Auto merge of #12428 - lowr:experimental/destructuring-assignment, r=flodiebold

feat: implement destructuring assignment

This is an attempt to implement destructuring assignments, or more specifically, type inference for [assignee expressions](https://doc.rust-lang.org/reference/expressions.html#place-expressions-and-value-expressions).

I'm not sure if this is the right approach, so I don't even expect this to be merged (hence the branch name 😉) but rather want to propose one direction we could choose. I don't mind getting merged if this is good enough though!

Some notes on the implementation choices:

- Assignee expressions are **not** desugared on HIR level unlike rustc, but are inferred directly along with other expressions. This matches the processing of other syntaxes that are desugared in rustc but not in r-a. I find this reasonable because r-a only needs to infer types and it's easier to relate AST nodes and HIR nodes, so I followed it.
- Assignee expressions obviously resemble patterns, so type inference for each kind of pattern and its corresponding assignee expressions share a significant amount of logic. I tried to reuse the type inference functions for patterns by introducing `PatLike` trait which generalizes assignee expressions and patterns.
  - This is not the most elegant solution I suspect (and I really don't like the name of the trait!), but it's cleaner and the change is smaller than other ways I experimented, like making the functions generic without such trait, or making them take `Either<ExprId, PatId>` in place of `PatId`.

in case this is merged:
Closes #11532
Closes #11839
Closes #12322

crates/hir-def/src/body/lower.rs

@@ -536,7 +536,7 @@ impl ExprCollector<'_> {
 
                 self.alloc_expr(Expr::MacroStmts { tail }, syntax_ptr)
             }
-            ast::Expr::UnderscoreExpr(_) => return None,
+            ast::Expr::UnderscoreExpr(_) => self.alloc_expr(Expr::Underscore, syntax_ptr),
         })
     }
 

crates/hir-def/src/expr.rs

@@ -203,6 +203,7 @@ pub enum Expr {
     },
     Array(Array),
     Literal(Literal),
+    Underscore,
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
@@ -345,6 +346,7 @@ impl Expr {
             },
             Expr::MacroStmts { tail } => f(*tail),
             Expr::Literal(_) => {}
+            Expr::Underscore => {}
         }
     }
 }

crates/hir-ty/src/infer.rs

@@ -125,6 +125,39 @@ impl Default for BindingMode {
     }
 }
 
+/// Used to generalize patterns and assignee expressions.
+trait PatLike: Into<ExprOrPatId> + Copy {
+    type BindingMode: Copy;
+
+    fn infer(
+        this: &mut InferenceContext,
+        id: Self,
+        expected_ty: &Ty,
+        default_bm: Self::BindingMode,
+    ) -> Ty;
+}
+
+impl PatLike for ExprId {
+    type BindingMode = ();
+
+    fn infer(this: &mut InferenceContext, id: Self, expected_ty: &Ty, _: Self::BindingMode) -> Ty {
+        this.infer_assignee_expr(id, expected_ty)
+    }
+}
+
+impl PatLike for PatId {
+    type BindingMode = BindingMode;
+
+    fn infer(
+        this: &mut InferenceContext,
+        id: Self,
+        expected_ty: &Ty,
+        default_bm: Self::BindingMode,
+    ) -> Ty {
+        this.infer_pat(id, expected_ty, default_bm)
+    }
+}
+
 #[derive(Debug)]
 pub(crate) struct InferOk<T> {
     value: T,

crates/hir-ty/src/infer/expr.rs

@@ -593,8 +593,8 @@ impl<'a> InferenceContext<'a> {
             }
             Expr::BinaryOp { lhs, rhs, op } => match op {
                 Some(BinaryOp::Assignment { op: None }) => {
-                    let lhs_ty = self.infer_expr(*lhs, &Expectation::none());
-                    self.infer_expr_coerce(*rhs, &Expectation::has_type(lhs_ty));
+                    let rhs_ty = self.infer_expr(*rhs, &Expectation::none());
+                    self.infer_assignee_expr(*lhs, &rhs_ty);
                     self.result.standard_types.unit.clone()
                 }
                 Some(BinaryOp::LogicOp(_)) => {
@@ -775,6 +775,12 @@ impl<'a> InferenceContext<'a> {
                 },
             },
             Expr::MacroStmts { tail } => self.infer_expr_inner(*tail, expected),
+            Expr::Underscore => {
+                // Underscore expressions may only appear in assignee expressions,
+                // which are handled by `infer_assignee_expr()`, so any underscore
+                // expression reaching this branch is an error.
+                self.err_ty()
+            }
         };
         // use a new type variable if we got unknown here
         let ty = self.insert_type_vars_shallow(ty);
@@ -811,6 +817,95 @@ impl<'a> InferenceContext<'a> {
         }
     }
 
+    pub(super) fn infer_assignee_expr(&mut self, lhs: ExprId, rhs_ty: &Ty) -> Ty {
+        let is_rest_expr = |expr| {
+            matches!(
+                &self.body[expr],
+                Expr::Range { lhs: None, rhs: None, range_type: RangeOp::Exclusive },
+            )
+        };
+
+        let rhs_ty = self.resolve_ty_shallow(rhs_ty);
+
+        let ty = match &self.body[lhs] {
+            Expr::Tuple { exprs } => {
+                // We don't consider multiple ellipses. This is analogous to
+                // `hir_def::body::lower::ExprCollector::collect_tuple_pat()`.
+                let ellipsis = exprs.iter().position(|e| is_rest_expr(*e));
+                let exprs: Vec<_> = exprs.iter().filter(|e| !is_rest_expr(**e)).copied().collect();
+
+                self.infer_tuple_pat_like(&rhs_ty, (), ellipsis, &exprs)
+            }
+            Expr::Call { callee, args } => {
+                // Tuple structs
+                let path = match &self.body[*callee] {
+                    Expr::Path(path) => Some(path),
+                    _ => None,
+                };
+
+                // We don't consider multiple ellipses. This is analogous to
+                // `hir_def::body::lower::ExprCollector::collect_tuple_pat()`.
+                let ellipsis = args.iter().position(|e| is_rest_expr(*e));
+                let args: Vec<_> = args.iter().filter(|e| !is_rest_expr(**e)).copied().collect();
+
+                self.infer_tuple_struct_pat_like(path, &rhs_ty, (), lhs, ellipsis, &args)
+            }
+            Expr::Array(Array::ElementList(elements)) => {
+                let elem_ty = match rhs_ty.kind(Interner) {
+                    TyKind::Array(st, _) => st.clone(),
+                    _ => self.err_ty(),
+                };
+
+                // There's no need to handle `..` as it cannot be bound.
+                let sub_exprs = elements.iter().filter(|e| !is_rest_expr(**e));
+
+                for e in sub_exprs {
+                    self.infer_assignee_expr(*e, &elem_ty);
+                }
+
+                match rhs_ty.kind(Interner) {
+                    TyKind::Array(_, _) => rhs_ty.clone(),
+                    // Even when `rhs_ty` is not an array type, this assignee
+                    // expression is infered to be an array (of unknown element
+                    // type and length). This should not be just an error type,
+                    // because we are to compute the unifiability of this type and
+                    // `rhs_ty` in the end of this function to issue type mismatches.
+                    _ => TyKind::Array(self.err_ty(), crate::consteval::usize_const(None))
+                        .intern(Interner),
+                }
+            }
+            Expr::RecordLit { path, fields, .. } => {
+                let subs = fields.iter().map(|f| (f.name.clone(), f.expr));
+
+                self.infer_record_pat_like(path.as_deref(), &rhs_ty, (), lhs.into(), subs)
+            }
+            Expr::Underscore => rhs_ty.clone(),
+            _ => {
+                // `lhs` is a place expression, a unit struct, or an enum variant.
+                let lhs_ty = self.infer_expr(lhs, &Expectation::none());
+
+                // This is the only branch where this function may coerce any type.
+                // We are returning early to avoid the unifiability check below.
+                let lhs_ty = self.insert_type_vars_shallow(lhs_ty);
+                let ty = match self.coerce(None, &rhs_ty, &lhs_ty) {
+                    Ok(ty) => ty,
+                    Err(_) => self.err_ty(),
+                };
+                self.write_expr_ty(lhs, ty.clone());
+                return ty;
+            }
+        };
+
+        let ty = self.insert_type_vars_shallow(ty);
+        if !self.unify(&ty, &rhs_ty) {
+            self.result
+                .type_mismatches
+                .insert(lhs.into(), TypeMismatch { expected: rhs_ty.clone(), actual: ty.clone() });
+        }
+        self.write_expr_ty(lhs, ty.clone());
+        ty
+    }
+
     fn infer_overloadable_binop(
         &mut self,
         lhs: ExprId,

crates/hir-ty/src/infer/pat.rs

@@ -4,7 +4,7 @@ use std::iter::repeat_with;
 
 use chalk_ir::Mutability;
 use hir_def::{
-    expr::{BindingAnnotation, Expr, Literal, Pat, PatId, RecordFieldPat},
+    expr::{BindingAnnotation, Expr, Literal, Pat, PatId},
     path::Path,
     type_ref::ConstScalar,
 };
@@ -17,15 +17,20 @@ use crate::{
     TyKind,
 };
 
+use super::PatLike;
+
 impl<'a> InferenceContext<'a> {
-    fn infer_tuple_struct_pat(
+    /// Infers type for tuple struct pattern or its corresponding assignee expression.
+    ///
+    /// Ellipses found in the original pattern or expression must be filtered out.
+    pub(super) fn infer_tuple_struct_pat_like<T: PatLike>(
         &mut self,
         path: Option<&Path>,
-        subpats: &[PatId],
         expected: &Ty,
-        default_bm: BindingMode,
-        id: PatId,
+        default_bm: T::BindingMode,
+        id: T,
         ellipsis: Option<usize>,
+        subs: &[T],
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path, true);
         let var_data = def.map(|it| it.variant_data(self.db.upcast()));
@@ -39,8 +44,8 @@ impl<'a> InferenceContext<'a> {
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
         let (pre, post) = match ellipsis {
-            Some(idx) => subpats.split_at(idx),
-            None => (subpats, &[][..]),
+            Some(idx) => subs.split_at(idx),
+            None => (subs, &[][..]),
         };
         let post_idx_offset = field_tys.iter().count().saturating_sub(post.len());
 
@@ -54,22 +59,22 @@ impl<'a> InferenceContext<'a> {
                     field_tys[field].clone().substitute(Interner, &substs)
                 });
             let expected_ty = self.normalize_associated_types_in(expected_ty);
-            self.infer_pat(subpat, &expected_ty, default_bm);
+            T::infer(self, subpat, &expected_ty, default_bm);
         }
 
         ty
     }
 
-    fn infer_record_pat(
+    /// Infers type for record pattern or its corresponding assignee expression.
+    pub(super) fn infer_record_pat_like<T: PatLike>(
         &mut self,
         path: Option<&Path>,
-        subpats: &[RecordFieldPat],
         expected: &Ty,
-        default_bm: BindingMode,
-        id: PatId,
+        default_bm: T::BindingMode,
+        id: T,
+        subs: impl Iterator<Item = (Name, T)>,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path, false);
-        let var_data = def.map(|it| it.variant_data(self.db.upcast()));
         if let Some(variant) = def {
             self.write_variant_resolution(id.into(), variant);
         }
@@ -80,18 +85,64 @@ impl<'a> InferenceContext<'a> {
             ty.as_adt().map(|(_, s)| s.clone()).unwrap_or_else(|| Substitution::empty(Interner));
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
-        for subpat in subpats {
-            let matching_field = var_data.as_ref().and_then(|it| it.field(&subpat.name));
-            let expected_ty = matching_field.map_or(self.err_ty(), |field| {
-                field_tys[field].clone().substitute(Interner, &substs)
-            });
+        let var_data = def.map(|it| it.variant_data(self.db.upcast()));
+
+        for (name, inner) in subs {
+            let expected_ty = var_data
+                .as_ref()
+                .and_then(|it| it.field(&name))
+                .map_or(self.err_ty(), |f| field_tys[f].clone().substitute(Interner, &substs));
             let expected_ty = self.normalize_associated_types_in(expected_ty);
-            self.infer_pat(subpat.pat, &expected_ty, default_bm);
+
+            T::infer(self, inner, &expected_ty, default_bm);
         }
 
         ty
     }
 
+    /// Infers type for tuple pattern or its corresponding assignee expression.
+    ///
+    /// Ellipses found in the original pattern or expression must be filtered out.
+    pub(super) fn infer_tuple_pat_like<T: PatLike>(
+        &mut self,
+        expected: &Ty,
+        default_bm: T::BindingMode,
+        ellipsis: Option<usize>,
+        subs: &[T],
+    ) -> Ty {
+        let expectations = match expected.as_tuple() {
+            Some(parameters) => &*parameters.as_slice(Interner),
+            _ => &[],
+        };
+
+        let ((pre, post), n_uncovered_patterns) = match ellipsis {
+            Some(idx) => (subs.split_at(idx), expectations.len().saturating_sub(subs.len())),
+            None => ((&subs[..], &[][..]), 0),
+        };
+        let mut expectations_iter = expectations
+            .iter()
+            .cloned()
+            .map(|a| a.assert_ty_ref(Interner).clone())
+            .chain(repeat_with(|| self.table.new_type_var()));
+
+        let mut inner_tys = Vec::with_capacity(n_uncovered_patterns + subs.len());
+
+        inner_tys.extend(expectations_iter.by_ref().take(n_uncovered_patterns + subs.len()));
+
+        // Process pre
+        for (ty, pat) in inner_tys.iter_mut().zip(pre) {
+            *ty = T::infer(self, *pat, ty, default_bm);
+        }
+
+        // Process post
+        for (ty, pat) in inner_tys.iter_mut().skip(pre.len() + n_uncovered_patterns).zip(post) {
+            *ty = T::infer(self, *pat, ty, default_bm);
+        }
+
+        TyKind::Tuple(inner_tys.len(), Substitution::from_iter(Interner, inner_tys))
+            .intern(Interner)
+    }
+
     pub(super) fn infer_pat(
         &mut self,
         pat: PatId,
@@ -129,42 +180,7 @@ impl<'a> InferenceContext<'a> {
 
         let ty = match &self.body[pat] {
             Pat::Tuple { args, ellipsis } => {
-                let expectations = match expected.as_tuple() {
-                    Some(parameters) => &*parameters.as_slice(Interner),
-                    _ => &[],
-                };
-
-                let ((pre, post), n_uncovered_patterns) = match ellipsis {
-                    Some(idx) => {
-                        (args.split_at(*idx), expectations.len().saturating_sub(args.len()))
-                    }
-                    None => ((&args[..], &[][..]), 0),
-                };
-                let mut expectations_iter = expectations
-                    .iter()
-                    .cloned()
-                    .map(|a| a.assert_ty_ref(Interner).clone())
-                    .chain(repeat_with(|| self.table.new_type_var()));
-
-                let mut inner_tys = Vec::with_capacity(n_uncovered_patterns + args.len());
-
-                inner_tys
-                    .extend(expectations_iter.by_ref().take(n_uncovered_patterns + args.len()));
-
-                // Process pre
-                for (ty, pat) in inner_tys.iter_mut().zip(pre) {
-                    *ty = self.infer_pat(*pat, ty, default_bm);
-                }
-
-                // Process post
-                for (ty, pat) in
-                    inner_tys.iter_mut().skip(pre.len() + n_uncovered_patterns).zip(post)
-                {
-                    *ty = self.infer_pat(*pat, ty, default_bm);
-                }
-
-                TyKind::Tuple(inner_tys.len(), Substitution::from_iter(Interner, inner_tys))
-                    .intern(Interner)
+                self.infer_tuple_pat_like(&expected, default_bm, *ellipsis, args)
             }
             Pat::Or(pats) => {
                 if let Some((first_pat, rest)) = pats.split_first() {
@@ -191,16 +207,18 @@ impl<'a> InferenceContext<'a> {
                 let subty = self.infer_pat(*pat, &expectation, default_bm);
                 TyKind::Ref(mutability, static_lifetime(), subty).intern(Interner)
             }
-            Pat::TupleStruct { path: p, args: subpats, ellipsis } => self.infer_tuple_struct_pat(
-                p.as_deref(),
-                subpats,
-                &expected,
-                default_bm,
-                pat,
-                *ellipsis,
-            ),
+            Pat::TupleStruct { path: p, args: subpats, ellipsis } => self
+                .infer_tuple_struct_pat_like(
+                    p.as_deref(),
+                    &expected,
+                    default_bm,
+                    pat,
+                    *ellipsis,
+                    subpats,
+                ),
             Pat::Record { path: p, args: fields, ellipsis: _ } => {
-                self.infer_record_pat(p.as_deref(), fields, &expected, default_bm, pat)
+                let subs = fields.iter().map(|f| (f.name.clone(), f.pat));
+                self.infer_record_pat_like(p.as_deref(), &expected, default_bm, pat.into(), subs)
             }
             Pat::Path(path) => {
                 // FIXME use correct resolver for the surrounding expression

crates/hir-ty/src/tests/coercion.rs

@@ -312,6 +312,24 @@ fn f(text: &str) {
     );
 }
 
+#[test]
+fn destructuring_assign_coerce() {
+    check_no_mismatches(
+        r"
+//- minicore: deref
+struct String;
+impl core::ops::Deref for String { type Target = str; }
+fn g(_text: &str) {}
+fn f(text: &str) {
+    let mut text = text;
+    let tmp = String;
+    [text, _] = [&tmp, &tmp];
+    g(text);
+}
+",
+    );
+}
+
 #[test]
 fn coerce_fn_item_to_fn_ptr() {
     check_no_mismatches(

crates/hir-ty/src/tests/simple.rs

@@ -1,6 +1,6 @@
 use expect_test::expect;
 
-use super::{check_infer, check_no_mismatches, check_types};
+use super::{check, check_infer, check_no_mismatches, check_types};
 
 #[test]
 fn infer_box() {
@@ -2745,3 +2745,301 @@ fn f() {
     "#,
     );
 }
+
+#[test]
+fn destructuring_assignment_slice() {
+    check_types(
+        r#"
+fn main() {
+    let a;
+      //^usize
+    [a,] = [0usize];
+
+    let a;
+      //^usize
+    [a, ..] = [0usize; 5];
+
+    let a;
+      //^usize
+    [.., a] = [0usize; 5];
+
+    let a;
+      //^usize
+    [.., a, _] = [0usize; 5];
+
+    let a;
+      //^usize
+    [_, a, ..] = [0usize; 5];
+
+    let a: &mut i64 = &mut 0;
+    [*a, ..] = [1, 2, 3];
+
+    let a: usize;
+    let b;
+      //^usize
+    [a, _, b] = [3, 4, 5];
+      //^usize
+
+    let a;
+      //^i64
+    let b;
+      //^i64
+    [[a, ..], .., [.., b]] = [[1, 2], [3i64, 4], [5, 6], [7, 8]];
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_tuple() {
+    check_types(
+        r#"
+fn main() {
+    let a;
+      //^char
+    let b;
+      //^i64
+    (a, b) = ('c', 0i64);
+
+    let a;
+      //^char
+    (a, ..) = ('c', 0i64);
+
+    let a;
+      //^i64
+    (.., a) = ('c', 0i64);
+
+    let a;
+      //^char
+    let b;
+      //^i64
+    (a, .., b) = ('c', 0i64);
+
+    let a;
+      //^char
+    let b;
+      //^bool
+    (a, .., b) = ('c', 0i64, true);
+
+    let a;
+      //^i64
+    let b;
+      //^bool
+    (_, a, .., b) = ('c', 0i64, true);
+
+    let a;
+      //^i64
+    let b;
+      //^usize
+    (_, a, .., b) = ('c', 0i64, true, 0usize);
+
+    let mut a = 1;
+      //^^^^^i64
+    let mut b: i64 = 0;
+    (a, b) = (b, a);
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_tuple_struct() {
+    check_types(
+        r#"
+struct S2(char, i64);
+struct S3(char, i64, bool);
+struct S4(char, i64, bool usize);
+fn main() {
+    let a;
+      //^char
+    let b;
+      //^i64
+    S2(a, b) = S2('c', 0i64);
+
+    let a;
+      //^char
+    let b;
+      //^i64
+    S2(a, .., b) = S2('c', 0i64);
+
+    let a;
+      //^char
+    let b;
+      //^bool
+    S3(a, .., b) = S3('c', 0i64, true);
+
+    let a;
+      //^i64
+    let b;
+      //^bool
+    S3(_, a, .., b) = S3('c', 0i64, true);
+
+    let a;
+      //^i64
+    let b;
+      //^usize
+    S4(_, a, .., b) = S4('c', 0i64, true, 0usize);
+
+    struct Swap(i64, i64);
+
+    let mut a = 1;
+      //^^^^^i64
+    let mut b = 0;
+      //^^^^^i64
+    Swap(a, b) = Swap(b, a);
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_struct() {
+    check_types(
+        r#"
+struct S {
+    a: usize,
+    b: char,
+}
+struct T {
+    s: S,
+    t: i64,
+}
+
+fn main() {
+    let a;
+      //^usize
+    let c;
+      //^char
+    S { a, b: c } = S { a: 3, b: 'b' };
+
+    let a;
+      //^char
+    S { b: a, .. } = S { a: 3, b: 'b' };
+
+    let a;
+      //^char
+    S { b: a, _ } = S { a: 3, b: 'b' };
+
+    let a;
+      //^usize
+    let c;
+      //^char
+    let t;
+      //^i64
+    T { s: S { a, b: c }, t } = T { s: S { a: 3, b: 'b' }, t: 0 };
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_nested() {
+    check_types(
+        r#"
+struct S {
+    a: TS,
+    b: [char; 3],
+}
+struct TS(usize, i64);
+
+fn main() {
+    let a;
+      //^i32
+    let b;
+      //^bool
+    ([.., a], .., b, _) = ([0, 1, 2], true, 'c');
+
+    let a;
+      //^i32
+    let b;
+      //^i32
+    [(.., a, _), .., (b, ..)] = [(1, 2); 5];
+
+    let a;
+      //^usize
+    let b;
+      //^char
+    S { a: TS(a, ..), b: [_, b, ..] } = S { a: TS(0, 0), b: ['a'; 3] };
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_unit_struct() {
+    // taken from rustc; see https://github.com/rust-lang/rust/pull/95380
+    check_no_mismatches(
+        r#"
+struct S;
+enum E { V, }
+type A = E;
+
+fn main() {
+    let mut a;
+
+    (S, a) = (S, ());
+
+    (E::V, a) = (E::V, ());
+
+    (<E>::V, a) = (E::V, ());
+    (A::V, a) = (E::V, ());
+}
+
+impl S {
+    fn check() {
+        let a;
+        (Self, a) = (S, ());
+    }
+}
+
+impl E {
+    fn check() {
+        let a;
+        (Self::V, a) = (E::V, ());
+    }
+}
+        "#,
+    );
+}
+
+#[test]
+fn destructuring_assignment_no_default_binding_mode() {
+    check(
+        r#"
+struct S { a: usize }
+struct TS(usize);
+fn main() {
+    let x;
+    [x,] = &[1,];
+  //^^^^expected &[i32; 1], got [{unknown}; _]
+
+    // FIXME we only want the outermost error, but this matches the current
+    // behavior of slice patterns
+    let x;
+    [(x,),] = &[(1,),];
+  // ^^^^expected {unknown}, got ({unknown},)
+  //^^^^^^^expected &[(i32,); 1], got [{unknown}; _]
+
+    let x;
+    ((x,),) = &((1,),);
+  //^^^^^^^expected &((i32,),), got (({unknown},),)
+
+    let x;
+    (x,) = &(1,);
+  //^^^^expected &(i32,), got ({unknown},)
+
+    let x;
+    (S { a: x },) = &(S { a: 42 },);
+  //^^^^^^^^^^^^^expected &(S,), got (S,)
+
+    let x;
+    S { a: x } = &S { a: 42 };
+  //^^^^^^^^^^expected &S, got S
+
+    let x;
+    TS(x) = &TS(42);
+  //^^^^^expected &TS, got TS
+}
+        "#,
+    );
+}