implement calling of const fn-methods in true constants

src/test/compile-fail/const-eval-span.rs

@@ -14,7 +14,7 @@
 struct S(i32);
 
 const CONSTANT: S = S(0);
-//~^ ERROR: constant evaluation error: non-constant path in constant expression [E0080]
+//~^ ERROR: constant evaluation error: call on struct [E0080]
 
 enum E {
     V = CONSTANT,

src/test/compile-fail/const-fn-stability-calls-2.rs

@@ -17,5 +17,6 @@ extern crate const_fn_lib;
 use const_fn_lib::foo;
 
 fn main() {
-    let x: [usize; foo()] = []; //~ ERROR non-constant path in constant expr
+    let x: [usize; foo()] = [];
+    //~^ ERROR unimplemented constant expression: calling non-local const fn [E0250]
 }

src/test/run-pass/const-fn-method.rs

@@ -22,4 +22,5 @@ const FOO: Foo = Foo::new();
 
 pub fn main() {
     assert_eq!(FOO.value, 22);
+    let _: [&'static str; Foo::new().value as usize] = ["hey"; 22];
 }

src/test/run-pass/const-fn.rs

@@ -29,4 +29,5 @@ fn main() {
 
     assert_eq!(DIFF, 22);
 
+    let _: [&'static str; sub(100, 99) as usize] = ["hi"];
 }