BuildRustCrate: more general overrides, and handling the "dylib" crate type (#35171)

* buildRustCrate: adding a symlink from libblah-xxxxx.so to libblah.so
* BuildRustCrate: overriding phases
* Carnix: 0.6.5 -> 0.6.6
* Fixing symlink_dependencies --buildDep
* Shorter symlink_dependencies
* running `runHook postBuild` *after* the build

doc/languages-frameworks/rust.md

@@ -58,6 +58,8 @@ To install crates with nix there is also an experimental project called
 
 ## Compiling Rust crates using Nix instead of Cargo
 
+### Simple operation
+
 When run, `cargo build` produces a file called `Cargo.lock`,
 containing pinned versions of all dependencies. Nixpkgs contains a
 tool called `carnix` (`nix-env -iA nixos.carnix`), which can be used
@@ -153,6 +155,8 @@ Here, the `libc` crate has no `src` attribute, so `buildRustCrate`
 will fetch it from [crates.io](https://crates.io). A `sha256`
 attribute is still needed for Nix purity.
 
+### Handling external dependencies
+
 Some crates require external libraries. For crates from
 [crates.io](https://crates.io), such libraries can be specified in
 `defaultCrateOverrides` package in nixpkgs itself.
@@ -210,7 +214,10 @@ with import <nixpkgs> {};
 }
 ```
 
-Three more parameters can be overridden:
+### Options and phases configuration
+
+Actually, the overrides introduced in the previous section are more
+general. A number of other parameters can be overridden:
 
 - The version of rustc used to compile the crate:
 
@@ -232,6 +239,30 @@ Three more parameters can be overridden:
   (hello {}).override { verbose = false; };
   ```
 
+- Extra arguments to be passed to `rustc`:
+
+  ```
+  (hello {}).override { extraRustcOpts = "-Z debuginfo=2"; };
+  ```
+
+- Phases, just like in any other derivation, can be specified using
+  the following attributes: `preUnpack`, `postUnpack`, `prePatch`,
+  `patches`, `postPatch`, `preConfigure` (in the case of a Rust crate,
+  this is run before calling the "build" script), `postConfigure`
+  (after the "build" script),`preBuild`, `postBuild`, `preInstall` and
+  `postInstall`. As an example, here is how to create a new module
+  before running the build script:
+
+  ```
+  (hello {}).override {
+    preConfigure = ''
+       echo "pub const PATH=\"${hi.out}\";" >> src/path.rs"
+    '';
+  };
+  ```
+
+### Features
+
 One can also supply features switches. For example, if we want to
 compile `diesel_cli` only with the `postgres` feature, and no default
 features, we would write:
@@ -243,7 +274,7 @@ features, we would write:
 }
 ```
 
-
+Where `diesel.nix` is the file generated by Carnix, as explained above.
 
 ## Using the Rust nightlies overlay