rustc: move most of lifetime elision to resolve_lifetimes.

Author: eddyb

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/lowering.rs

@@ -314,9 +314,10 @@ impl<'a> LoweringContext<'a> {
                 TyKind::Slice(ref ty) => hir::TySlice(self.lower_ty(ty)),
                 TyKind::Ptr(ref mt) => hir::TyPtr(self.lower_mt(mt)),
                 TyKind::Rptr(ref region, ref mt) => {
+                    let span = Span { hi: t.span.lo, ..t.span };
                     let lifetime = match *region {
                         Some(ref lt) => self.lower_lifetime(lt),
-                        None => self.elided_lifetime(t.span)
+                        None => self.elided_lifetime(span)
                     };
                     hir::TyRptr(lifetime, self.lower_mt(mt))
                 }