Handle destructuring assignments uniformly

Instead of lowering them to `<expr> = <expr>`, then hacking on-demand to resolve them, we lower them to `<pat> = <expr>`, and use the pattern infrastructure to handle them. It turns out, destructuring assignments are surprisingly similar to pattern bindings, and so only minor modifications are needed.

This fixes few bugs that arose because of the non-uniform handling (for example, MIR lowering not handling slice and record patterns, and closure capture calculation not handling destructuring assignments at all), and furthermore, guarantees we won't have such bugs in the future, since the programmer will always have to explicitly handle `Expr::Assignment`.

Tests don't pass yet; that's because the generated patterns do not exist in the source map. The next commit will fix that.

src/tools/rust-analyzer/crates/hir-def/src/body.rs

@@ -22,7 +22,8 @@ use crate::{
     db::DefDatabase,
     expander::Expander,
     hir::{
-        dummy_expr_id, Binding, BindingId, Expr, ExprId, Label, LabelId, Pat, PatId, RecordFieldPat,
+        dummy_expr_id, Array, AsmOperand, Binding, BindingId, Expr, ExprId, Label, LabelId, Pat,
+        PatId, RecordFieldPat, Statement,
     },
     item_tree::AttrOwner,
     nameres::DefMap,
@@ -286,7 +287,8 @@ impl Body {
             | Pat::Path(..)
             | Pat::ConstBlock(..)
             | Pat::Wild
-            | Pat::Missing => {}
+            | Pat::Missing
+            | Pat::Expr(_) => {}
             &Pat::Bind { subpat, .. } => {
                 if let Some(subpat) = subpat {
                     f(subpat);
@@ -322,6 +324,143 @@ impl Body {
             None => true,
         }
     }
+
+    pub fn walk_child_exprs(&self, expr_id: ExprId, mut f: impl FnMut(ExprId)) {
+        let expr = &self[expr_id];
+        match expr {
+            Expr::Continue { .. }
+            | Expr::Const(_)
+            | Expr::Missing
+            | Expr::Path(_)
+            | Expr::OffsetOf(_)
+            | Expr::Literal(_)
+            | Expr::Underscore => {}
+            Expr::InlineAsm(it) => it.operands.iter().for_each(|(_, op)| match op {
+                AsmOperand::In { expr, .. }
+                | AsmOperand::Out { expr: Some(expr), .. }
+                | AsmOperand::InOut { expr, .. } => f(*expr),
+                AsmOperand::SplitInOut { in_expr, out_expr, .. } => {
+                    f(*in_expr);
+                    if let Some(out_expr) = out_expr {
+                        f(*out_expr);
+                    }
+                }
+                AsmOperand::Out { expr: None, .. }
+                | AsmOperand::Const(_)
+                | AsmOperand::Label(_)
+                | AsmOperand::Sym(_) => (),
+            }),
+            Expr::If { condition, then_branch, else_branch } => {
+                f(*condition);
+                f(*then_branch);
+                if let &Some(else_branch) = else_branch {
+                    f(else_branch);
+                }
+            }
+            Expr::Let { expr, .. } => {
+                f(*expr);
+            }
+            Expr::Block { statements, tail, .. }
+            | Expr::Unsafe { statements, tail, .. }
+            | Expr::Async { statements, tail, .. } => {
+                for stmt in statements.iter() {
+                    match stmt {
+                        Statement::Let { initializer, else_branch, pat, .. } => {
+                            if let &Some(expr) = initializer {
+                                f(expr);
+                            }
+                            if let &Some(expr) = else_branch {
+                                f(expr);
+                            }
+                            walk_exprs_in_pat(self, *pat, &mut f);
+                        }
+                        Statement::Expr { expr: expression, .. } => f(*expression),
+                        Statement::Item => (),
+                    }
+                }
+                if let &Some(expr) = tail {
+                    f(expr);
+                }
+            }
+            Expr::Loop { body, .. } => f(*body),
+            Expr::Call { callee, args, .. } => {
+                f(*callee);
+                args.iter().copied().for_each(f);
+            }
+            Expr::MethodCall { receiver, args, .. } => {
+                f(*receiver);
+                args.iter().copied().for_each(f);
+            }
+            Expr::Match { expr, arms } => {
+                f(*expr);
+                arms.iter().map(|arm| arm.expr).for_each(f);
+            }
+            Expr::Break { expr, .. }
+            | Expr::Return { expr }
+            | Expr::Yield { expr }
+            | Expr::Yeet { expr } => {
+                if let &Some(expr) = expr {
+                    f(expr);
+                }
+            }
+            Expr::Become { expr } => f(*expr),
+            Expr::RecordLit { fields, spread, .. } => {
+                for field in fields.iter() {
+                    f(field.expr);
+                }
+                if let &Some(expr) = spread {
+                    f(expr);
+                }
+            }
+            Expr::Closure { body, .. } => {
+                f(*body);
+            }
+            Expr::BinaryOp { lhs, rhs, .. } => {
+                f(*lhs);
+                f(*rhs);
+            }
+            Expr::Range { lhs, rhs, .. } => {
+                if let &Some(lhs) = rhs {
+                    f(lhs);
+                }
+                if let &Some(rhs) = lhs {
+                    f(rhs);
+                }
+            }
+            Expr::Index { base, index, .. } => {
+                f(*base);
+                f(*index);
+            }
+            Expr::Field { expr, .. }
+            | Expr::Await { expr }
+            | Expr::Cast { expr, .. }
+            | Expr::Ref { expr, .. }
+            | Expr::UnaryOp { expr, .. }
+            | Expr::Box { expr } => {
+                f(*expr);
+            }
+            Expr::Tuple { exprs, .. } => exprs.iter().copied().for_each(f),
+            Expr::Array(a) => match a {
+                Array::ElementList { elements, .. } => elements.iter().copied().for_each(f),
+                Array::Repeat { initializer, repeat } => {
+                    f(*initializer);
+                    f(*repeat)
+                }
+            },
+            &Expr::Assignment { target, value } => {
+                walk_exprs_in_pat(self, target, &mut f);
+                f(value);
+            }
+        }
+
+        fn walk_exprs_in_pat(this: &Body, pat_id: PatId, f: &mut impl FnMut(ExprId)) {
+            this.walk_pats(pat_id, &mut |pat| {
+                if let Pat::Expr(expr) | Pat::ConstBlock(expr) = this[pat] {
+                    f(expr);
+                }
+            });
+        }
+    }
 }
 
 impl Default for Body {