buildRustPackage: add verifyCargoDeps option

One issue with cargoSha256 is that it's hard to detect when it needs to
be updated or not. It's possible to upgrade a package and forget to
update cargoSha256 and run with old versions of the program or
libraries.

This commit introduces `verifyCargoDeps` which, when enabled, will check
that the Cargo.lock is not out of date in the cargoDeps by comparing it
with the package source.

doc/languages-frameworks/rust.section.md

@@ -43,6 +43,7 @@ rustPlatform.buildRustPackage rec {
   };
 
   cargoSha256 = "0q68qyl2h6i0qsz82z840myxlnjay8p1w5z7hfyr8fqp7wgwa9cx";
+  verifyCargoDeps = true;
 
   meta = with stdenv.lib; {
     description = "A fast line-oriented regex search tool, similar to ag and ack";
@@ -64,6 +65,9 @@ When the `Cargo.lock`, provided by upstream, is not in sync with the
 added in `cargoPatches` will also be prepended to the patches in `patches` at
 build-time.
 
+When `verifyCargoDeps` is set to `true`, the build will also verify that the
+`cargoSha256` is not out of date by comparing the `Cargo.lock` file in both the `cargoDeps` and `src`. Note that this option changes the value of `cargoSha256` since it also copies the `Cargo.lock` in it. To avoid breaking backward-compatibility this option is not enabled by default but hopefully will be in the future.
+
 ## Compiling Rust crates using Nix instead of Cargo
 
 ### Simple operation