Support infallible EnumString for enums with a default variant#432
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scovich wants to merge 2 commits intoPeternator7:masterfrom
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Support infallible EnumString for enums with a default variant#432scovich wants to merge 2 commits intoPeternator7:masterfrom
scovich wants to merge 2 commits intoPeternator7:masterfrom
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## What changes are proposed in this pull request? There's a semantic gap in strum's `EnumString`: The parsing API it derives is always fallible (impl `FromString` and `TryFrom`, both with strum parsing error), but enums with a default variant have infallible parsing (because any unknown value goes into the default). Define and use a new trait, `IntoTableFeature`, which mimicks `Into`. We can't "just" impl From for TableFeature because the blanket `impl<T: From> TryFrom for T` conflicts with the `impl TryFrom for TableFeature` strum emits. Result: Code that works with table features no longer needs to parse+unwrap, and there's now a narrow waist to update some day if/when this gets fixed upstream (just remove the extension trait and `impl From for TableFeature` instead). See also * Peternator7/strum#430 * Peternator7/strum#432 ## How was this change tested? Updated unit tests.
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Any enum with a
#[strum(default)]variant has infallible parsing in practice, but the compiler does not know this because the error type is stillstrum::ParseError.This PR proposes a non-breaking change to address the issue: Define a new type-level attribute
parse_infallible, which instructsEnumStringto usestd::convert::InfallibleforFromStr::Err, and to deriveFrom<&str>(always) instead ofTryFrom<&str>(1.34+). This allows e.g.and (in 1.82+)
Existing
EnumStringderivations remain unchanged unless the user adds theparse_infallibleattribute. As a future breaking change, we could start inferringparse_infallibleautomatically even for enums that do not carry the annotation. If a breaking change is immediately acceptable, we don't need to define the new attribute at all, and this PR becomes simpler.NOTE:
TryFromis still available (for 1.34+) withparse_infallible, because theFrom<&str>derived byEnumStringactivates a blanket implementation of reverseInto, which in turn activates a blanket implementation ofTryFrom.Also update docs and add new unit tests that exercise infallible parsing for both normal and PHF enums.
Fixes #307