Add E0316.md

Author: FabianWolff

compiler/rustc_error_codes/src/error_codes.rs

@@ -157,6 +157,7 @@ E0308: include_str!("./error_codes/E0308.md"),
 E0309: include_str!("./error_codes/E0309.md"),
 E0310: include_str!("./error_codes/E0310.md"),
 E0312: include_str!("./error_codes/E0312.md"),
+E0316: include_str!("./error_codes/E0316.md"),
 E0317: include_str!("./error_codes/E0317.md"),
 E0321: include_str!("./error_codes/E0321.md"),
 E0322: include_str!("./error_codes/E0322.md"),
@@ -555,7 +556,6 @@ E0783: include_str!("./error_codes/E0783.md"),
            // variable
 //  E0314, // closure outlives stack frame
 //  E0315, // cannot invoke closure outside of its lifetime
-    E0316, // nested quantification of lifetimes
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
     E0320, // recursive overflow during dropck
 //  E0372, // coherence not object safe

compiler/rustc_error_codes/src/error_codes/E0316.md

@@ -0,0 +1,32 @@
+A `where` clause contains a nested quantification over lifetimes.
+
+Erroneous code example:
+
+```compile_fail,E0316
+trait Tr<'a, 'b> {}
+
+fn foo<T>(t: T)
+where
+    for<'a> &'a T: for<'b> Tr<'a, 'b>, // error: nested quantification
+{
+}
+```
+
+Rust syntax allows lifetime quantifications in two places within
+`where` clauses: Quantifying over the trait bound only (as in
+`Ty: for<'l> Trait<'l>`) and quantifying over the whole clause
+(as in `for<'l> &'l Ty: Trait<'l>`). Using both in the same clause
+leads to a nested lifetime quantification, which is not supported.
+
+The following example compiles, because the clause with the nested
+quantification has been rewritten to use only one `for<>`:
+
+```
+trait Tr<'a, 'b> {}
+
+fn foo<T>(t: T)
+where
+    for<'a, 'b> &'a T: Tr<'a, 'b>, // ok
+{
+}
+```