Reorganize the instruction builder traits.

Leave the primary InstBuilderBase trait alone, but add an alternative
InstInserterBase trait that can be implemented instead by builders that
always allocate new instructions with dfg.make_inst().

Any implementation of InstInserterBase can be used as an instruction
builder by wrapping it in an InsertBuilder. The InsertBuilder type adds
additional functionality via the with_results() method which makes it
possible to override the result values on the instruction that is built.

The motivation for this shuffle is that the with_result() functionality
can now be reused by different kinds of instruction builders, as long as
they insert new instructions. So ReplaceBuilder doesn't get
with_results().

lib/cretonne/src/ir/layout.rs

@@ -7,7 +7,8 @@ use std::cmp;
 use std::iter::{Iterator, IntoIterator};
 use entity_map::EntityMap;
 use packed_option::PackedOption;
-use ir::entities::{Ebb, Inst};
+use ir::{Ebb, Inst, Type, DataFlowGraph};
+use ir::builder::InstInserterBase;
 use ir::progpoint::{ProgramOrder, ExpandedProgramPoint};
 
 /// The `Layout` struct determines the layout of EBBs and instructions in a function. It does not
@@ -1044,6 +1045,38 @@ impl<'f> Cursor<'f> {
     }
 }
 
+/// An instruction inserter which can be used to build and insert instructions at a cursor
+/// position.
+///
+/// This is used by `dfg.ins()`.
+pub struct LayoutCursorInserter<'c, 'fc: 'c, 'fd> {
+    pos: &'c mut Cursor<'fc>,
+    dfg: &'fd mut DataFlowGraph,
+}
+
+impl<'c, 'fc: 'c, 'fd> LayoutCursorInserter<'c, 'fc, 'fd> {
+    /// Create a new inserter. Don't use this, use `dfg.ins(pos)`.
+    pub fn new(pos: &'c mut Cursor<'fc>,
+               dfg: &'fd mut DataFlowGraph)
+               -> LayoutCursorInserter<'c, 'fc, 'fd> {
+        LayoutCursorInserter { pos, dfg }
+    }
+}
+
+impl<'c, 'fc: 'c, 'fd> InstInserterBase<'fd> for LayoutCursorInserter<'c, 'fc, 'fd> {
+    fn data_flow_graph(&self) -> &DataFlowGraph {
+        self.dfg
+    }
+
+    fn data_flow_graph_mut(&mut self) -> &mut DataFlowGraph {
+        self.dfg
+    }
+
+    fn insert_built_inst(self, inst: Inst, _ctrl_typevar: Type) -> &'fd mut DataFlowGraph {
+        self.pos.insert_inst(inst);
+        self.dfg
+    }
+}
 
 #[cfg(test)]
 mod tests {