Ensure that users cannot use generated arguments.

This commit gensyms the generated ident for replacement arguments so
that users cannot refer to them. It also ensures that levenshtein
distance suggestions do not suggest gensymed identifiers.

src/librustc_resolve/lib.rs

@@ -4183,7 +4183,7 @@ impl<'a> Resolver<'a> {
         let add_module_candidates = |module: Module<'_>, names: &mut Vec<TypoSuggestion>| {
             for (&(ident, _), resolution) in module.resolutions.borrow().iter() {
                 if let Some(binding) = resolution.borrow().binding {
-                    if filter_fn(binding.def()) {
+                    if !ident.name.is_gensymed() && filter_fn(binding.def()) {
                         names.push(TypoSuggestion {
                             candidate: ident.name,
                             article: binding.def().article(),
@@ -4201,7 +4201,7 @@ impl<'a> Resolver<'a> {
             for rib in self.ribs[ns].iter().rev() {
                 // Locals and type parameters
                 for (ident, def) in &rib.bindings {
-                    if filter_fn(*def) {
+                    if !ident.name.is_gensymed() && filter_fn(*def) {
                         names.push(TypoSuggestion {
                             candidate: ident.name,
                             article: def.article(),
@@ -4228,7 +4228,7 @@ impl<'a> Resolver<'a> {
                                             index: CRATE_DEF_INDEX,
                                         });
 
-                                        if filter_fn(crate_mod) {
+                                        if !ident.name.is_gensymed() && filter_fn(crate_mod) {
                                             Some(TypoSuggestion {
                                                 candidate: ident.name,
                                                 article: "a",
@@ -4251,13 +4251,16 @@ impl<'a> Resolver<'a> {
             // Add primitive types to the mix
             if filter_fn(Def::PrimTy(Bool)) {
                 names.extend(
-                    self.primitive_type_table.primitive_types.iter().map(|(name, _)| {
-                        TypoSuggestion {
-                            candidate: *name,
-                            article: "a",
-                            kind: "primitive type",
-                        }
-                    })
+                    self.primitive_type_table.primitive_types
+                        .iter()
+                        .filter(|(name, _)| !name.is_gensymed())
+                        .map(|(name, _)| {
+                            TypoSuggestion {
+                                candidate: *name,
+                                article: "a",
+                                kind: "primitive type",
+                            }
+                        })
                 )
             }
         } else {

src/libsyntax/parse/parser.rs

@@ -8878,7 +8878,7 @@ impl<'a> Parser<'a> {
 
                 // Construct a name for our temporary argument.
                 let name = format!("__arg{}", index);
-                let ident = Ident::from_str(&name);
+                let ident = Ident::from_str(&name).gensym();
 
                 // Check if this is a ident pattern, if so, we can optimize and avoid adding a
                 // `let <pat> = __argN;` statement, instead just adding a `let <pat> = <pat>;`

src/test/ui/async-await-drop-order-locals-are-hidden.rs

@@ -0,0 +1,11 @@
+// edition:2018
+
+#![allow(unused_variables)]
+#![feature(async_await)]
+
+async fn foobar_async(x: u32, (a, _, _c): (u32, u32, u32), _: u32, _y: u32) {
+    assert_eq!(__arg1, (1, 2, 3)); //~ ERROR cannot find value `__arg1` in this scope [E0425]
+    assert_eq!(__arg2, 4); //~ ERROR cannot find value `__arg2` in this scope [E0425]
+}
+
+fn main() {}

src/test/ui/async-await-drop-order-locals-are-hidden.stderr

@@ -0,0 +1,15 @@
+error[E0425]: cannot find value `__arg1` in this scope
+  --> $DIR/async-await-drop-order-locals-are-hidden.rs:7:16
+   |
+LL |     assert_eq!(__arg1, (1, 2, 3));
+   |                ^^^^^^ not found in this scope
+
+error[E0425]: cannot find value `__arg2` in this scope
+  --> $DIR/async-await-drop-order-locals-are-hidden.rs:8:16
+   |
+LL |     assert_eq!(__arg2, 4);
+   |                ^^^^^^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0425`.

src/test/ui/auxiliary/arc_wake.rs

@@ -0,0 +1,64 @@
+// edition:2018
+
+use std::sync::Arc;
+use std::task::{
+    Waker, RawWaker, RawWakerVTable,
+};
+
+macro_rules! waker_vtable {
+    ($ty:ident) => {
+        &RawWakerVTable::new(
+            clone_arc_raw::<$ty>,
+            wake_arc_raw::<$ty>,
+            wake_by_ref_arc_raw::<$ty>,
+            drop_arc_raw::<$ty>,
+        )
+    };
+}
+
+pub trait ArcWake {
+    fn wake(self: Arc<Self>);
+
+    fn wake_by_ref(arc_self: &Arc<Self>) {
+        arc_self.clone().wake()
+    }
+
+    fn into_waker(wake: Arc<Self>) -> Waker where Self: Sized
+    {
+        let ptr = Arc::into_raw(wake) as *const ();
+
+        unsafe {
+            Waker::from_raw(RawWaker::new(ptr, waker_vtable!(Self)))
+        }
+    }
+}
+
+unsafe fn increase_refcount<T: ArcWake>(data: *const ()) {
+    // Retain Arc by creating a copy
+    let arc: Arc<T> = Arc::from_raw(data as *const T);
+    let arc_clone = arc.clone();
+    // Forget the Arcs again, so that the refcount isn't decrased
+    let _ = Arc::into_raw(arc);
+    let _ = Arc::into_raw(arc_clone);
+}
+
+unsafe fn clone_arc_raw<T: ArcWake>(data: *const ()) -> RawWaker {
+    increase_refcount::<T>(data);
+    RawWaker::new(data, waker_vtable!(T))
+}
+
+unsafe fn drop_arc_raw<T: ArcWake>(data: *const ()) {
+    // Drop Arc
+    let _: Arc<T> = Arc::from_raw(data as *const T);
+}
+
+unsafe fn wake_arc_raw<T: ArcWake>(data: *const ()) {
+    let arc: Arc<T> = Arc::from_raw(data as *const T);
+    ArcWake::wake(arc);
+}
+
+unsafe fn wake_by_ref_arc_raw<T: ArcWake>(data: *const ()) {
+    let arc: Arc<T> = Arc::from_raw(data as *const T);
+    ArcWake::wake_by_ref(&arc);
+    let _ = Arc::into_raw(arc);
+}