Add back the E0018 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

@@ -23,6 +23,7 @@ E0014: include_str!("./error_codes/E0014.md"),
 E0015: include_str!("./error_codes/E0015.md"),
 E0016: include_str!("./error_codes/E0016.md"),
 E0017: include_str!("./error_codes/E0017.md"),
+E0018: include_str!("./error_codes/E0018.md"),
 E0019: include_str!("./error_codes/E0019.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),

src/librustc_error_codes/error_codes/E0018.md

@@ -0,0 +1,29 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+The value of static and constant integers must be known at compile time. You
+can't cast a pointer to an integer because the address of a pointer can vary.
+
+For example, if you write:
+
+```compile_fail,E0658
+static MY_STATIC: u32 = 42;
+static MY_STATIC_ADDR: usize = &MY_STATIC as *const _ as usize;
+static WHAT: usize = (MY_STATIC_ADDR^17) + MY_STATIC_ADDR;
+```
+
+Then `MY_STATIC_ADDR` would contain the address of `MY_STATIC`. However,
+the address can change when the program is linked, as well as change
+between different executions due to ASLR, and many linkers would
+not be able to calculate the value of `WHAT`.
+
+On the other hand, static and constant pointers can point either to
+a known numeric address or to the address of a symbol.
+
+```
+static MY_STATIC: u32 = 42;
+static MY_STATIC_ADDR: &'static u32 = &MY_STATIC;
+const CONST_ADDR: *const u8 = 0x5f3759df as *const u8;
+```
+
+This does not pose a problem by itself because they can't be
+accessed directly.