Revert "Permit multiple lifetime bounds on parameter types",

which was pushed accidentally.

This reverts commit 5e8d273.

Author: nikomatsakis

active/0049-bounds-on-object-and-generic-types.md

@@ -69,6 +69,24 @@ declared:
 Here, the constraint `T:'a` indicates that the data being iterated
 over must live at least as long as the collection (logically enough).
 
+### At most one explicit lifetime bound is permitted
+
+For simplicity, we permit at most one *explicit* lifetime bound on any
+given parameter type. That means that the following function is illegal:
+
+    fn foo<'a,'b,A:'a+'b>() { ... }
+
+Remember that if there are multiple lifetime bounds, it implies that
+all of them must hold. That means that if, in fact, `A` outlives both
+`'a` and `'b` then either one of them is shorter than the other, the
+two are the same, or there is a third lifetime that outlives them
+both. Therefore, the function above can be rewritten as follows (using
+explicit lifetime bounds, specified below):
+
+    fn foo<'a,'b,'c:'a+'b,A:'c>() { ... }
+
+As far as I know, this situation has not arisen once in the codebase.
+
 ## Lifetime bounds on object types
 
 Like parameters, all object types have a lifetime bound. Unlike
@@ -95,9 +113,9 @@ Here are some examples:
     // IsStatic+'a        'static+'a
     // IsStatic+Is<'a>+'b 'static,'a,'b
 
-Object types must have exactly one bound -- zero bounds is not
-acceptable. Therefore, if an object type with no derivable bounds
-appears, we will supply a default lifetime using the normal rules:
+In general no object type is permitted to have zero bounds. Therefore,
+if an object type with no derivable bounds appears, we will supply a
+default lifetime using the normal rules:
 
     trait Writer { /* no derivable bounds */ }
     struct Foo<'a> {