Change precedence of + in type grammar

text/0000-precedence-of-plus.md

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Change the precedence of `+` (object bounds) in type grammar so that
+it is similar to the precedence in the expression grammars.
+
+# Motivation
+
+Currently `+` in types has a much higher precedence than it does in expressions.
+This means that for example one can write a type like the following:
+
+```
+&Object+Send
+```
+
+Whereas if that were an expression, parentheses would be required:
+
+```rust
+&(Object+Send)
+````
+
+Besides being confusing in its own right, this loose approach with
+regard to precedence yields ambiguities with unboxed closure bounds:
+
+```rust
+fn foo<F>(f: F)
+    where F: FnOnce(&int) -> &Object + Send
+{ }
+```
+
+In this example, it is unclear whether `F` returns an object which is
+`Send`, or whether `F` itself is `Send`.
+
+# Detailed design
+
+This RFC proposes that the precedence of `+` be made lower than unary
+type operators. In addition, the grammar is segregated such that in
+"open-ended" contexts (e.g., after `->`), parentheses are required to
+use a `+`, whereas in others (e.g., inside `<>`), parentheses are not.
+Here are some examples:
+
+```rust
+// Before                             After                         Note
+// ~~~~~~                             ~~~~~                         ~~~~
+   &Object+Send                       &(Object+Send)
+   &'a Object+'a                      &'a (Object+'a)
+   Box<Object+Send>                   Box<Object+Send>
+   foo::<Object+Send,int>(...)        foo::<Object+Send,int>(...)
+   Fn() -> Object+Send                Fn() -> (Object+Send)         // (*)
+   Fn() -> &Object+Send               Fn() -> &(Object+Send)
+
+// (*) Must yield a type error, as return type must be `Sized`.
+```
+
+More fully, the type grammar is as follows (EBNF notation):
+
+    TYPE = PATH
+         | '&' [LIFETIME] TYPE
+         | '&' [LIFETIME] 'mut' TYPE
+         | '*' 'const' TYPE
+         | '*' 'mut' TYPE
+         | ...
+         | '(' SUM ')'
+    SUM  = TYPE { '+' TYPE }
+    PATH = IDS '<' SUM { ',' SUM } '>'
+         | IDS '(' SUM { ',' SUM } ')' '->' TYPE
+    IDS  = ['::'] ID { '::' ID }
+
+Where clauses would use the following grammar:
+
+    WHERE_CLAUSE = PATH { '+' PATH }
+
+One property of this grammar is that the `TYPE` nonterminal does not
+require a terminator as it has no "open-ended" expansions. `SUM`, in
+contrast, can be extended any number of times via the `+` token. Hence
+is why `SUM` must be enclosed in parens to make it into a `TYPE`.
+
+# Drawbacks
+
+Common types like `&'a Foo+'a` become slightly longer (`&'a (Foo+'a)`).
+
+# Alternatives
+
+We could live with the inconsistency between the type/expression
+grammars and disambiguate where clauses in an ad-hoc way.
+
+# Unresolved questions
+
+None.