Add long description and test for E0311

compiler/rustc_error_codes/src/error_codes.rs

@@ -159,6 +159,7 @@ E0307: include_str!("./error_codes/E0307.md"),
 E0308: include_str!("./error_codes/E0308.md"),
 E0309: include_str!("./error_codes/E0309.md"),
 E0310: include_str!("./error_codes/E0310.md"),
+E0311: include_str!("./error_codes/E0311.md"),
 E0312: include_str!("./error_codes/E0312.md"),
 E0316: include_str!("./error_codes/E0316.md"),
 E0317: include_str!("./error_codes/E0317.md"),
@@ -568,7 +569,6 @@ E0790: include_str!("./error_codes/E0790.md"),
 //  E0300, // unexpanded macro
 //  E0304, // expected signed integer constant
 //  E0305, // expected constant
-    E0311, // thing may not live long enough
     E0313, // lifetime of borrowed pointer outlives lifetime of captured
            // variable
 //  E0314, // closure outlives stack frame

compiler/rustc_error_codes/src/error_codes/E0311.md

@@ -0,0 +1,44 @@
+This error occurs when there is an unsatisfied outlives bound involving an
+elided region on a generic type parameter or associated type.
+
+Erroneous code example:
+
+```compile_fail,E0311
+fn no_restriction<T>(x: &()) -> &() {
+    with_restriction::<T>(x)
+}
+
+fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
+    x
+}
+```
+
+Why doesn't this code compile? It helps to look at the lifetime bounds that are
+automatically added by the compiler. For more details see the documentation for
+[lifetime elision]( https://doc.rust-lang.org/reference/lifetime-elision.html).
+
+The compiler elides the lifetime of `x` and the return type to some arbitrary
+lifetime `'anon` in `no_restriction()`. The only information available to the
+compiler is that `'anon` is valid for the duration of the function. When
+calling `with_restriction()`, the compiler requires the completely unrelated
+type parameter `T` to outlive `'anon` because of the `T: 'a bound` in
+`with_restriction()`. This causes an error because `T` is not required to
+outlive `'anon` in `no_restriction()`.
+
+If `no_restriction()` were to use `&T` instead of `&()` as an argument, the
+compiler would have added an implied bound [implied
+bound](https://rust-lang.github.io/rfcs/2089-implied-bounds.html), causing this
+to compile.
+
+This error can be resolved by explicitly naming the elided lifetime for `x` and
+then explicily requiring that the generic parameter `T` outlives that lifetime:
+
+```
+fn no_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
+    with_restriction::<T>(x)
+}
+
+fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
+    x
+}
+```

src/test/ui/error-codes/E0311.rs

@@ -0,0 +1,9 @@
+fn no_restriction<T>(x: &()) -> &() {
+    with_restriction::<T>(x) //~ ERROR E0311
+}
+
+fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
+    x
+}
+
+fn main() {}

src/test/ui/error-codes/E0311.stderr

@@ -0,0 +1,24 @@
+error[E0311]: the parameter type `T` may not live long enough
+  --> $DIR/E0311.rs:2:5
+   |
+LL |     with_restriction::<T>(x)
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the parameter type `T` must be valid for the anonymous lifetime defined here...
+  --> $DIR/E0311.rs:1:25
+   |
+LL | fn no_restriction<T>(x: &()) -> &() {
+   |                         ^^^
+note: ...so that the type `T` will meet its required lifetime bounds
+  --> $DIR/E0311.rs:2:5
+   |
+LL |     with_restriction::<T>(x)
+   |     ^^^^^^^^^^^^^^^^^^^^^
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn no_restriction<T: 'a>(x: &()) -> &() {
+   |                    ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0311`.

src/test/ui/suggestions/lifetimes/missing-lifetimes-in-signature-2.stderr

@@ -27,3 +27,4 @@ LL | fn func<T: Test + 'a>(foo: &Foo, t: T) {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0311`.

src/test/ui/suggestions/lifetimes/missing-lifetimes-in-signature.stderr

@@ -162,5 +162,5 @@ LL |     G: Get<T> + 'a,
 
 error: aborting due to 8 previous errors
 
-Some errors have detailed explanations: E0261, E0309, E0621, E0700.
+Some errors have detailed explanations: E0261, E0309, E0311, E0621, E0700.
 For more information about an error, try `rustc --explain E0261`.