currency: Implement runtime fallback for missing patterns and remove constructor validation by younies · Pull Request #8066 · unicode-org/icu4x

younies · 2026-06-11T14:06:02Z

This PR implements runtime fallback for currency patterns in CurrencyEssentials and relaxes validation in formatters to support custom data that may lack some patterns.

Changelog

Runtime Fallback:
- Positive pattern getters in CurrencyEssentials now return non-optional &DoublePlaceholderPattern. If a pattern index is out of bounds (representing a missing pattern in custom data), they fall back according to the hierarchy, with FALLBACK_PATTERN ("{1}{0}") as the ultimate fallback.
- Implemented positive pattern fallback hierarchies:
  - standard_alpha_next_to_number_positive ➔ standard
  - accounting_positive ➔ standard
  - accounting_alpha_next_to_number_positive ➔ standard_alpha_next_to_number ➔ standard
- Negative pattern getters return Option<&DoublePlaceholderPattern>, with accounting_negative falling back to standard_negative if missing (None).
Relaxed Validation:
- Removed standard pattern validation from CurrencyFormatter and CompactCurrencyFormatter constructors. They now initialize successfully even if the standard pattern is missing in the data, relying on runtime fallback.
Documentation:
- Added comments detailing the fallback hierarchies for all pattern getters.
- Added comments in datagen explaining the datagen-time fallback resolution.
Data:
- Regenerated experimental baked data.

…ternIndices Make standard_alpha_next_to_number, accounting_positive, and accounting_alpha_next_to_number_positive optional (Option<u8>) in PatternIndices to support true runtime fallback and avoid datagen-time fallback duplication. Also updated getters in CurrencyEssentials to implement the fallback hierarchies at runtime and added documentation comments for them. Regenerated experimental test data. TAG=agy CONV=7de8b65e-8ff1-43aa-9dbd-1a73ee7b45ff

younies · 2026-06-11T14:07:17Z

+        debug_assert!(
+            (self.indices.standard as usize) < self.patterns.len(),
+            "Standard pattern index {} is out of bounds for patterns of length {}",
+            self.indices.standard,
+            self.patterns.len()
+        );


@robertbastian : https://github.com/unicode-org/icu4x/pull/8050/changes/BASE..36013c38afcdb0e14dbca1c66a9e515400bd8b5d#r3396052964

please see my comment on the other PR

add by @younies #8050 (comment)

robertbastian

no, for several reasons:

an Option<u8> takes more space than a u8
it shifts work that can be done at datagen time to runtime

…alidation Revert the changes that made positive patterns optional in PatternIndices. Positive patterns are now u8 (non-optional) again, and fallback is resolved at datagen time. Added runtime fallback to FALLBACK_PATTERN in CurrencyEssentials getters for custom data support. Removed standard pattern validation from CurrencyFormatter and CompactCurrencyFormatter constructors, allowing them to succeed and use runtime fallback if the pattern is missing. Regenerated experimental baked data. TAG=agy CONV=2329ca53-2c21-4bb8-bc21-4b69db92e9cc

- Remove `.unwrap()` from positive pattern getters in `icu_provider_source` tests, as positive patterns are now non-optional. - Add `#[cfg(feature = "chrono")]` to `test_against_chrono` in `zoneinfo64` to fix compilation failure when `chrono` feature is not enabled. TAG=agy CONV=2329ca53-2c21-4bb8-bc21-4b69db92e9cc

younies · 2026-06-15T13:56:26Z

I have made all pattern indices u8 except for the negative ones. At runtime, we need to know if a negative pattern does not exist so we can fall back to the decimal formatter.

Regarding the struct size, we will address that when we add all the patterns and implement the ULE.

robertbastian · 2026-06-15T15:00:42Z

+// we have the fallback here for users feeding their own data without
+// handling the fallback logic in their data generation.


You should document the invariant that the standard pattern always exists, and phrase any case where it doesn't as a condition that we don't support.

Suggested change

// we have the fallback here for users feeding their own data without

// handling the fallback logic in their data generation.

// we have the fallback here in case the data struct is corrupted.

robertbastian · 2026-06-15T15:03:36Z

+        debug_assert!(
+            (self.indices.standard as usize) < self.patterns.len(),
+            "Standard pattern index {} is out of bounds for patterns of length {}",
+            self.indices.standard,
+            self.patterns.len()
+        );
        self.patterns
            .get(self.indices.standard as usize)
-            .or_else(|| self.patterns.get(0))
+            .unwrap_or(FALLBACK_PATTERN)


Suggested change

debug_assert!(

(self.indices.standard as usize) < self.patterns.len(),

"Standard pattern index {} is out of bounds for patterns of length {}",

self.indices.standard,

self.patterns.len()

);

self.patterns

.get(self.indices.standard as usize)

.or_else(|| self.patterns.get(0))

.unwrap_or(FALLBACK_PATTERN)

self.patterns

.get(self.indices.standard as usize)

.unwrap_or_else(|| {

debug_assert!(

false,

"Standard pattern index {} is out of bounds for patterns of length {}",

self.indices.standard,

self.patterns.len()

);

// GIGO

FALLBACK_PATTERN

})

robertbastian · 2026-06-15T15:05:29Z

    }

    #[test]
+    #[cfg(feature = "chrono")]


don't make unrelated changes.

robertbastian · 2026-06-15T15:11:14Z

+    /// Even though the baked data handles the fallback at data generation time,
+    /// we have the fallback here for users feeding their own data without
+    /// handling the fallback logic in their data generation.


there is no fallback here

robertbastian · 2026-06-15T15:13:07Z

        self.patterns
            .get(self.indices.standard_alpha_next_to_number as usize)
-            .or_else(|| self.standard_pattern())
+            .unwrap_or_else(|| self.standard_pattern())


Suggested change

.unwrap_or_else(|| self.standard_pattern())

.unwrap_or_else(|| {

debug_assert!(

false,

"Standard-alpha-next-to-number pattern index {} is out of bounds for patterns of length {}",

self.indices.standard,

self.patterns.len()

);

// GIGO

FALLBACK_PATTERN

})

robertbastian · 2026-06-15T15:14:39Z

+    /// Fallback hierarchy:
+    /// `standard_alpha_next_to_number` -> `standard`
+    ///
+    /// Even though the baked data handles the fallback at data generation time,
+    /// we have the fallback here for users feeding their own data without
+    /// handling the fallback logic in their data generation.


don't document like this. we should treat the fallback as a datagen concern, here we just assume we have a value for each type

Suggested change

/// Fallback hierarchy:

/// `standard_alpha_next_to_number` -> `standard`

///

/// Even though the baked data handles the fallback at data generation time,

/// we have the fallback here for users feeding their own data without

/// handling the fallback logic in their data generation.

robertbastian · 2026-06-15T15:15:26Z


    /// Returns the `standard_alpha_next_to_number` negative pattern if specified, falling back to standard negative.
+    ///
+    /// Fallback hierarchy:


issue: handle the negative fallbacks at datagen time as well. they can still be Option<u8>

younies requested review from a team, Manishearth, robertbastian and sffc as code owners June 11, 2026 14:06

younies removed request for Manishearth and sffc June 11, 2026 14:06

younies mentioned this pull request Jun 11, 2026

currency: Pack distinct patterns in CurrencyEssentialsV1 and support explicit standard negative patterns #8050

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younies commented Jun 11, 2026

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fix fmt

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younies added 6 commits June 11, 2026 15:28

fix comment

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fix fmt

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fix fmt

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fix fmt

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younies changed the title ~~currency: Make positive pattern variants optional and implement runtime fallback~~ currency: Implement runtime fallback for missing patterns and remove constructor validation Jun 15, 2026

younies requested a review from robertbastian June 15, 2026 13:56

robertbastian requested changes Jun 15, 2026

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currency: Implement runtime fallback for missing patterns and remove constructor validation#8066

currency: Implement runtime fallback for missing patterns and remove constructor validation#8066
younies wants to merge 8 commits into
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younies:fix-r3396052964

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		// we have the fallback here for users feeding their own data without
		// handling the fallback logic in their data generation.

	// we have the fallback here for users feeding their own data without
	// handling the fallback logic in their data generation.
	// we have the fallback here in case the data struct is corrupted.

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younies commented Jun 11, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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younies commented Jun 15, 2026

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younies commented Jun 11, 2026 •

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