Implement destructuring assignment for structs and slices

Co-authored-by: varkor <github@varkor.com>

1 files changed, 119 insertions, 7 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 1f2aba2b27e..330776fc8c5 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -187,8 +187,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(e.span, asm),
                 ExprKind::LlvmInlineAsm(ref asm) => self.lower_expr_llvm_asm(asm),
-                ExprKind::Struct(ref path, ref fields, ref maybe_expr) => {
-                    let maybe_expr = maybe_expr.as_ref().map(|x| self.lower_expr(x));
+                ExprKind::Struct(ref path, ref fields, ref rest) => {
+                    let rest = match rest {
+                        StructRest::Base(e) => Some(self.lower_expr(e)),
+                        StructRest::Rest(sp) => {
+                            self.sess
+                                .struct_span_err(*sp, "base expression required after `..`")
+                                .span_label(*sp, "add a base expression here")
+                                .emit();
+                            Some(&*self.arena.alloc(self.expr_err(*sp)))
+                        }
+                        StructRest::None => None,
+                    };
                     hir::ExprKind::Struct(
                         self.arena.alloc(self.lower_qpath(
                             e.id,
@@ -198,7 +208,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             ImplTraitContext::disallowed(),
                         )),
                         self.arena.alloc_from_iter(fields.iter().map(|x| self.lower_field(x))),
-                        maybe_expr,
+                        rest,
                     )
                 }
                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
@@ -851,20 +861,22 @@ impl<'hir> LoweringContext<'_, 'hir> {
         whole_span: Span,
     ) -> hir::ExprKind<'hir> {
         // Return early in case of an ordinary assignment.
-        fn is_ordinary(lhs: &Expr) -> bool {
+        fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
             match &lhs.kind {
-                ExprKind::Tup(..) => false,
+                ExprKind::Array(..) | ExprKind::Struct(..) | ExprKind::Tup(..) => false,
+                // Check for tuple struct constructor.
+                ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
                 ExprKind::Paren(e) => {
                     match e.kind {
                         // We special-case `(..)` for consistency with patterns.
                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
-                        _ => is_ordinary(e),
+                        _ => is_ordinary(lower_ctx, e),
                     }
                 }
                 _ => true,
             }
         }
-        if is_ordinary(lhs) {
+        if is_ordinary(self, lhs) {
             return hir::ExprKind::Assign(self.lower_expr(lhs), self.lower_expr(rhs), eq_sign_span);
         }
         if !self.sess.features_untracked().destructuring_assignment {
@@ -902,6 +914,26 @@ impl<'hir> LoweringContext<'_, 'hir> {
         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
     }
 
+    /// If the given expression is a path to a tuple struct, returns that path.
+    /// It is not a complete check, but just tries to reject most paths early
+    /// if they are not tuple structs.
+    /// Type checking will take care of the full validation later.
+    fn extract_tuple_struct_path<'a>(&mut self, expr: &'a Expr) -> Option<&'a Path> {
+        // For tuple struct destructuring, it must be a non-qualified path (like in patterns).
+        if let ExprKind::Path(None, path) = &expr.kind {
+            // Does the path resolves to something disallowed in a tuple struct/variant pattern?
+            if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
+                if partial_res.unresolved_segments() == 0
+                    && !partial_res.base_res().expected_in_tuple_struct_pat()
+                {
+                    return None;
+                }
+            }
+            return Some(path);
+        }
+        None
+    }
+
     /// Convert the LHS of a destructuring assignment to a pattern.
     /// Each sub-assignment is recorded in `assignments`.
     fn destructure_assign(
@@ -911,6 +943,86 @@ impl<'hir> LoweringContext<'_, 'hir> {
         assignments: &mut Vec<hir::Stmt<'hir>>,
     ) -> &'hir hir::Pat<'hir> {
         match &lhs.kind {
+            // Slice patterns.
+            ExprKind::Array(elements) => {
+                let (pats, rest) =
+                    self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
+                let slice_pat = if let Some((i, span)) = rest {
+                    let (before, after) = pats.split_at(i);
+                    hir::PatKind::Slice(
+                        before,
+                        Some(self.pat_without_dbm(span, hir::PatKind::Wild)),
+                        after,
+                    )
+                } else {
+                    hir::PatKind::Slice(pats, None, &[])
+                };
+                return self.pat_without_dbm(lhs.span, slice_pat);
+            }
+            // Tuple structs.
+            ExprKind::Call(callee, args) => {
+                if let Some(path) = self.extract_tuple_struct_path(callee) {
+                    let (pats, rest) = self.destructure_sequence(
+                        args,
+                        "tuple struct or variant",
+                        eq_sign_span,
+                        assignments,
+                    );
+                    let qpath = self.lower_qpath(
+                        callee.id,
+                        &None,
+                        path,
+                        ParamMode::Optional,
+                        ImplTraitContext::disallowed(),
+                    );
+                    // Destructure like a tuple struct.
+                    let tuple_struct_pat =
+                        hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
+                    return self.pat_without_dbm(lhs.span, tuple_struct_pat);
+                }
+            }
+            // Structs.
+            ExprKind::Struct(path, fields, rest) => {
+                let field_pats = self.arena.alloc_from_iter(fields.iter().map(|f| {
+                    let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
+                    hir::FieldPat {
+                        hir_id: self.next_id(),
+                        ident: f.ident,
+                        pat,
+                        is_shorthand: f.is_shorthand,
+                        span: f.span,
+                    }
+                }));
+                let qpath = self.lower_qpath(
+                    lhs.id,
+                    &None,
+                    path,
+                    ParamMode::Optional,
+                    ImplTraitContext::disallowed(),
+                );
+                let fields_omitted = match rest {
+                    StructRest::Base(e) => {
+                        self.sess
+                            .struct_span_err(
+                                e.span,
+                                "functional record updates are not allowed in destructuring \
+                                    assignments",
+                            )
+                            .span_suggestion(
+                                e.span,
+                                "consider removing the trailing pattern",
+                                String::new(),
+                                rustc_errors::Applicability::MachineApplicable,
+                            )
+                            .emit();
+                        true
+                    }
+                    StructRest::Rest(_) => true,
+                    StructRest::None => false,
+                };
+                let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
+                return self.pat_without_dbm(lhs.span, struct_pat);
+            }
             // Tuples.
             ExprKind::Tup(elements) => {
                 let (pats, rest) =