Provide a public API to obtain `IngredientIndex` · salsa-rs/salsa@d0ca81e

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useless conversion to the same type: `impl std::iter::Iterator<Item = &salsa_struct::SalsaField>`

the borrowed expression implements the required traits

usage of `Iterator::fold` on a type that implements `Try`

`salsa-2022-macros` (lib) generated 3 warnings (run `cargo clippy --fix --lib -p salsa-2022-macros` to apply 2 suggestions)

the following explicit lifetimes could be elided: 'b

the following explicit lifetimes could be elided: 'u

the following explicit lifetimes could be elided: 'u

`salsa-2022` (lib) generated 3 warnings (run `cargo clippy --fix --lib -p salsa-2022` to apply 3 suggestions)

`salsa-2022-macros` (lib test) generated 3 warnings (3 duplicates)

this `impl` can be derived

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