SE-0377: Clarify that passing a non-implicitly-copyable binding as an argument is an error if it copies.

proposals/0377-parameter-ownership-modifiers.md

@@ -232,7 +232,6 @@ extension String {
 }
 ```
 
-
 A value would need to be implicitly copied if:
 
 - a *consuming operation* is applied to a `borrowing` binding, or
@@ -289,6 +288,28 @@ func baz(a: consuming String) {
 }
 ```
 
+To clarify the boundary within which the no-implicit-copy constraint applies, a
+parameter binding's value *is* noncopyable as part of the *call expression* in
+the caller, so if forming the call requires copying, that will raise an error,
+even if the parameter would be implicitly copyable in the callee. The function
+body serves as the boundary for the no-implicit-copy constraint:
+
+```
+struct Bar {
+    var a: String
+    var b: String
+    init(ab: String) {
+        // OK, ab is implicitly copyable here
+        a = ab
+        b = ab
+    }
+}
+
+func foo(x: borrowing String) {
+    _ = Bar(ab: x) // ERROR: would need to copy `x` to let `Bar.init` consume it
+}
+```
+
 ## Source compatibility
 
 Adding `consuming` or `borrowing` to a parameter in the language today does not