Add more info for warnings promoted to errors

src/rust-2021/warnings-promoted-to-error.md

@@ -2,18 +2,48 @@
 
 ## Summary
 
-- `bare-trait-objects` and `ellipsis-inclusive-range-patters` are now hard errors.
+- Code that triggered the `bare_trait_objects` and `ellipsis_inclusive_range_patterns` lints will error in Rust 2021.
 
 ## Details
 
-Two existing lints are becoming hard errors in Rust 2021.
-These lints will remain warnings in older editions.
+Two existing lints are becoming hard errors in Rust 2021, but these lints will remain warnings in older editions.
 
-* `bare-trait-objects`:
-  The use of the `dyn` keyword to identify [trait objects](https://doc.rust-lang.org/book/ch17-02-trait-objects.html)
-  will be mandatory in Rust 2021.
+### `bare_trait_objects`:
 
-* `ellipsis-inclusive-range-patterns`:
-  The [deprecated `...` syntax](https://doc.rust-lang.org/stable/reference/patterns.html#range-patterns)
-  for inclusive range patterns is no longer accepted in Rust 2021.
-  It has been superseded by `..=`, which is consistent with expressions.
+The use of the `dyn` keyword to identify [trait objects](https://doc.rust-lang.org/book/ch17-02-trait-objects.html)
+will be mandatory in Rust 2021.
+
+For example, the following code which does not include the `dyn` keyword in `&MyTrait`
+will produce an error instead of just a lint in Rust 2021:
+
+```rust
+pub trait MyTrait {}
+
+pub fn my_function(_trait_object: &MyTrait) { // should be `&dyn MyTrait`
+  unimplemented!()
+}
+```
+
+### `ellipsis_inclusive_range_patterns`:
+
+The [deprecated `...` syntax](https://doc.rust-lang.org/stable/reference/patterns.html#range-patterns)
+for inclusive range patterns (i.e., ranges where the end value is *included* in the range) is no longer 
+accepted in Rust 2021. It has been superseded by `..=`, which is consistent with expressions.
+
+For example, the following code which uses `...` in a pattern will produce an error instead of 
+just a lint in Rust 2021:
+
+```rust
+pub fn less_or_eq_to_100(n: u8) -> bool {
+  matches!(n, 0...100) // should be `0..=100`
+}
+```
+
+## Migrations 
+
+If your Rust 2015 or 2018 code does not produce any warnings for `bare_trait_objects` 
+or `ellipsis_inclusive_range_patterns` and you've not allowed these lints through the 
+use of `#![allow()]` or some other mechanism, then there's no need to migrate.
+
+To automatically migrate any crate that uses `...` in patterns or does not use `dyn` with
+trait objects, you can run `cargo fix --edition`.