Rollup merge of rust-lang#78836 - fanzier:struct-and-slice-destructuring, r=petrochenkov

Implement destructuring assignment for structs and slices

This is the second step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: rust-lang#71126). This PR is the second part of rust-lang#71156, which was split up to allow for easier review.

Note that the first PR (rust-lang#78748) is not merged yet, so it is included as the first commit in this one. I thought this would allow the review to start earlier because I have some time this weekend to respond to reviews. If ``@petrochenkov`` prefers to wait until the first PR is merged, I totally understand, of course.

This PR implements destructuring assignment for (tuple) structs and slices. In order to do this, the following *parser change* was necessary: struct expressions are not required to have a base expression, i.e. `Struct { a: 1, .. }` becomes legal (in order to act like a struct pattern).

Unfortunately, this PR slightly regresses the diagnostics implemented in rust-lang#77283. However, it is only a missing help message in `src/test/ui/issues/issue-77218.rs`. Other instances of this diagnostic are not affected. Since I don't exactly understand how this help message works and how to fix it yet, I was hoping it's OK to regress this temporarily and fix it in a follow-up PR.

Thanks to ``@varkor`` who helped with the implementation, particularly around the struct rest changes.

r? ``@petrochenkov``

Author: m-ou-se

compiler/rustc_ast/src/ast.rs

@@ -1061,7 +1061,7 @@ pub struct Expr {
 
 // `Expr` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-rustc_data_structures::static_assert_size!(Expr, 112);
+rustc_data_structures::static_assert_size!(Expr, 120);
 
 impl Expr {
     /// Returns `true` if this expression would be valid somewhere that expects a value;
@@ -1218,6 +1218,16 @@ pub enum RangeLimits {
     Closed,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum StructRest {
+    /// `..x`.
+    Base(P<Expr>),
+    /// `..`.
+    Rest(Span),
+    /// No trailing `..` or expression.
+    None,
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum ExprKind {
     /// A `box x` expression.
@@ -1312,7 +1322,7 @@ pub enum ExprKind {
     Field(P<Expr>, Ident),
     /// An indexing operation (e.g., `foo[2]`).
     Index(P<Expr>, P<Expr>),
-    /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`).
+    /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`; and `..` in destructuring assingment).
     Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
 
     /// Variable reference, possibly containing `::` and/or type
@@ -1340,9 +1350,8 @@ pub enum ExprKind {
 
     /// A struct literal expression.
     ///
-    /// E.g., `Foo {x: 1, y: 2}`, or `Foo {x: 1, .. base}`,
-    /// where `base` is the `Option<Expr>`.
-    Struct(Path, Vec<Field>, Option<P<Expr>>),
+    /// E.g., `Foo {x: 1, y: 2}`, or `Foo {x: 1, .. rest}`.
+    Struct(Path, Vec<Field>, StructRest),
 
     /// An array literal constructed from one repeated element.
     ///

compiler/rustc_ast/src/mut_visit.rs

@@ -1288,7 +1288,11 @@ pub fn noop_visit_expr<T: MutVisitor>(
         ExprKind::Struct(path, fields, expr) => {
             vis.visit_path(path);
             fields.flat_map_in_place(|field| vis.flat_map_field(field));
-            visit_opt(expr, |expr| vis.visit_expr(expr));
+            match expr {
+                StructRest::Base(expr) => vis.visit_expr(expr),
+                StructRest::Rest(_span) => {}
+                StructRest::None => {}
+            }
         }
         ExprKind::Paren(expr) => {
             vis.visit_expr(expr);

compiler/rustc_ast/src/visit.rs

@@ -719,7 +719,11 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Struct(ref path, ref fields, ref optional_base) => {
             visitor.visit_path(path, expression.id);
             walk_list!(visitor, visit_field, fields);
-            walk_list!(visitor, visit_expr, optional_base);
+            match optional_base {
+                StructRest::Base(expr) => visitor.visit_expr(expr),
+                StructRest::Rest(_span) => {}
+                StructRest::None => {}
+            }
         }
         ExprKind::Tup(ref subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);

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) =

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -630,6 +630,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
     gate_all!(inline_const, "inline-const is experimental");
+    gate_all!(destructuring_assignment, "destructuring assignments are unstable");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1729,7 +1729,7 @@ impl<'a> State<'a> {
         &mut self,
         path: &ast::Path,
         fields: &[ast::Field],
-        wth: &Option<P<ast::Expr>>,
+        rest: &ast::StructRest,
         attrs: &[ast::Attribute],
     ) {
         self.print_path(path, true, 0);
@@ -1750,22 +1750,21 @@ impl<'a> State<'a> {
             },
             |f| f.span,
         );
-        match *wth {
-            Some(ref expr) => {
+        match rest {
+            ast::StructRest::Base(_) | ast::StructRest::Rest(_) => {
                 self.ibox(INDENT_UNIT);
                 if !fields.is_empty() {
                     self.s.word(",");
                     self.s.space();
                 }
                 self.s.word("..");
-                self.print_expr(expr);
-                self.end();
-            }
-            _ => {
-                if !fields.is_empty() {
-                    self.s.word(",")
+                if let ast::StructRest::Base(ref expr) = *rest {
+                    self.print_expr(expr);
                 }
+                self.end();
             }
+            ast::StructRest::None if !fields.is_empty() => self.s.word(","),
+            _ => {}
         }
         self.s.word("}");
     }
@@ -1891,8 +1890,8 @@ impl<'a> State<'a> {
             ast::ExprKind::Repeat(ref element, ref count) => {
                 self.print_expr_repeat(element, count, attrs);
             }
-            ast::ExprKind::Struct(ref path, ref fields, ref wth) => {
-                self.print_expr_struct(path, &fields[..], wth, attrs);
+            ast::ExprKind::Struct(ref path, ref fields, ref rest) => {
+                self.print_expr_struct(path, &fields[..], rest, attrs);
             }
             ast::ExprKind::Tup(ref exprs) => {
                 self.print_expr_tup(&exprs[..], attrs);

compiler/rustc_expand/src/build.rs

@@ -298,7 +298,7 @@ impl<'a> ExtCtxt<'a> {
         path: ast::Path,
         fields: Vec<ast::Field>,
     ) -> P<ast::Expr> {
-        self.expr(span, ast::ExprKind::Struct(path, fields, None))
+        self.expr(span, ast::ExprKind::Struct(path, fields, ast::StructRest::None))
     }
     pub fn expr_struct_ident(
         &self,

compiler/rustc_hir/src/def.rs

@@ -484,4 +484,9 @@ impl<Id> Res<Id> {
     pub fn matches_ns(&self, ns: Namespace) -> bool {
         self.ns().map_or(true, |actual_ns| actual_ns == ns)
     }
+
+    /// Returns whether such a resolved path can occur in a tuple struct/variant pattern
+    pub fn expected_in_tuple_struct_pat(&self) -> bool {
+        matches!(self, Res::Def(DefKind::Ctor(_, CtorKind::Fn), _) | Res::SelfCtor(..))
+    }
 }

compiler/rustc_parse/src/parser/expr.rs

@@ -2087,7 +2087,7 @@ impl<'a> Parser<'a> {
         recover: bool,
     ) -> PResult<'a, P<Expr>> {
         let mut fields = Vec::new();
-        let mut base = None;
+        let mut base = ast::StructRest::None;
         let mut recover_async = false;
 
         attrs.extend(self.parse_inner_attributes()?);
@@ -2102,8 +2102,14 @@ impl<'a> Parser<'a> {
         while self.token != token::CloseDelim(token::Brace) {
             if self.eat(&token::DotDot) {
                 let exp_span = self.prev_token.span;
+                // We permit `.. }` on the left-hand side of a destructuring assignment.
+                if self.check(&token::CloseDelim(token::Brace)) {
+                    self.sess.gated_spans.gate(sym::destructuring_assignment, self.prev_token.span);
+                    base = ast::StructRest::Rest(self.prev_token.span.shrink_to_hi());
+                    break;
+                }
                 match self.parse_expr() {
-                    Ok(e) => base = Some(e),
+                    Ok(e) => base = ast::StructRest::Base(e),
                     Err(mut e) if recover => {
                         e.emit();
                         self.recover_stmt();

compiler/rustc_resolve/src/late.rs

@@ -298,9 +298,7 @@ impl<'a> PathSource<'a> {
                     _,
                 )
                 | Res::SelfCtor(..)),
-            PathSource::TupleStruct(..) => {
-                matches!(res, Res::Def(DefKind::Ctor(_, CtorKind::Fn), _) | Res::SelfCtor(..))
-            }
+            PathSource::TupleStruct(..) => res.expected_in_tuple_struct_pat(),
             PathSource::Struct => matches!(res, Res::Def(
                     DefKind::Struct
                     | DefKind::Union