Emit spanned diagnostics for invalid #[setting] attributes instead of panicking

• Status: Todo
• Kind: Chore
• Authors: jp
• Date: 2026-08-24
• Label: package=schematic
• Label: type=enhancement

Field::from (crates/contrib/schematic_macros/src/common/field.rs) treats an invalid #[setting] / #[schema] attribute as fatal:

let args = FieldArgs::from_attributes(&field.attrs).unwrap_or_else(|error| {
    panic!("Invalid `#[setting]` or `#[schema]` attribute on field `{name}`: {error}");
});

The hard failure is correct and has already earned its keep — it surfaced four fields whose #[setting] keys had been silently discarded, two of which had a declared merge strategy that never applied. What it loses is the span: a proc-macro panic surfaces as

error: proc-macro derive panicked
  --> crates/jp_config/src/lib.rs:96:35
   |
96 | #[derive(Debug, Clone, PartialEq, Config)]
   |                                   ^^^^^^
   = help: message: Invalid `#[setting]` ... on field `inherit`: Unknown field: `optional`

anchored on the whole #[derive(Config)], with darling's span discarded and the real message demoted to a help: note. On a struct with thirty fields the compiler cannot point at the offending key.

In practice the struct location plus the field name in the message is usually enough to find it, which is why this is a papercut rather than a bug.

Fix

The idiomatic shape is Field::from returning darling::Result<Field> and the derive entry point emitting error.write_errors(), which produces a spanned compile_error! at the offending key.

That threads Result through Container::from, the struct and enum builders, and both derive entry points — a refactor across the macro crate, larger than the schematic changes in #887 combined. It should carry its own compile-fail test coverage to be worth doing.

Explicitly not worth a shortcut: a thread-local error accumulator would avoid the signature churn but is exactly the kind of subtle statefulness this crate does not need.

Context

Raised in review on #887 (comment 3659116544), where the reviewer scoped it as a follow-up.