Update posts/2025-01-09-Rust-1.84.0.md

Co-authored-by: Ed Page <eopage@gmail.com>

Author: Mark-Simulacrum

posts/2025-01-09-Rust-1.84.0.md

@@ -28,6 +28,30 @@ aware selection allows library authors to easily adopt newer Rust versions
 while allowing consumers of the library to automatically use old versions
 if they need compatibility with older toolchains.
 
+You can opt-in to the MSRV-aware resolver via [`.cargo/config.toml`](https://doc.rust-lang.org/cargo/reference/config.html#resolverincompatible-rust-versions):
+```toml
+[resolver]
+incompatible-rust-versions = "fallback"
+```
+Then when adding a dependency:
+```console
+$ cargo add clap
+    Updating crates.io index
+warning: ignoring clap@4.5.23 (which requires rustc 1.74) to maintain demo's rust-version of 1.60
+      Adding clap v4.0.32 to dependencies
+    Updating crates.io index
+     Locking 33 packages to latest Rust 1.60 compatible versions
+      Adding clap v4.0.32 (available: v4.5.23, requires Rust 1.74)
+```
+
+When [verifying the latest dependencies in CI](https://doc.rust-lang.org/cargo/guide/continuous-integration.html#verifying-latest-dependencies), you can override this:
+```console
+$ CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS=allow cargo update
+    Updating crates.io index
+     Locking 12 packages to latest compatible versions
+    Updating clap v4.0.32 -> v4.5.23
+```
+
 Library authors should take the MSRV-aware resolver into account when deciding
 their policy on adopting new Rust toolchain features. Previously, a library
 adopting features from a new Rust toolchain would force downstream users of