config list-builder: Allow overriding the per-item build method

This will be useful especially for simple lists where the entry
doesn't need a separate builder type.

Cargo.lock

@@ -3342,6 +3342,7 @@ dependencies = [
  "serde",
  "shellexpand-fork",
  "thiserror",
+ "tor-basic-utils",
  "tor-error",
  "tracing",
  "tracing-test",

crates/tor-config/Cargo.toml

@@ -15,6 +15,7 @@ default = ["expand-paths"]
 expand-paths = ["shellexpand", "directories"]
 
 [dependencies]
+tor-basic-utils = { path="../tor-basic-utils", version = "0.2.0"}
 tor-error = { path="../tor-error", version = "0.2.0"}
 
 thiserror = "1"

crates/tor-config/src/lib.rs

@@ -55,6 +55,8 @@ pub use err::{ConfigBuildError, ReconfigureError};
 pub use mut_cfg::MutCfg;
 pub use path::CfgPath;
 
+pub use tor_basic_utils::macro_coalesce_args;
+
 /// Rules for reconfiguring a running Arti instance.
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
 #[non_exhaustive]

crates/tor-config/src/list_builder.rs

@@ -17,6 +17,10 @@
 /// nontrivial, you should put the actual defaulting functionality in a (probably-private)
 /// function, as the macro will expand it twice.
 ///
+/// The `item_build` clause, if supplied, provides a closure with type
+/// `FnMut(&ThingBuilder) -> Result<Thing, ConfigBuildErro>`; the default is to call
+/// `thing_builder.build()`.
+///
 /// ```
 /// use derive_builder::Builder;
 /// use serde::Deserialize;
@@ -59,6 +63,7 @@ macro_rules! define_list_config_builder {
         }
         built: $Built:ty = $built:expr;
         default = $default:expr;
+        $( item_build: $item_build:expr; )?
     } => {
         $($docs_and_attrs)*
         #[derive(Default, Clone, Deserialize)]
@@ -100,10 +105,16 @@ macro_rules! define_list_config_builder {
                         &default_buffer
                     }
                 };
+
                 let $things = $things
                     .iter()
-                    .map(|item| item.build())
+                    .map(
-                    .collect::<Result<_, _>>()?;
+                        $crate::macro_coalesce_args!{
+                            [ $( $item_build )? ],
+                            [ |item| item.build() ],
+                        }
+                    )
+                    .collect::<Result<_, ConfigBuildError>>()?;
                 Ok($built)
             }
         }