Rollup merge of #86096 - FabianWolff:ec-E0316, r=GuillaumeGomez

Comment out unused error codes and add description for E0316

I have added an extended description of `E0316` and commented out a bunch of unused error codes to make clear the fact that they are no longer in use. You can check for yourself with
```shell
for ec in                                                \
    E0314 E0315   E0473 E0474 E0475   E0479 E0480 E0481  \
    E0483 E0484 E0485 E0486 E0487 E0488 E0489
do
    if [ ! -z "`grep -r $ec compiler/* --exclude-dir=rustc_error_codes`" ]
    then
        echo $ec
        false
    fi
done
```
i.e. these error codes appear nowhere in the compiler code and thus cannot be emitted.

r? ```@GuillaumeGomez```

3 files changed, 49 insertions, 16 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index c343809179b..f10efd83236 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/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"),
@@ -553,9 +554,8 @@ E0783: include_str!("./error_codes/E0783.md"),
     E0311, // thing may not live long enough
     E0313, // lifetime of borrowed pointer outlives lifetime of captured
            // variable
-    E0314, // closure outlives stack frame
-    E0315, // cannot invoke closure outside of its lifetime
-    E0316, // nested quantification of lifetimes
+//  E0314, // closure outlives stack frame
+//  E0315, // cannot invoke closure outside of its lifetime
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
     E0320, // recursive overflow during dropck
 //  E0372, // coherence not object safe
@@ -584,21 +584,21 @@ E0783: include_str!("./error_codes/E0783.md"),
 //  E0470, removed
 //  E0471, // constant evaluation error (in pattern)
     E0472, // llvm_asm! is unsupported on this target
-    E0473, // dereference of reference outside its lifetime
-    E0474, // captured variable `..` does not outlive the enclosing closure
-    E0475, // index of slice outside its lifetime
+//  E0473, // dereference of reference outside its lifetime
+//  E0474, // captured variable `..` does not outlive the enclosing closure
+//  E0475, // index of slice outside its lifetime
     E0476, // lifetime of the source pointer does not outlive lifetime bound...
-    E0479, // the type `..` (provided as the value of a type parameter) is...
-    E0480, // lifetime of method receiver does not outlive the method call
-    E0481, // lifetime of function argument does not outlive the function call
+//  E0479, // the type `..` (provided as the value of a type parameter) is...
+//  E0480, // lifetime of method receiver does not outlive the method call
+//  E0481, // lifetime of function argument does not outlive the function call
     E0482, // lifetime of return value does not outlive the function call
-    E0483, // lifetime of operand does not outlive the operation
-    E0484, // reference is not valid at the time of borrow
-    E0485, // automatically reference is not valid at the time of borrow
-    E0486, // type of expression contains references that are not valid during..
-    E0487, // unsafe use of destructor: destructor might be called while...
-    E0488, // lifetime of variable does not enclose its declaration
-    E0489, // type/lifetime parameter not in scope here
+//  E0483, // lifetime of operand does not outlive the operation
+//  E0484, // reference is not valid at the time of borrow
+//  E0485, // automatically reference is not valid at the time of borrow
+//  E0486, // type of expression contains references that are not valid during..
+//  E0487, // unsafe use of destructor: destructor might be called while...
+//  E0488, // lifetime of variable does not enclose its declaration
+//  E0489, // type/lifetime parameter not in scope here
     E0490, // a value of type `..` is borrowed for too long
     E0498,  // malformed plugin attribute
     E0514, // metadata version mismatch
diff --git a/compiler/rustc_error_codes/src/error_codes/E0316.md b/compiler/rustc_error_codes/src/error_codes/E0316.md
new file mode 100644
index 00000000000..4368c321737
--- /dev/null
+++ b/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
+{
+}
+```
diff --git a/src/test/ui/where-clauses/where-for-self.stderr b/src/test/ui/where-clauses/where-for-self.stderr
index 84430ffcf88..d06afc1e423 100644
--- a/src/test/ui/where-clauses/where-for-self.stderr
+++ b/src/test/ui/where-clauses/where-for-self.stderr
@@ -6,3 +6,4 @@ LL |     where for<'a> &'a T: for<'b> Bar<'b>
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0316`.