1 files changed, 110 insertions, 0 deletions
diff --git a/src/lifetime.rs b/src/lifetime.rs
new file mode 100644
index 0000000..1e5a658
--- /dev/null
+++ b/src/lifetime.rs
@@ -0,0 +1,110 @@
+use proc_macro2::{Span, TokenStream};
+use std::mem;
+use syn::visit_mut::{self, VisitMut};
+use syn::{
+    parse_quote_spanned, token, Expr, GenericArgument, Lifetime, Receiver, ReturnType, Token, Type,
+    TypeBareFn, TypeImplTrait, TypeParen, TypePtr, TypeReference,
+};
+
+pub struct CollectLifetimes {
+    pub elided: Vec<Lifetime>,
+    pub explicit: Vec<Lifetime>,
+}
+
+impl CollectLifetimes {
+    pub fn new() -> Self {
+        CollectLifetimes {
+            elided: Vec::new(),
+            explicit: Vec::new(),
+        }
+    }
+
+    fn visit_opt_lifetime(&mut self, reference: Token![&], lifetime: &mut Option<Lifetime>) {
+        match lifetime {
+            None => *lifetime = Some(self.next_lifetime(reference.span)),
+            Some(lifetime) => self.visit_lifetime(lifetime),
+        }
+    }
+
+    fn visit_lifetime(&mut self, lifetime: &mut Lifetime) {
+        if lifetime.ident == "_" {
+            *lifetime = self.next_lifetime(lifetime.span());
+        } else {
+            self.explicit.push(lifetime.clone());
+        }
+    }
+
+    fn next_lifetime(&mut self, span: Span) -> Lifetime {
+        let name = format!("'life{}", self.elided.len());
+        let life = Lifetime::new(&name, span);
+        self.elided.push(life.clone());
+        life
+    }
+}
+
+impl VisitMut for CollectLifetimes {
+    fn visit_receiver_mut(&mut self, arg: &mut Receiver) {
+        if let Some((reference, lifetime)) = &mut arg.reference {
+            self.visit_opt_lifetime(*reference, lifetime);
+        }
+    }
+
+    fn visit_type_reference_mut(&mut self, ty: &mut TypeReference) {
+        self.visit_opt_lifetime(ty.and_token, &mut ty.lifetime);
+        visit_mut::visit_type_reference_mut(self, ty);
+    }
+
+    fn visit_generic_argument_mut(&mut self, gen: &mut GenericArgument) {
+        if let GenericArgument::Lifetime(lifetime) = gen {
+            self.visit_lifetime(lifetime);
+        }
+        visit_mut::visit_generic_argument_mut(self, gen);
+    }
+}
+
+pub struct AddLifetimeToImplTrait;
+
+impl VisitMut for AddLifetimeToImplTrait {
+    fn visit_type_impl_trait_mut(&mut self, ty: &mut TypeImplTrait) {
+        let span = ty.impl_token.span;
+        let lifetime = parse_quote_spanned!(span=> 'async_trait);
+        ty.bounds.insert(0, lifetime);
+        if let Some(punct) = ty.bounds.pairs_mut().next().unwrap().punct_mut() {
+            punct.span = span;
+        }
+        visit_mut::visit_type_impl_trait_mut(self, ty);
+    }
+
+    fn visit_type_reference_mut(&mut self, ty: &mut TypeReference) {
+        parenthesize_impl_trait(&mut ty.elem, ty.and_token.span);
+        visit_mut::visit_type_reference_mut(self, ty);
+    }
+
+    fn visit_type_ptr_mut(&mut self, ty: &mut TypePtr) {
+        parenthesize_impl_trait(&mut ty.elem, ty.star_token.span);
+        visit_mut::visit_type_ptr_mut(self, ty);
+    }
+
+    fn visit_type_bare_fn_mut(&mut self, ty: &mut TypeBareFn) {
+        if let ReturnType::Type(arrow, return_type) = &mut ty.output {
+            parenthesize_impl_trait(return_type, arrow.spans[0]);
+        }
+        visit_mut::visit_type_bare_fn_mut(self, ty);
+    }
+
+    fn visit_expr_mut(&mut self, _e: &mut Expr) {
+        // Do not recurse into impl Traits inside of an array length expression.
+        //
+        //    fn outer(arg: [u8; { fn inner(_: impl Trait) {}; 0 }]);
+    }
+}
+
+fn parenthesize_impl_trait(elem: &mut Type, paren_span: Span) {
+    if let Type::ImplTrait(_) = *elem {
+        let placeholder = Type::Verbatim(TokenStream::new());
+        *elem = Type::Paren(TypeParen {
+            paren_token: token::Paren(paren_span),
+            elem: Box::new(mem::replace(elem, placeholder)),
+        });
+    }
+}