doc/test: add UI test and reword docs for E0013 and E0015

Author: Ezrashaw

compiler/rustc_error_codes/src/error_codes/E0015.md

@@ -1,5 +1,4 @@
-A constant item was initialized with something that is not a constant
-expression.
+A non-`const` function was called in a `const` context.
 
 Erroneous code example:
 
@@ -8,26 +7,20 @@ fn create_some() -> Option<u8> {
     Some(1)
 }
 
-const FOO: Option<u8> = create_some(); // error!
+// error: cannot call non-const fn `create_some` in constants
+const FOO: Option<u8> = create_some();
 ```
 
-The only functions that can be called in static or constant expressions are
-`const` functions, and struct/enum constructors.
+All functions used in a `const` context (constant or static expression) must
+be marked `const`.
 
 To fix this error, you can declare `create_some` as a constant function:
 
 ```
-const fn create_some() -> Option<u8> { // declared as a const function
+// declared as a `const` function:
+const fn create_some() -> Option<u8> {
     Some(1)
 }
 
-const FOO: Option<u8> = create_some(); // ok!
-
-// These are also working:
-struct Bar {
-    x: u8,
-}
-
-const OTHER_FOO: Option<u8> = Some(1);
-const BAR: Bar = Bar {x: 1};
+const FOO: Option<u8> = create_some(); // no error!
 ```

src/test/ui/error-codes/E0013.rs

@@ -0,0 +1,4 @@
+static X: i32 = 42;
+const Y: i32 = X; //~ ERROR constants cannot refer to statics [E0013]
+
+fn main() {}

src/test/ui/error-codes/E0013.stderr

@@ -0,0 +1,11 @@
+error[E0013]: constants cannot refer to statics
+  --> $DIR/E0013.rs:2:16
+   |
+LL | const Y: i32 = X;
+   |                ^
+   |
+   = help: consider extracting the value of the `static` to a `const`, and referring to that
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0013`.

src/test/ui/error-codes/E0015.rs

@@ -0,0 +1,8 @@
+fn create_some() -> Option<u8> {
+    Some(1)
+}
+
+const FOO: Option<u8> = create_some();
+//~^ ERROR cannot call non-const fn `create_some` in constants [E0015]
+
+fn main() {}

src/test/ui/error-codes/E0015.stderr

@@ -0,0 +1,11 @@
+error[E0015]: cannot call non-const fn `create_some` in constants
+  --> $DIR/E0015.rs:5:25
+   |
+LL | const FOO: Option<u8> = create_some();
+   |                         ^^^^^^^^^^^^^
+   |
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.