Modernize field!/sign!/hashmap! macros to emit typed SignatureSchema

[darinkishore]

Context

lib.rs defines three macros:

• field! (lines 46-133) — generates serde_json::Value maps via schemars. Only used in tests/test_field_macro.rs.
• sign! (lines 135-171) — inline signature + Predict creation. Zero callers currently.
• hashmap! (lines 178-187) — convenience HashMap literal. Used in tests/examples.

These are ergonomic features for the untyped path. field! and sign! use schemars::schema_for!() to generate JSON schemas, which is the pre-V1 approach. The typed path uses #[derive(Signature)] + SignatureSchema::of::<S>().

What's needed

field! macro

• Should emit typed FieldSchema values instead of serde_json::Value maps
• Or consider if #[derive(Signature)] makes this macro redundant

sign! macro

• Should work with the typed Signature derive and produce a Predict<__InlineSignature> that's fully wired (accessor registration, etc.)
• Currently generates a derive but doesn't call register_predict_accessor or build()

hashmap! macro

• Low priority. HashMap::from([...]) is standard Rust. Keep or kill, doesn't matter much.

schemars dependency

• __macro_support re-exports schemars solely for field!
• Once field! is modernized, schemars can be dropped from Cargo.toml (also requires specials.rs migration, see related issue)

Related

• fully remove untyped signatures #52 (remove untyped signatures)
• specials.rs migration issue

[darinkishore]

Codebase audit — current macro state

field! (lib.rs:148-236):

• Emits serde_json::Value objects via schemars::schema_for!()
• Only caller: tests/test_field_macro.rs
• Useful ergonomic: declare fields inline without a full #[derive(Signature)] struct

sign! (lib.rs:238-274):

• Creates an anonymous __InlineSignature struct with #[derive(Signature)] and returns Predict::<__InlineSignature>::new()
• Zero callers currently
• Would be great for quick prototyping: let p = sign!((question: String) -> answer: String);

hashmap! (lib.rs:276-290):

• Standard convenience macro. Works fine, no changes needed.

Modernization path for field!:

• Should emit FieldSchema (from core/schema.rs) instead of serde_json::Value
• Use facet::Shape or BamlType::baml_type_ir() instead of schemars::schema_for!()
• Example new output:

  field! { input["Description"] => question: String }
  // → FieldSchema { name: "question", description: "Description", type_ir: String::baml_type_ir(), ... }

Modernization path for sign!:

• Currently works since it does #[derive(Signature)] — but it doesn't call register_predict_accessor or wire up the builder pattern
• Should produce a fully functional Predict that works with optimizers
• Consider if this needs to produce a named type (for optimizer walker to find) or if anonymous is fine for non-optimized use

schemars removal: once field! uses BamlType::baml_type_ir() instead of schemars::schema_for!(), the schemars dep in __macro_support and Cargo.toml can be dropped (assuming #72 is also done).