Add do yeet expressions to allow experimentation in nightly

Using an obviously-placeholder syntax.  An RFC would still be needed before this could have any chance at stabilization, and it might be removed at any point.

But I'd really like to have it in nightly at least to ensure it works well with try_trait_v2, especially as we refactor the traits.

compiler/rustc_ast_lowering/src/expr.rs

@@ -221,6 +221,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let e = e.as_ref().map(|x| self.lower_expr(x));
                     hir::ExprKind::Ret(e)
                 }
+                ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
                 ExprKind::InlineAsm(ref asm) => {
                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
                 }
@@ -1543,6 +1544,44 @@ impl<'hir> LoweringContext<'_, 'hir> {
         )
     }
 
+    /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
+    /// ```rust
+    /// // If there is an enclosing `try {...}`:
+    /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
+    /// // Otherwise:
+    /// return FromResidual::from_residual(Yeet(residual)),
+    /// ```
+    /// But to simplify this, there's a `from_yeet` lang item function which
+    /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
+    fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
+        // The expression (if present) or `()` otherwise.
+        let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
+            (sub_expr.span, self.lower_expr(sub_expr))
+        } else {
+            (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
+        };
+
+        let unstable_span = self.mark_span_with_reason(
+            DesugaringKind::YeetExpr,
+            span,
+            self.allow_try_trait.clone(),
+        );
+
+        let from_yeet_expr = self.wrap_in_try_constructor(
+            hir::LangItem::TryTraitFromYeet,
+            unstable_span,
+            yeeted_expr,
+            yeeted_span,
+        );
+
+        if let Some(catch_node) = self.catch_scope {
+            let target_id = Ok(self.lower_node_id(catch_node));
+            hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
+        } else {
+            hir::ExprKind::Ret(Some(from_yeet_expr))
+        }
+    }
+
     // =========================================================================
     // Helper methods for building HIR.
     // =========================================================================