Add test for const "equivalent"

Author: estebank

tests/ui/structs/manual-default-impl-could-be-derived.fixed

@@ -79,12 +79,27 @@ fn foo() -> K {
 }
 
 // Verify that cross-crate tracking of "equivalences" keeps working.
-#[derive(PartialEq, Debug)]
 #[derive(Default)] struct L {
     x: Vec<i32>,
 }
 
 
+// Account for `const`s
+#[derive(Default)] struct M {
+    x: N,
+}
+
+
+struct N;
+
+impl Default for N { // ok
+    fn default() -> Self {
+        N_CONST
+    }
+}
+
+const N_CONST: N = N;
+
 fn main() {
     let _ = A::default();
     let _ = B::default();
@@ -98,4 +113,6 @@ fn main() {
     let _ = J::default();
     let _ = K::default();
     let _ = L::default();
+    let _ = M::default();
+    let _ = N::default();
 }

tests/ui/structs/manual-default-impl-could-be-derived.rs

@@ -137,7 +137,6 @@ fn foo() -> K {
 }
 
 // Verify that cross-crate tracking of "equivalences" keeps working.
-#[derive(PartialEq, Debug)]
 struct L {
     x: Vec<i32>,
 }
@@ -150,6 +149,29 @@ impl Default for L { //~ ERROR
     }
 }
 
+// Account for `const`s
+struct M {
+    x: N,
+}
+
+impl Default for M { //~ ERROR
+    fn default() -> Self {
+        M {
+            x: N_CONST,
+        }
+    }
+}
+
+struct N;
+
+impl Default for N { // ok
+    fn default() -> Self {
+        N_CONST
+    }
+}
+
+const N_CONST: N = N;
+
 fn main() {
     let _ = A::default();
     let _ = B::default();
@@ -163,4 +185,6 @@ fn main() {
     let _ = J::default();
     let _ = K::default();
     let _ = L::default();
+    let _ = M::default();
+    let _ = N::default();
 }

tests/ui/structs/manual-default-impl-could-be-derived.stderr

@@ -176,7 +176,7 @@ LL ~ #[derive(Default)] struct J {
    |
 
 error: `impl Default` that could be derived
-  --> $DIR/manual-default-impl-could-be-derived.rs:145:1
+  --> $DIR/manual-default-impl-could-be-derived.rs:144:1
    |
 LL | / impl Default for L {
 LL | |     fn default() -> Self {
@@ -192,5 +192,22 @@ help: you don't need to manually `impl Default`, you can derive it
 LL ~ #[derive(Default)] struct L {
    |
 
-error: aborting due to 11 previous errors
+error: `impl Default` that could be derived
+  --> $DIR/manual-default-impl-could-be-derived.rs:157:1
+   |
+LL | / impl Default for M {
+LL | |     fn default() -> Self {
+LL | |         M {
+LL | |             x: N_CONST,
+LL | |         }
+LL | |     }
+LL | | }
+   | |_^
+   |
+help: you don't need to manually `impl Default`, you can derive it
+   |
+LL ~ #[derive(Default)] struct M {
+   |
+
+error: aborting due to 12 previous errors