Perform lifetime elision (more) syntactically, before type-checking.

src/librustc/diagnostics.rs

@@ -327,6 +327,69 @@ struct ListNode {
 This works because `Box` is a pointer, so its size is well-known.
 "##,
 
+E0106: r##"
+This error indicates that a lifetime is missing from a type. If it is an error
+inside a function signature, the problem may be with failing to adhere to the
+lifetime elision rules (see below).
+
+Here are some simple examples of where you'll run into this error:
+
+```compile_fail,E0106
+struct Foo { x: &bool }        // error
+struct Foo<'a> { x: &'a bool } // correct
+
+enum Bar { A(u8), B(&bool), }        // error
+enum Bar<'a> { A(u8), B(&'a bool), } // correct
+
+type MyStr = &str;        // error
+type MyStr<'a> = &'a str; // correct
+```
+
+Lifetime elision is a special, limited kind of inference for lifetimes in
+function signatures which allows you to leave out lifetimes in certain cases.
+For more background on lifetime elision see [the book][book-le].
+
+The lifetime elision rules require that any function signature with an elided
+output lifetime must either have
+
+ - exactly one input lifetime
+ - or, multiple input lifetimes, but the function must also be a method with a
+   `&self` or `&mut self` receiver
+
+In the first case, the output lifetime is inferred to be the same as the unique
+input lifetime. In the second case, the lifetime is instead inferred to be the
+same as the lifetime on `&self` or `&mut self`.
+
+Here are some examples of elision errors:
+
+```compile_fail,E0106
+// error, no input lifetimes
+fn foo() -> &str { }
+
+// error, `x` and `y` have distinct lifetimes inferred
+fn bar(x: &str, y: &str) -> &str { }
+
+// error, `y`'s lifetime is inferred to be distinct from `x`'s
+fn baz<'a>(x: &'a str, y: &str) -> &str { }
+```
+
+Here's an example that is currently an error, but may work in a future version
+of Rust:
+
+```compile_fail,E0106
+struct Foo<'a>(&'a str);
+
+trait Quux { }
+impl Quux for Foo { }
+```
+
+Lifetime elision in implementation headers was part of the lifetime elision
+RFC. It is, however, [currently unimplemented][iss15872].
+
+[book-le]: https://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision
+[iss15872]: https://github.com/rust-lang/rust/issues/15872
+"##,
+
 E0109: r##"
 You tried to give a type parameter to a type which doesn't need it. Erroneous
 code example:

src/librustc/hir/intravisit.rs

@@ -301,7 +301,7 @@ pub trait Visitor<'v> : Sized {
     fn visit_ty_param_bound(&mut self, bounds: &'v TyParamBound) {
         walk_ty_param_bound(self, bounds)
     }
-    fn visit_poly_trait_ref(&mut self, t: &'v PolyTraitRef, m: &'v TraitBoundModifier) {
+    fn visit_poly_trait_ref(&mut self, t: &'v PolyTraitRef, m: TraitBoundModifier) {
         walk_poly_trait_ref(self, t, m)
     }
     fn visit_variant_data(&mut self,
@@ -421,7 +421,7 @@ pub fn walk_lifetime_def<'v, V: Visitor<'v>>(visitor: &mut V, lifetime_def: &'v
 
 pub fn walk_poly_trait_ref<'v, V>(visitor: &mut V,
                                   trait_ref: &'v PolyTraitRef,
-                                  _modifier: &'v TraitBoundModifier)
+                                  _modifier: TraitBoundModifier)
     where V: Visitor<'v>
 {
     walk_list!(visitor, visit_lifetime_def, &trait_ref.bound_lifetimes);
@@ -547,8 +547,8 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
         TyPtr(ref mutable_type) => {
             visitor.visit_ty(&mutable_type.ty)
         }
-        TyRptr(ref opt_lifetime, ref mutable_type) => {
-            walk_list!(visitor, visit_lifetime, opt_lifetime);
+        TyRptr(ref lifetime, ref mutable_type) => {
+            visitor.visit_lifetime(lifetime);
             visitor.visit_ty(&mutable_type.ty)
         }
         TyNever => {},
@@ -566,8 +566,11 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
             visitor.visit_ty(ty);
             visitor.visit_nested_body(length)
         }
-        TyTraitObject(ref bounds) => {
-            walk_list!(visitor, visit_ty_param_bound, bounds);
+        TyTraitObject(ref bounds, ref lifetime) => {
+            for bound in bounds {
+                visitor.visit_poly_trait_ref(bound, TraitBoundModifier::None);
+            }
+            visitor.visit_lifetime(lifetime);
         }
         TyImplTrait(ref bounds) => {
             walk_list!(visitor, visit_ty_param_bound, bounds);
@@ -695,7 +698,7 @@ pub fn walk_foreign_item<'v, V: Visitor<'v>>(visitor: &mut V, foreign_item: &'v
 
 pub fn walk_ty_param_bound<'v, V: Visitor<'v>>(visitor: &mut V, bound: &'v TyParamBound) {
     match *bound {
-        TraitTyParamBound(ref typ, ref modifier) => {
+        TraitTyParamBound(ref typ, modifier) => {
             visitor.visit_poly_trait_ref(typ, modifier);
         }
         RegionTyParamBound(ref lifetime) => {