nix hash convert: Add manpage

src/nix/hash-convert.md

@@ -0,0 +1,40 @@
+R""(
+
+# Examples
+
+* Convert a hash to `nix32` (a base-32 encoding with a Nix-specific character set).
+
+  ```console
+  $ nix hash convert --hash-algo sha1 --to nix32 800d59cfcd3c05e900cb4e214be48f6b886a08df
+  vw46m23bizj4n8afrc0fj19wrp7mj3c0
+  ```
+
+* Convert a hash to [the `sri` format](https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity) that includes an algorithm specification:
+
+  ```console
+  # nix hash convert --hash-algo sha1 800d59cfcd3c05e900cb4e214be48f6b886a08df
+  sha1-gA1Zz808BekAy04hS+SPa4hqCN8=
+  ```
+
+  or with an explicit `--to` format:
+
+  ```console
+  # nix hash convert --hash-algo sha1 --to sri 800d59cfcd3c05e900cb4e214be48f6b886a08df
+  sha1-gA1Zz808BekAy04hS+SPa4hqCN8=
+  ```
+
+* Assert the input format of the hash:
+
+  ```console
+  # nix hash convert --hash-algo sha256 --from nix32 ungWv48Bz+pBQUDeXa4iI7ADYaOWF3qctBD/YfIAFa0=
+  error: input hash 'ungWv48Bz+pBQUDeXa4iI7ADYaOWF3qctBD/YfIAFa0=' does not have the expected format '--from nix32'
+
+  # nix hash convert --hash-algo sha256 --from nix32 1b8m03r63zqhnjf7l5wnldhh7c134ap5vpj0850ymkq1iyzicy5s
+  sha256-ungWv48Bz+pBQUDeXa4iI7ADYaOWF3qctBD/YfIAFa0=
+  ```
+
+# Description
+
+`nix hash convert` converts hashes from one encoding to another.
+
+)""

src/nix/hash.cc

@@ -150,15 +150,14 @@ struct CmdHashConvert : Command
 
     std::string description() override
     {
-        std::string descr( "convert between different hash formats. Choose from: ");
-        auto iter = hashFormats.begin();
-        assert(iter != hashFormats.end());
-        descr += *iter++;
-        while (iter != hashFormats.end()) {
-            descr += ", " + *iter++;
-        }
+        return "convert between hash formats";
+    }
 
-        return descr;
+    std::string doc() override
+    {
+        return
+          #include "hash-convert.md"
+          ;
     }
 
     Category category() override { return catUtility; }