Add E0788 for improper #[no_coverage] usage

compiler/rustc_error_codes/src/error_codes/E0788.md

@@ -0,0 +1,21 @@
+A `#[no_coverage]` attribute was incorrectly placed on something that couldn't
+be covered.
+
+Example of erroneous code:
+
+```compile_fail,E0788
+#[no_coverage]
+struct Foo;
+
+#[no_coverage]
+const FOO: Foo = Foo;
+```
+
+`#[no_coverage]` tells the compiler to not generate coverage instrumentation for
+a piece of code when the `-C instrument-coverage` flag is passed. Things like
+structs and consts are not coverable code, and thus cannot do anything with this
+attribute.
+
+If you wish to apply this attribute to all methods in an impl or module,
+manually annotate each method; it is not possible to annotate the entire impl
+with a `#[no_coverage]` attribute.