Improve README.md

Author: smoelius

README.md

@@ -41,11 +41,7 @@ fn as_u16(x: u64) -> u16 {
 
 ### With a nightly compiler
 
-**We recommend enabling Rust features [`custom_inner_attributes`] and [`proc_macro_hygiene`], and compiling with the [`procmacro2_semver_exempt`] config flag, e.g.:**
-
-```sh
-RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
-```
+**We recommend enabling Rust features [`custom_inner_attributes`] and [`proc_macro_hygiene`].**
 
 If you enable the [`custom_inner_attributes`] and [`proc_macro_hygiene`] features, you can use `cast_checks::enable` as an inner [attribute]. Example:
 
@@ -59,17 +55,15 @@ mod m {
 }
 ```
 
-However, in our experience, this can cause panics to refer to the wrong locations.
+## `CAST_CHECKS_LOG`
 
-To help counter the above, if you compile with the [`procmacro2_semver_exempt`] config flag, `cast_checks` will include more elaborate `expect` messages. Example:
+If you are concerned that some casts are not being checked, try setting `CAST_CHECKS_LOG` and passing the [`procmacro2_semver_exempt`] config flag when compiling, e.g.:
 
-```text
-thread 'checked_truncation' panicked at 'invalid cast in `x as u8` at cast_checks/tests/basic.rs:30:13: TryFromIntError(())', cast_checks/tests/basic.rs:30:13
+```sh
+CAST_CHECKS_LOG=1 RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
 ```
 
-## `CAST_CHECKS_LOG`
-
-If you are concerned that some casts are not being checked, try setting `CAST_CHECKS_LOG` when compiling. This will cause `cast_checks` to dump to standard output:
+This will cause `cast_checks` to dump to standard output:
 
 - all rewritten locations
 - all modules whose contents are not visited because they are not inlined