Merge pull request #1261 from lcnr/implied-bounds

add section about implied bounds

Author: ehuss

src/trait-bounds.md

@@ -156,6 +156,79 @@ fn call_on_ref_zero<F>(f: F) where F: for<'a> Fn(&'a i32) {
 }
 ```
 
+## Implied bounds
+
+Lifetime bounds required for types to be well-formed are sometimes inferred.
+
+```rust
+fn requires_t_outlives_a<'a, T>(x: &'a T) {}
+```
+The type parameter `T` is required to outlive `'a` for the type `&'a T` to be well-formed.
+This is inferred because the function signature contains the type `&'a T` which is
+only valid if `T: 'a` holds.
+
+Implied bounds are added for all parameters and outputs of functions. Inside of `requires_t_outlives_a`
+you can assume `T: 'a` to hold even if you don't explicitly specify this:
+
+```rust
+fn requires_t_outlives_a_not_implied<'a, T: 'a>() {}
+
+fn requires_t_outlives_a<'a, T>(x: &'a T) {
+    // This compiles, because `T: 'a` is implied by
+    // the reference type `&'a T`.
+    requires_t_outlives_a_not_implied::<'a, T>();
+}
+```
+
+```rust,compile_fail,E0309
+# fn requires_t_outlives_a_not_implied<'a, T: 'a>() {}
+fn not_implied<'a, T>() {
+    // This errors, because `T: 'a` is not implied by
+    // the function signature.
+    requires_t_outlives_a_not_implied::<'a, T>();
+}
+```
+
+Only lifetime bounds are implied, trait bounds still have to be explicitly added.
+The following example therefore causes an error:
+
+```rust,compile_fail,E0277
+use std::fmt::Debug;
+struct IsDebug<T: Debug>(T);
+// error[E0277]: `T` doesn't implement `Debug`
+fn doesnt_specify_t_debug<T>(x: IsDebug<T>) {}
+```
+
+Lifetime bounds are also inferred for type definitions and impl blocks for any type:
+
+```rust
+struct Struct<'a, T> {
+    // This requires `T: 'a` to be well-formed
+    // which is inferred by the compiler.
+    field: &'a T,
+}
+
+enum Enum<'a, T> {
+    // This requires `T: 'a` to be well-formed,
+    // which is inferred by the compiler.
+    //
+    // Note that `T: 'a` is required even when only
+    // using `Enum::OtherVariant`.
+    SomeVariant(&'a T),
+    OtherVariant,
+}
+
+trait Trait<'a, T: 'a> {}
+
+// This would error because `T: 'a` is not implied by any type
+// in the impl header.
+//     impl<'a, T> Trait<'a, T> for () {}
+
+// This compiles as `T: 'a` is implied by the self type `&'a T`.
+impl<'a, T> Trait<'a, T> for &'a T {}
+```
+
+
 [LIFETIME_OR_LABEL]: tokens.md#lifetimes-and-loop-labels
 [_GenericParams_]: items/generics.md
 [_TypePath_]: paths.md#paths-in-types