Auto merge of rust-lang#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 rust-lang#11532
Closes rust-lang#11839
Closes rust-lang#12322

Author: bors

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,