compiler: assume external functions don't let pointers escape

Assume any external function won't let pointers live longer than the
call itself. This is true in the vast majority of cases (apparently
everywhere currently) but might not always be true.

TODO: add a //go:noescape (or maybe //go:escape) to handle this, instead
of this assumption.

compiler/optimizer.go

@@ -219,6 +219,16 @@ func (c *Compiler) doesEscape(value llvm.Value) bool {
 			// Call only escapes when the (pointer) parameter is not marked
 			// "nocapture". This flag means that the parameter does not escape
 			// the give function.
+			if use.CalledValue().IsAFunction() != nilValue {
+				if use.CalledValue().IsDeclaration() {
+					// Kind of dirty: assume external functions don't let
+					// pointers escape.
+					// TODO: introduce //go:noescape that sets the 'nocapture'
+					// flag on each input parameter.
+					println("declaration:", use.CalledValue().Name())
+					continue
+				}
+			}
 			if !c.hasFlag(use, value, "nocapture") {
 				return true
 			}