Fix dependency graph test cases to have correct commments and use -Z incr-comp

src/test/compile-fail/dep-graph-caller-callee.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Test that two unrelated functions have no trans dependency.
+// Test that immediate callers have to change when callee changes, but
+// not callers' callers.
+
+// compile-flags: -Z incr-comp
 
 #![feature(rustc_attrs)]
 #![allow(dead_code)]

src/test/compile-fail/dep-graph-struct-signature.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Test that two unrelated functions have no trans dependency.
+// Test cases where a changing struct appears in the signature of fns
+// and methods.
+
+// compile-flags: -Z incr-comp
 
 #![feature(rustc_attrs)]
 #![allow(dead_code)]
@@ -68,7 +71,7 @@ mod signatures {
 
     #[rustc_then_this_would_need(ItemSignature)] //~ ERROR OK
     #[rustc_then_this_would_need(CollectItem)] //~ ERROR OK
-    fn indirect(x: WillChange) { }
+    fn indirect(x: WillChanges) { }
 }
 
 // these are invalid dependencies, though sometimes we create edges

src/test/compile-fail/dep-graph-trait-impl-two-traits-same-method.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Test that two unrelated functions have no trans dependency.
+// Test that adding an impl to a trait `Foo` DOES affect functions
+// that only use `Bar` if they have methods in common.
+
+// compile-flags: -Z incr-comp
 
 #![feature(rustc_attrs)]
 #![allow(dead_code)]

src/test/compile-fail/dep-graph-trait-impl-two-traits.rs

@@ -9,7 +9,9 @@
 // except according to those terms.
 
 // Test that adding an impl to a trait `Foo` does not affect functions
-// that only use `Bar`.
+// that only use `Bar`, so long as they do not have methods in common.
+
+// compile-flags: -Z incr-comp
 
 #![feature(rustc_attrs)]
 #![allow(warnings)]

src/test/compile-fail/dep-graph-trait-impl.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Test that two unrelated functions have no trans dependency.
+// Test that when a trait impl changes, fns whose body uses that trait
+// must also be recompiled.
+
+// compile-flags: -Z incr-comp
 
 #![feature(rustc_attrs)]
 #![allow(warnings)]

src/test/compile-fail/dep-graph-unrelated.rs

@@ -10,6 +10,8 @@
 
 // Test that two unrelated functions have no trans dependency.
 
+// compile-flags: -Z incr-comp
+
 #![feature(rustc_attrs)]
 #![allow(dead_code)]