long error explanation for E0313

compiler/rustc_error_codes/src/error_codes.rs

@@ -161,6 +161,7 @@ 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"),
+E0313: include_str!("./error_codes/E0313.md"),
 E0316: include_str!("./error_codes/E0316.md"),
 E0317: include_str!("./error_codes/E0317.md"),
 E0321: include_str!("./error_codes/E0321.md"),
@@ -569,8 +570,6 @@ E0790: include_str!("./error_codes/E0790.md"),
 //  E0300, // unexpanded macro
 //  E0304, // expected signed integer constant
 //  E0305, // expected constant
-    E0313, // lifetime of borrowed pointer outlives lifetime of captured
-           // variable
 //  E0314, // closure outlives stack frame
 //  E0315, // cannot invoke closure outside of its lifetime
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums

compiler/rustc_error_codes/src/error_codes/E0313.md

@@ -0,0 +1,43 @@
+Lifetime of borrowed pointer outlives lifetime of captured variable.
+
+Erroneous code example:
+
+```compile_fail,E0313
+fn foo(bar: &'static mut ()) {
+    || foo(bar);
+}
+```
+
+This error is raised because the body of the closure, which implements `FnMut`,
+has a lifetime shorter than `'static`. Let's name this lifetime `'fooc`. This is
+problematic because `foo`'s argument requires that `'fooc` must outlive
+`'static`, but references to captured variables in a `FnMut` closure can't
+escape the closure.
+
+To fix this error, define a temporary variable inside the closure that takes
+ownership of `foo`'s argument. Then, pass this into `foo`:
+
+```
+fn foo(bar: &'static mut ()) {
+    || {
+        let baz = bar;
+        foo(baz);
+    };
+}
+```
+
+This works because `bar` must be moved into the closure for the body to be
+sensical. The closure now implements `FnOnce` because the value is moved in
+then moved out in order to call the function, meaning it can only be called
+once.
+
+Additionally, it seems like the following would also work:
+
+```
+fn foo(bar: &'static mut ()) {
+    move || foo(bar);
+}
+```
+
+Although the closure is forced to capture its environment by value, which is
+what `FnOnce` does, `move` closures can still implement `FnMut` (or `Fn`).

src/tools/tidy/src/error_codes_check.rs

@@ -11,8 +11,8 @@ use regex::Regex;
 
 // A few of those error codes can't be tested but all the others can and *should* be tested!
 const EXEMPTED_FROM_TEST: &[&str] = &[
-    "E0313", "E0377", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519", "E0523",
-    "E0554", "E0640", "E0717", "E0729", "E0789",
+    "E0377", "E0461", "E0462", "E0465", "E0476", "E0490", "E0514", "E0519", "E0523", "E0554",
+    "E0640", "E0717", "E0729", "E0789",
 ];
 
 // Some error codes don't have any tests apparently...