ); } #[test] fn enum_containing_bool() { check_diagnostics( r#" enum Either { A(bool), B } fn main() { match Either::B { } //^^^^^^^^^ Missing match arm match Either::B { //^^^^^^^^^ Missing match arm Either::A(true) => (), Either::B => () } match Either::B { Either::A(true) => (), Either::A(false) => (), Either::B => (), } match Either::B { Either::B => (), _ => (), } match Either::B { Either::A(_) => (), Either::B => (), } } "#, ); } #[test] fn enum_different_sizes() { check_diagnostics( r#" enum Either { A(bool), B(bool, bool) } fn main() { match Either::A(false) { //^^^^^^^^^^^^^^^^ Missing match arm Either::A(_) => (), Either::B(false, _) => (), } match Either::A(false) { Either::A(_) => (), Either::B(true, _) => (), Either::B(false, _) => (), } match Either::A(false) { Either::A(true) | Either::A(false) => (), Either::B(true, _) => (), Either::B(false, _) => (), } } "#, ); } #[test] fn tuple_of_enum_no_diagnostic() { check_diagnostics( r#" enum Either { A(bool), B(bool, bool) } enum Either2 { C, D } fn main() { match (Either::A(false), Either2::C) { (Either::A(true), _) | (Either::A(false), _) => (), (Either::B(true, _), Either2::C) => (), (Either::B(false, _), Either2::C) => (), (Either::B(_, _), Either2::D) => (), } } "#, ); } #[test] fn mismatched_types() { // Match statements with arms that don't match the // expression pattern do not fire this diagnostic. check_diagnostics( r#" enum Either { A, B } enum Either2 { C, D } fn main() { match Either::A { Either2::C => (), Either2::D => (), } match (true, false) { (true, false, true) => (), (true) => (), } match (0) { () => () } match Unresolved::Bar { Unresolved::Baz => () } } "#, ); } #[test] fn malformed_match_arm_tuple_enum_missing_pattern() { // We are testing to be sure we don't panic here when the match // arm `Either::B` is missing its pattern. check_diagnostics( r#" enum Either { A, B(u32) } fn main() { match Either::A { Either::A => (), Either::B() => (), } } "#, ); } #[test] fn expr_diverges() { check_diagnostics( r#" enum Either { A, B } fn main() { match loop {} { Either::A => (), Either::B => (), } match loop {} { Either::A => (), } match loop { break Foo::A } { //^^^^^^^^^^^^^^^^^^^^^ Missing match arm Either::A => (), } match loop { break Foo::A } { Either::A => (), Either::B => (), } } "#, ); } #[test] fn expr_partially_diverges() { check_diagnostics( r#" enum Either<T> { A(T), B } fn foo() -> Either<!> { Either::B } fn main() -> u32 { match foo() { Either::A(val) => val, Either::B => 0, } } "#, ); } #[test] fn enum_record() { check_diagnostics( r#" enum Either { A { foo: bool }, B } fn main() { let a = Either::A { foo: true }; match a { } //^ Missing match arm match a { Either::A { foo: true } => () } //^ Missing match arm match a { Either::A { } => (), //^^^^^^^^^ Missing structure fields: // | - foo Either::B => (), } match a { //^ Missing match arm Either::A { } => (), } //^^^^^^^^^ Missing structure fields: // | - foo match a { Either::A { foo: true } => (), Either::A { foo: false } => (), Either::B => (), } match a { Either::A { foo: _ } => (), Either::B => (), } } "#, ); } #[test] fn enum_record_fields_out_of_order() { check_diagnostics( r#" enum Either { A { foo: bool, bar: () }, B, } fn main() { let a = Either::A { foo: true, bar: () }; match a { //^ Missing match arm Either::A { bar: (), foo: false } => (), Either::A { foo: true, bar: () } => (), } match a { Either::A { bar: (), foo: false } => (), Either::A { foo: true, bar: () } => (), Either::B => (), } } "#, ); } #[test] fn enum_record_ellipsis() { check_diagnostics( r#" enum Either { A { foo: bool, bar: bool }, B, } fn main() { let a = Either::B; match a { //^ Missing match arm Either::A { foo: true, .. } => (), Either::B => (), } match a { //^ Missing match arm Either::A { .. } => (), } match a { Either::A { foo: true, .. } => (), Either::A { foo: false, .. } => (), Either::B => (), } match a { Either::A { .. } => (), Either::B => (), } } "#, ); } #[test] fn enum_tuple_partial_ellipsis() { check_diagnostics( r#" enum Either { A(bool, bool, bool, bool), B, } fn main() { match Either::B { //^^^^^^^^^ Missing match arm Either::A(true, .., true) => (), Either::A(true, .., false) => (), Either::A(false, .., false) => (), Either::B => (), } match Either::B { //^^^^^^^^^ Missing match arm Either::A(true, .., true) => (), Either::A(true, .., false) => (), Either::A(.., true) => (), Either::B => (), } match Either::B { Either::A(true, .., true) => (), Either::A(true, .., false) => (), Either::A(false, .., true) => (), Either::A(false, .., false) => (), Either::B => (), } match Either::B { Either::A(true, .., true) => (), Either::A(true, .., false) => (), Either::A(.., true) => (), Either::A(.., false) => (), Either::B => (), } } "#, ); } #[test] fn never() { check_diagnostics( r#" enum Never {} fn enum_(never: Never) { match never {} } fn enum_ref(never: &Never) { match never {} } fn bang(never: !) { match never {} } "#, ); } #[test] fn or_pattern_panic() { check_diagnostics( r#" pub enum Category { Infinity, Zero } fn panic(a: Category, b: Category) { match (a, b) { (Category::Zero | Category::Infinity, _) => (), (_, Category::Zero | Category::Infinity) => (), } // FIXME: This is a false positive, but the code used to cause a panic in the match checker, // so this acts as a regression test for that. match (a, b) { //^^^^^^ Missing match arm (Category::Infinity, Category::Infinity) | (Category::Zero, Category::Zero) => (), (Category::Infinity | Category::Zero, _) => (), } } "#, ); } #[test] fn unknown_type() { check_diagnostics( r#" enum Option<T> { Some(T), None } fn main() { // `Never` is deliberately not defined so that it's an uninferred type. match Option::<Never>::None { None => (), Some(never) => match never {}, } } "#, ); } #[test] fn tuple_of_bools_with_ellipsis_at_end_missing_arm() { check_diagnostics( r#" fn main() { match (false, true, false) { //^^^^^^^^^^^^^^^^^^^^ Missing match arm (false, ..) => (), } }"#, ); } #[test] fn tuple_of_bools_with_ellipsis_at_beginning_missing_arm() { check_diagnostics( r#" fn main() { match (false, true, false) { //^^^^^^^^^^^^^^^^^^^^ Missing match arm (.., false) => (), } }"#, ); } #[test] fn tuple_of_bools_with_ellipsis_in_middle_missing_arm() { check_diagnostics( r#" fn main() { match (false, true, false) { //^^^^^^^^^^^^^^^^^^^^ Missing match arm (true, .., false) => (), } }"#, ); } #[test] fn record_struct() { check_diagnostics( r#"struct Foo { a: bool } fn main(f: Foo) { match f {} //^ Missing match arm match f { Foo { a: true } => () } //^ Missing match arm match &f { Foo { a: true } => () } //^^ Missing match arm match f { Foo { a: _ } => () } match f { Foo { a: true } => (), Foo { a: false } => (), } match &f { Foo { a: true } => (), Foo { a: false } => (), } } "#, ); } #[test] fn tuple_struct() { check_diagnostics( r#"struct Foo(bool); fn main(f: Foo) { match f {} //^ Missing match arm match f { Foo(true) => () } //^ Missing match arm match f { Foo(true) => (), Foo(false) => (), } } "#, ); } #[test] fn unit_struct() { check_diagnostics( r#"struct Foo; fn main(f: Foo) { match f {} //^ Missing match arm match f { Foo => () } } "#, ); } #[test] fn record_struct_ellipsis() { check_diagnostics( r#"struct Foo { foo: bool, bar: bool } fn main(f: Foo) { match f { Foo { foo: true, .. } => () } //^ Missing match arm match f { //^ Missing match arm Foo { foo: true, .. } => (), Foo { bar: false, .. } => () } match f { Foo { .. } => () } match f { Foo { foo: true, .. } => (), Foo { foo: false, .. } => () } } "#, ); } mod false_negatives { //! The implementation of match checking here is a work in progress. As we roll this out, we //! prefer false negatives to false positives (ideally there would be no false positives). This //! test module should document known false negatives. Eventually we will have a complete //! implementation of match checking and this module will be empty. //! //! The reasons for documenting known false negatives: //! //! 1. It acts as a backlog of work that can be done to improve the behavior of the system. //! 2. It ensures the code doesn't panic when handling these cases. use super::*; #[test] fn integers() { // We don't currently check integer exhaustiveness. check_diagnostics( r#" fn main() { match 5 { 10 => (), 11..20 => (), } } "#, ); } #[test] fn internal_or() { // We do not currently handle patterns with internal `or`s. check_diagnostics( r#" fn main() { enum Either { A(bool), B } match Either::B { Either::A(true | false) => (), } } "#, ); } } }