fix: transform JSX macros nested inside expression placeholders

[cpruijsen]

Fixes #16

<Trans>, <Plural>, <Select> and <SelectOrdinal> inside a JSX expression placeholder are now transformed. Before this change, they were emitted as-is, for example in {cond ? <Trans>a</Trans> : <Trans>b</Trans>}, {cond && <Trans>a</Trans>} or {items.map(i => <Trans>{i}</Trans>)}. The macro import was still removed, so the output referenced an element with no import.

Cause

fold_jsx_element returned the result of transform_jsx_macro without folding it again. transform_jsx_macro copies placeholder expressions verbatim into values and components, so the folder never visited macros inside them. The other fold_* methods in the same impl Fold call fold_children_with(self) after they transform.

Fix

Both macro branches in fold_jsx_element now call fold_children_with(self) on the produced element. The fold runs after the message is built, so a direct child macro such as <Trans>You have <Plural .../></Trans> is still inlined into the parent message. The produced element uses the runtime Trans identifier, so the fold does not match it again.

Before:

values: { 0: props.world ? <Trans>world</Trans> : <Trans>guys</Trans> }

After:

values: {
  0: props.world
    ? <Trans_ {.../*i18n*/ { id: "ELi2P3", message: "world" }}/>
    : <Trans_ {.../*i18n*/ { id: "39nd-2", message: "guys" }}/>
}

The nested descriptors keep their /*i18n*/ comments, so the extractor can find these messages too.

Scope

• The fold runs on the produced element, not the source element. Folding the source children first would compile a direct child <Plural> or <Trans> into a runtime element, and it would no longer be inlined into the parent message.
• The fold covers all children of the produced element, not only values and components. This also fixes macros in the preserved render and component attributes, for example render={() => <Trans>x</Trans>}. I can limit it to the descriptor if you prefer a narrower change.

Tests

• New snapshot test jsx_macros_inside_expression_are_transformed covers the repro from the issue and the && form. Without the fix, it shows the untransformed <Trans> in values.
• All existing lingui_macro and lingui_extractor snapshots are unchanged.
• cargo fmt --check and cargo clippy -p lingui_macro --all-targets --all-features -- -D warnings pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (673f2fd) to head (911fe12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #279   +/-   ##
=======================================
  Coverage   94.44%   94.44%           
=======================================
  Files          11       11           
  Lines        2518     2518           
=======================================
  Hits         2378     2378           
  Misses        140      140           

Flag	Coverage Δ
unittests	94.44% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/lingui_macro/src/lib.rs	91.47% <100.00%> (ø)

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