Desugars contract into the internal AST extensions

Check ensures on early return due to Try / Yeet

Expand these two expressions to include a call to contract checking

core/src/contracts.rs

@@ -1,5 +1,10 @@
 //! Unstable module containing the unstable contracts lang items and attribute macros.
 
+#[cfg(not(bootstrap))]
+pub use crate::macros::builtin::contracts_ensures as ensures;
+#[cfg(not(bootstrap))]
+pub use crate::macros::builtin::contracts_requires as requires;
+
 /// Emitted by rustc as a desugaring of `#[requires(PRED)] fn foo(x: X) { ... }`
 /// into: `fn foo(x: X) { check_requires(|| PRED) ... }`
 #[cfg(not(bootstrap))]

core/src/macros/mod.rs

@@ -1777,6 +1777,32 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Attribute macro applied to a function to give it a post-condition.
+    ///
+    /// The attribute carries an argument token-tree which is
+    /// eventually parsed as a unary closure expression that is
+    /// invoked on a reference to the return value.
+    #[cfg(not(bootstrap))]
+    #[unstable(feature = "rustc_contracts", issue = "none")]
+    #[allow_internal_unstable(core_intrinsics)]
+    #[rustc_builtin_macro]
+    pub macro contracts_ensures($item:item) {
+        /* compiler built-in */
+    }
+
+    /// Attribute macro applied to a function to give it a precondition.
+    ///
+    /// The attribute carries an argument token-tree which is
+    /// eventually parsed as an boolean expression with access to the
+    /// function's formal parameters
+    #[cfg(not(bootstrap))]
+    #[unstable(feature = "rustc_contracts", issue = "none")]
+    #[allow_internal_unstable(core_intrinsics)]
+    #[rustc_builtin_macro]
+    pub macro contracts_requires($item:item) {
+        /* compiler built-in */
+    }
+
     /// Attribute macro applied to a function to register it as a handler for allocation failure.
     ///
     /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).