Auto merge of #49251 - nikomatsakis:issue-15872-elision-impl-header, r=cramertj

support elision in impl headers

You can now do things like:

```
impl MyTrait<'_> for &u32 { ... }
```

Each `'_` or elided lifetime is a fresh parameter. `'_` and elision are still not permitted in associated type values. (Plausibly we could support that if there is a single input lifetime.) The original lifetime elision RFC was a bit unclear on this point: [as documented here, I think this is the correct interpretation, both because it fits existing impls and it's most analogous to the behavior in fns](#15872 (comment)).

We do not support elision with deprecated forms:

```
impl MyTrait for std::cell::Ref<u32> { } // ERROR
```

Builds on the in-band lifetime stuff.

r? @cramertj

Fixes #15872

Author: bors

src/librustc/hir/intravisit.rs

@@ -420,7 +420,10 @@ pub fn walk_lifetime<'v, V: Visitor<'v>>(visitor: &mut V, lifetime: &'v Lifetime
         LifetimeName::Name(name) => {
             visitor.visit_name(lifetime.span, name);
         }
-        LifetimeName::Static | LifetimeName::Implicit | LifetimeName::Underscore => {}
+        LifetimeName::Fresh(_) |
+        LifetimeName::Static |
+        LifetimeName::Implicit |
+        LifetimeName::Underscore => {}
     }
 }
 

src/librustc/hir/lowering.rs

@@ -203,9 +203,31 @@ pub struct Lifetime {
 
 #[derive(Debug, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Copy)]
 pub enum LifetimeName {
+    /// User typed nothing. e.g. the lifetime in `&u32`.
     Implicit,
+
+    /// User typed `'_`.
     Underscore,
+
+    /// Synthetic name generated when user elided a lifetime in an impl header,
+    /// e.g. the lifetimes in cases like these:
+    ///
+    ///     impl Foo for &u32
+    ///     impl Foo<'_> for u32
+    ///
+    /// in that case, we rewrite to
+    ///
+    ///     impl<'f> Foo for &'f u32
+    ///     impl<'f> Foo<'f> for u32
+    ///
+    /// where `'f` is something like `Fresh(0)`. The indices are
+    /// unique per impl, but not necessarily continuous.
+    Fresh(usize),
+
+    /// User wrote `'static`
     Static,
+
+    /// Some user-given name like `'x`
     Name(Name),
 }
 
@@ -214,7 +236,7 @@ impl LifetimeName {
         use self::LifetimeName::*;
         match *self {
             Implicit => keywords::Invalid.name(),
-            Underscore => keywords::UnderscoreLifetime.name(),
+            Fresh(_) | Underscore => keywords::UnderscoreLifetime.name(),
             Static => keywords::StaticLifetime.name(),
             Name(name) => name,
         }
@@ -235,7 +257,13 @@ impl Lifetime {
         use self::LifetimeName::*;
         match self.name {
             Implicit | Underscore => true,
-            Static | Name(_) => false,
+
+            // It might seem surprising that `Fresh(_)` counts as
+            // *not* elided -- but this is because, as far as the code
+            // in the compiler is concerned -- `Fresh(_)` variants act
+            // equivalently to "some fresh name". They correspond to
+            // early-bound regions on an impl, in other words.
+            Fresh(_) | Static | Name(_) => false,
         }
     }
 

src/librustc/hir/mod.rs

@@ -145,6 +145,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for hir::ImplItemId {
 impl_stable_hash_for!(enum hir::LifetimeName {
     Implicit,
     Underscore,
+    Fresh(index),
     Static,
     Name(name)
 });