Rollup merge of #59823 - davidtwco:issue-54716, r=cramertj

[wg-async-await] Drop `async fn` arguments in async block

Fixes #54716.

This PR modifies the HIR lowering (and some other places to make this work) so that unused arguments to a async function are always dropped inside the async move block and not at the end of the function body.

```
async fn foo(<pattern>: <type>) {
  async move {
  }
} // <-- dropped as you "exit" the fn

// ...becomes...
fn foo(__arg0: <ty>) {
  async move {
    let <pattern>: <ty> = __arg0;
  } // <-- dropped as you "exit" the async block
}
```

However, the exact ordering of drops is not the same as a regular function, [as visible in this playground example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=be39af1a58e5d430be1eb3c722cb1ec3) - I believe this to be an unrelated issue. There is a [Zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/187312-t-compiler.2Fwg-async-await/topic/.2354716.20drop.20order) for this.

r? @cramertj
cc @nikomatsakis

1 files changed, 11 insertions, 12 deletions

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index d94e4762e67..7ce3951f13e 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -372,7 +372,7 @@ pub fn vis_to_string(v: &ast::Visibility) -> String {
 }
 
 pub fn fun_to_string(decl: &ast::FnDecl,
-                     header: ast::FnHeader,
+                     header: &ast::FnHeader,
                      name: ast::Ident,
                      generics: &ast::Generics)
                      -> String {
@@ -1133,7 +1133,7 @@ impl<'a> State<'a> {
         match item.node {
             ast::ForeignItemKind::Fn(ref decl, ref generics) => {
                 self.head("")?;
-                self.print_fn(decl, ast::FnHeader::default(),
+                self.print_fn(decl, &ast::FnHeader::default(),
                               Some(item.ident),
                               generics, &item.vis)?;
                 self.end()?; // end head-ibox
@@ -1263,7 +1263,7 @@ impl<'a> State<'a> {
                 self.s.word(";")?;
                 self.end()?; // end the outer cbox
             }
-            ast::ItemKind::Fn(ref decl, header, ref param_names, ref body) => {
+            ast::ItemKind::Fn(ref decl, ref header, ref param_names, ref body) => {
                 self.head("")?;
                 self.print_fn(
                     decl,
@@ -1615,7 +1615,7 @@ impl<'a> State<'a> {
                             vis: &ast::Visibility)
                             -> io::Result<()> {
         self.print_fn(&m.decl,
-                      m.header,
+                      &m.header,
                       Some(ident),
                       &generics,
                       vis)
@@ -2213,7 +2213,7 @@ impl<'a> State<'a> {
                 self.bclose_(expr.span, INDENT_UNIT)?;
             }
             ast::ExprKind::Closure(
-                capture_clause, asyncness, movability, ref decl, ref body, _) => {
+                capture_clause, ref asyncness, movability, ref decl, ref body, _) => {
                 self.print_movability(movability)?;
                 self.print_asyncness(asyncness)?;
                 self.print_capture_clause(capture_clause)?;
@@ -2798,7 +2798,7 @@ impl<'a> State<'a> {
 
     pub fn print_fn(&mut self,
                     decl: &ast::FnDecl,
-                    header: ast::FnHeader,
+                    header: &ast::FnHeader,
                     name: Option<ast::Ident>,
                     generics: &ast::Generics,
                     vis: &ast::Visibility) -> io::Result<()> {
@@ -2853,8 +2853,7 @@ impl<'a> State<'a> {
         }
     }
 
-    pub fn print_asyncness(&mut self, asyncness: ast::IsAsync)
-                                -> io::Result<()> {
+    pub fn print_asyncness(&mut self, asyncness: &ast::IsAsync) -> io::Result<()> {
         if asyncness.is_async() {
             self.word_nbsp("async")?;
         }
@@ -3126,7 +3125,7 @@ impl<'a> State<'a> {
             span: syntax_pos::DUMMY_SP,
         };
         self.print_fn(decl,
-                      ast::FnHeader { unsafety, abi, ..ast::FnHeader::default() },
+                      &ast::FnHeader { unsafety, abi, ..ast::FnHeader::default() },
                       name,
                       &generics,
                       &source_map::dummy_spanned(ast::VisibilityKind::Inherited))?;
@@ -3189,7 +3188,7 @@ impl<'a> State<'a> {
     }
 
     pub fn print_fn_header_info(&mut self,
-                                header: ast::FnHeader,
+                                header: &ast::FnHeader,
                                 vis: &ast::Visibility) -> io::Result<()> {
         self.s.word(visibility_qualified(vis, ""))?;
 
@@ -3198,7 +3197,7 @@ impl<'a> State<'a> {
             ast::Constness::Const => self.word_nbsp("const")?
         }
 
-        self.print_asyncness(header.asyncness.node)?;
+        self.print_asyncness(&header.asyncness.node)?;
         self.print_unsafety(header.unsafety)?;
 
         if header.abi != Abi::Rust {
@@ -3247,7 +3246,7 @@ mod tests {
             assert_eq!(
                 fun_to_string(
                     &decl,
-                    ast::FnHeader {
+                    &ast::FnHeader {
                         unsafety: ast::Unsafety::Normal,
                         constness: source_map::dummy_spanned(ast::Constness::NotConst),
                         asyncness: source_map::dummy_spanned(ast::IsAsync::NotAsync),