Make `Yoke` work on types with borrowing generics

[sffc]

Same kind of change as #3770. It's a bit more involved due to Yoke's associated types

[Manishearth]

The WIP #3771 change that this needs to work with is below. #4120 has a WIP patch, we should write a test similar to #3770

commit c61033391354f51df4d226cecd3b986cd3b1d304
Author: Manish Goregaokar <manishsmail@gmail.com>
Date:   Wed Aug 2 17:08:26 2023 -0700

    wip

diff --git a/components/datetime/src/provider/calendar/symbols.rs b/components/datetime/src/provider/calendar/symbols.rs
index 563d9d229..4c85bf4e6 100644
--- a/components/datetime/src/provider/calendar/symbols.rs
+++ b/components/datetime/src/provider/calendar/symbols.rs
@@ -126,6 +126,97 @@ pub struct Eras<'data> {
     pub narrow: ZeroMap<'data, UnvalidatedStr, str>,
 }
 
+// UTS 35 specifies that `format` widths are mandatory,
+// except for `short`.
+#[derive(Debug, PartialEq, Clone, Default, yoke::Yokeable, zerofrom::ZeroFrom)]
+#[cfg_attr(
+    feature = "datagen",
+    derive(serde::Serialize, databake::Bake),
+    databake(path = icu_datetime::provider::calendar::$name),
+)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize))]
+#[yoke(prove_covariance_manually)]
+#[zerofrom(may_borrow(SymbolsV1))]
+#[yoke(may_borrow(SymbolsV1))]
+/// Symbol data for the “format” style formatting of Month.
+///
+/// <div class="stab unstable">
+/// 🚧 This code is considered unstable; it may change at any time, in breaking or non-breaking ways,
+/// including in SemVer minor releases. While the serde representation of data structs is guaranteed
+/// to be stable, their Rust representation might not be. Use with caution.
+/// </div>
+pub struct FormatWidthsV1<SymbolsV1> {
+    /// Abbreviated length symbol for “format” style symbol.
+    #[cfg_attr(feature = "serde", serde(borrow))]
+    pub abbreviated: SymbolsV1,
+    /// Narrow length symbol for “format” style symbol.
+    #[cfg_attr(feature = "serde", serde(borrow))]
+    pub narrow: SymbolsV1,
+    /// Short length symbol for “format” style symbol.
+    #[cfg_attr(feature = "serde", serde(borrow))]
+    pub short: Option<SymbolsV1>,
+    /// Wide length symbol for “format” style symbol.
+    #[cfg_attr(feature = "serde", serde(borrow))]
+    pub wide: SymbolsV1,
+}
+
+// // UTS 35 specifies that `stand_alone` widths are optional
+// #[derive(Debug, PartialEq, Clone, Default, yoke::Yokeable, zerofrom::ZeroFrom)]
+// #[cfg_attr(
+//     feature = "datagen",
+//     derive(serde::Serialize, databake::Bake),
+//     databake(path = icu_datetime::provider::calendar::$name),
+// )]
+// #[cfg_attr(feature = "serde", derive(serde::Deserialize))]
+// #[yoke(prove_covariance_manually)]
+// #[doc = concat!("Symbol data for the \"stand-alone\" style formatting of ", stringify!($field_id),
+//     ".\n\nThe stand-alone style is used in contexts where the field is displayed by itself.")]
+// ///
+// /// <div class="stab unstable">
+// /// 🚧 This code is considered unstable; it may change at any time, in breaking or non-breaking ways,
+// /// including in SemVer minor releases. While the serde representation of data structs is guaranteed
+// /// to be stable, their Rust representation might not be. Use with caution.
+// /// </div>
+// pub struct StandAloneWidthsV1<'data> {
+//     #[doc = concat!("Abbreviated length symbol for \"stand-alone\" style symbol for ", stringify!($name), ".")]
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub abbreviated: Option<SymbolsV1<'data>>,
+//     #[doc = concat!("Narrow length symbol for \"stand-alone\" style symbol for ", stringify!($name), ".")]
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub narrow: Option<SymbolsV1<'data>>,
+//     #[doc = concat!("Short length symbol for \"stand-alone\" style symbol for ", stringify!($name), ".")]
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub short: Option<SymbolsV1<'data>>,
+//     #[doc = concat!("Wide length symbol for \"stand-alone\" style symbol for ", stringify!($name), ".")]
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub wide: Option<SymbolsV1<'data>>,
+// }
+
+// #[derive(Debug, PartialEq, Clone, Default, yoke::Yokeable, zerofrom::ZeroFrom)]
+// #[cfg_attr(
+//     feature = "datagen",
+//     derive(serde::Serialize, databake::Bake),
+//     databake(path = icu_datetime::provider::calendar::$name),
+// )]
+// #[cfg_attr(feature = "serde", derive(serde::Deserialize))]
+// #[yoke(prove_covariance_manually)]
+// #[doc = concat!("The struct containing the symbol data for ", stringify!($field_id),
+//     " that contains the \"format\" style symbol data ([`FormatWidthsV1`]) and \"stand-alone\" style symbol data ([`StandAloneWidthsV1`]).")]
+// ///
+// /// <div class="stab unstable">
+// /// 🚧 This code is considered unstable; it may change at any time, in breaking or non-breaking ways,
+// /// including in SemVer minor releases. While the serde representation of data structs is guaranteed
+// /// to be stable, their Rust representation might not be. Use with caution.
+// /// </div>
+// pub struct ContextsV1<'data> {
+//     /// The symbol data for "format" style symbols.
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub format: FormatWidthsV1<'data>,
+//     /// The symbol data for "stand-alone" style symbols.
+//     #[cfg_attr(feature = "serde", serde(borrow))]
+//     pub stand_alone: Option<StandAloneWidthsV1<'data>>,
+// }
+
 // Note: the SymbolsV* struct doc strings metadata are attached to `$name` in the macro invocation to
 // avoid macro parsing ambiguity caused by other metadata already attached to `$symbols`.
 macro_rules! symbols {
@@ -158,39 +249,7 @@ macro_rules! symbols {
             /// </div>
             $symbols
 
-            // UTS 35 specifies that `format` widths are mandatory,
-            // except for `short`.
-            #[derive(Debug, PartialEq, Clone, Default, yoke::Yokeable, zerofrom::ZeroFrom)]
-            #[cfg_attr(
-                feature = "datagen",
-                derive(serde::Serialize, databake::Bake),
-                databake(path = icu_datetime::provider::calendar::$name),
-            )]
-            #[cfg_attr(feature = "serde", derive(serde::Deserialize))]
-            #[yoke(prove_covariance_manually)]
-            #[doc = concat!("Symbol data for the \"format\" style formatting of ", stringify!($field_id),
-                ".\n\nThe format style is used in contexts where it is different from the stand-alone form, ex: ",
-                "a case inflected form where the stand-alone form is the nominative case.")]
-            ///
-            /// <div class="stab unstable">
-            /// 🚧 This code is considered unstable; it may change at any time, in breaking or non-breaking ways,
-            /// including in SemVer minor releases. While the serde representation of data structs is guaranteed
-            /// to be stable, their Rust representation might not be. Use with caution.
-            /// </div>
-            pub struct FormatWidthsV1<'data> {
-                #[doc = concat!("Abbreviated length symbol for \"format\" style symbol for ", stringify!($name), ".")]
-                #[cfg_attr(feature = "serde", serde(borrow))]
-                pub abbreviated: SymbolsV1<'data>,
-                #[doc = concat!("Narrow length symbol for \"format\" style symbol for ", stringify!($name), ".")]
-                #[cfg_attr(feature = "serde", serde(borrow))]
-                pub narrow: SymbolsV1<'data>,
-                #[doc = concat!("Short length symbol for \"format\" style symbol for ", stringify!($name), ", if present.")]
-                #[cfg_attr(feature = "serde", serde(borrow))]
-                pub short: Option<SymbolsV1<'data>>,
-                #[doc = concat!("Wide length symbol for \"format\" style symbol for ", stringify!($name), ".")]
-                #[cfg_attr(feature = "serde", serde(borrow))]
-                pub wide: SymbolsV1<'data>,
-            }
+            pub type FormatWidthsV1<'data>  = super::FormatWidthsV1<SymbolsV1<'data>>;
 
             // UTS 35 specifies that `stand_alone` widths are optional
             #[derive(Debug, PartialEq, Clone, Default, yoke::Yokeable, zerofrom::ZeroFrom)]

[Manishearth]

When we handle this we should also correctly handle ?Sized on generics in Yoke. #4659 is implementing a stopgap solution.

Longterm solutions are:

• Distinguish between may_borrow and non-may_borrow parameters, and use Sized on the former ones.
• (easier) Implement yoke(additional_bounds(T: Sized)), which we may want anyway.