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 +1,70 @@
+This error occurs when there is insufficient information for the rust compiler
+to prove that a type has a long enough lifetime.
+
+Erroneous code example:
+
+```compile_fail,E0311
+use std::borrow::BorrowMut;
+
+trait NestedBorrowMut<U, V> {
+    fn nested_borrow_mut(&mut self) -> &mut V;
+}
+
+impl<T, U, V> NestedBorrowMut<U, V> for T
+where
+    T: BorrowMut<U>,
+    U: BorrowMut<V>, // error: missing lifetime specifier
+{
+    fn nested_borrow_mut(&mut self) -> &mut V {
+        self.borrow_mut().borrow_mut()
+    }
+}
+```
+
+Why doesn't this code compile? The problem has to do with Rust's rules for
+lifetime elision in functions (Chapter 10.3 in the Rust book). One of the
+inputs is a reference to `self`, so the compiler attempts to assign the
+the same lifetime to the `&mut self` input and `&mut V` output to the
+`nested_borrow_mut()` function. The problem is that there is no way for the
+compiler to directly figure out how these two lifetimes are related in the
+implementation of the function. We're implementing the `NextedBorrowMut`
+trait for a type `T`, so the `&mut self` reference has the lifetime of `T`.
+We know that `T` implements the `BorrowMut` trait returning a reference to `U`,
+and that `U` implements the `BorrowMut` trait returning a reference to `V`.
+The key is that we have not told the compiler that those two `U` lifetimes
+are the same: for all it knows, we could be that the first `BorrowMut` trait
+on `T` works with a lifetime `'a` and the second `BorrowMut` trait on `U`
+works on a lifetime `'b`.
+
+The fix here is to add explicit lifetime annotations that tell the compiler
+that the lifetime of the output is in fact the same as the lifetime of the
+input (`self`). There are three references involved, to objects of type `T`
+(`self`), `U` (the intermediate type), and `V` (the return type). In the
+working code below, we see that all have been given the same lifetime `'a`:
+- `&'a mut self` in the function argument list for `T`
+- `U: BorrowMut<V> + 'a` in the trait bounds for `U`
+- `&'a mut V` in the function return for `V`.
+
+The compiler can the check that the implementation of the
+`nested_borrow_mut()` function satisfies these lifetimes. There are two
+functions being called inside of `nested_borrow_mut()`, both of which are
+the `borrow_mut()` function, which promises that the output lifetime is
+the same as the input lifetime (see lifetime elision rules), which checks out.
+
+```
+use std::borrow::BorrowMut;
+
+trait NestedBorrowMut<'a, U, V> {
+    fn nested_borrow_mut(& 'a mut self) -> &'a mut V;
+}
+
+impl<'a, T, U, V> NestedBorrowMut<'a, U, V> for T
+where
+    T: BorrowMut<U>,
+    U: BorrowMut<V> + 'a, // Adding lifetime specifier
+{
+    fn nested_borrow_mut(&'a mut self) -> &'a mut V {
+        self.borrow_mut().borrow_mut()
+    }
+}
+```

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

@@ -0,0 +1,18 @@
+use std::borrow::BorrowMut;
+
+trait NestedBorrowMut<U, V> {
+    fn nested_borrow_mut(&mut self) -> &mut V;
+}
+
+impl<T, U, V> NestedBorrowMut<U, V> for T
+where
+    T: BorrowMut<U>,
+    U: BorrowMut<V>, // Error is caused by missing lifetime here
+{
+    fn nested_borrow_mut(&mut self) -> &mut V {
+        let u_ref = self.borrow_mut(); //~ ERROR E0311
+        u_ref.borrow_mut() //~ ERROR E0311
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,45 @@
+error[E0311]: the parameter type `U` may not live long enough
+  --> $DIR/E0311.rs:13:21
+   |
+LL |         let u_ref = self.borrow_mut();
+   |                     ^^^^^^^^^^^^^^^^^
+   |
+note: the parameter type `U` must be valid for the anonymous lifetime defined here...
+  --> $DIR/E0311.rs:12:26
+   |
+LL |     fn nested_borrow_mut(&mut self) -> &mut V {
+   |                          ^^^^^^^^^
+note: ...so that the type `U` will meet its required lifetime bounds
+  --> $DIR/E0311.rs:13:21
+   |
+LL |         let u_ref = self.borrow_mut();
+   |                     ^^^^^^^^^^^^^^^^^
+help: consider adding an explicit lifetime bound...
+   |
+LL |     U: BorrowMut<V> + 'a, // Error is caused by missing lifetime here
+   |                     ++++
+
+error[E0311]: the parameter type `U` may not live long enough
+  --> $DIR/E0311.rs:14:9
+   |
+LL |         u_ref.borrow_mut()
+   |         ^^^^^^^^^^^^^^^^^^
+   |
+note: the parameter type `U` must be valid for the anonymous lifetime defined here...
+  --> $DIR/E0311.rs:12:26
+   |
+LL |     fn nested_borrow_mut(&mut self) -> &mut V {
+   |                          ^^^^^^^^^
+note: ...so that the type `U` will meet its required lifetime bounds
+  --> $DIR/E0311.rs:14:9
+   |
+LL |         u_ref.borrow_mut()
+   |         ^^^^^^^^^^^^^^^^^^
+help: consider adding an explicit lifetime bound...
+   |
+LL |     U: BorrowMut<V> + 'a, // Error is caused by missing lifetime here
+   |                     ++++
+
+error: aborting due to 2 previous errors
+
+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`.