Introduce move expressions (move($expr))

compiler/rustc_ast/src/ast.rs

@@ -1595,6 +1595,7 @@ impl Expr {
             // need parens sometimes. E.g. we can print `(let _ = a) && b` as `let _ = a && b`
             // but we need to print `(let _ = a) < b` as-is with parens.
             | ExprKind::Let(..)
+            | ExprKind::Move(..)
             | ExprKind::Unary(..) => ExprPrecedence::Prefix,
 
             // Need parens if and only if there are prefix attributes.
@@ -1764,6 +1765,8 @@ pub enum ExprKind {
     Binary(BinOp, Box<Expr>, Box<Expr>),
     /// A unary operation (e.g., `!x`, `*x`).
     Unary(UnOp, Box<Expr>),
+    /// A `move(expr)` expression.
+    Move(Box<Expr>, Span),
     /// A literal (e.g., `1`, `"foo"`).
     Lit(token::Lit),
     /// A cast (e.g., `foo as f64`).

compiler/rustc_ast/src/util/classify.rs

@@ -108,6 +108,7 @@ pub fn leading_labeled_expr(mut expr: &ast::Expr) -> bool {
             Assign(e, _, _)
             | AssignOp(_, e, _)
             | Await(e, _)
+            | Move(e, _)
             | Use(e, _)
             | Binary(_, e, _)
             | Call(e, _)
@@ -183,6 +184,7 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | Ret(Some(e))
             | Unary(_, e)
             | Yeet(Some(e))
+            | Move(e, _)
             | Become(e) => {
                 expr = e;
             }

compiler/rustc_ast/src/visit.rs

@@ -1024,7 +1024,9 @@ macro_rules! common_visitor_and_walkers {
                     visit_visitable!($($mut)? vis, block, opt_label),
                 ExprKind::Gen(capt, body, kind, decl_span) =>
                     visit_visitable!($($mut)? vis, capt, body, kind, decl_span),
-                ExprKind::Await(expr, span) | ExprKind::Use(expr, span) =>
+                ExprKind::Await(expr, span)
+                | ExprKind::Move(expr, span)
+                | ExprKind::Use(expr, span) =>
                     visit_visitable!($($mut)? vis, expr, span),
                 ExprKind::Assign(lhs, rhs, span) =>
                     visit_visitable!($($mut)? vis, lhs, rhs, span),

compiler/rustc_ast_lowering/src/errors.rs

@@ -136,6 +136,13 @@ pub(crate) struct ClosureCannotBeStatic {
     pub fn_decl_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag("`move(expr)` is only supported in plain closures")]
+pub(crate) struct MoveExprOnlyInPlainClosures {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("functional record updates are not allowed in destructuring assignments")]
 pub(crate) struct FunctionalRecordUpdateDestructuringAssignment {