Rollup merge of #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: #71126). This PR is the second part of #71156, which was split up to allow for easier review.

Note that the first PR (#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 #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``

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);