librustc: Tie up loose ends in unboxed closures.

This patch primarily does two things: (1) it prevents lifetimes from
leaking out of unboxed closures; (2) it allows unboxed closure type
notation, call notation, and construction notation to construct closures
matching any of the three traits.

This breaks code that looked like:

    let mut f;
    {
        let x = &5i;
        f = |&mut:| *x + 10;
    }

Change this code to avoid having a reference escape. For example:

    {
        let x = &5i;
        let mut f; // <-- move here to avoid dangling reference
        f = |&mut:| *x + 10;
    }

I believe this is enough to consider unboxed closures essentially
implemented. Further issues (for example, higher-rank lifetimes) should
be filed as followups.

Closes #14449.

[breaking-change]

src/libsyntax/ast.rs

@@ -528,7 +528,7 @@ pub enum Expr_ {
     ExprMatch(Gc<Expr>, Vec<Arm>),
     ExprFnBlock(CaptureClause, P<FnDecl>, P<Block>),
     ExprProc(P<FnDecl>, P<Block>),
-    ExprUnboxedFn(CaptureClause, P<FnDecl>, P<Block>),
+    ExprUnboxedFn(CaptureClause, UnboxedClosureKind, P<FnDecl>, P<Block>),
     ExprBlock(P<Block>),
 
     ExprAssign(Gc<Expr>, Gc<Expr>),
@@ -900,6 +900,7 @@ pub struct BareFnTy {
 
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub struct UnboxedFnTy {
+    pub kind: UnboxedClosureKind,
     pub decl: P<FnDecl>,
 }
 
@@ -1297,6 +1298,13 @@ pub enum ForeignItem_ {
     ForeignItemStatic(P<Ty>, /* is_mutbl */ bool),
 }
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub enum UnboxedClosureKind {
+    FnUnboxedClosureKind,
+    FnMutUnboxedClosureKind,
+    FnOnceUnboxedClosureKind,
+}
+
 /// The data we save and restore about an inlined item or method.  This is not
 /// part of the AST that we parse from a file, but it becomes part of the tree
 /// that we trans.