[ybai08]

With compact_ranges(true), a range whose bounds are not numbers falls back to the struct form, but the fallback in SerializeStruct::serialize_field always wrote a bare ( itself. It skipped two things serialize_struct does:

• the struct name when struct_names or the explicit_struct_names extension is on, and
• leaving out the struct's own parentheses when the range is the content of a newtype variant under unwrap_variant_newtypes.

Two of those outputs can't be read back:

let r = String::from("a")..String::from("z");
// PrettyConfig::new().compact_ranges(true).extensions(Extensions::EXPLICIT_STRUCT_NAMES)
// master: #![enable(explicit_struct_names)](start:"a",end:"z")  -> ExpectedStructName("Range")

enum E { B(Range<String>) }
// PrettyConfig::new().compact_ranges(true).extensions(Extensions::UNWRAP_VARIANT_NEWTYPES)
// master: #![enable(unwrap_variant_newtypes)]B((start:"a",end:"z"))  -> ExpectedIdentifier

With struct_names(true) alone the name is dropped ((start:"a",end:"z") instead of Range(start:"a",end:"z")), which still parses.

RangeCompound now keeps the struct name and whether it was entered as an unwrapped newtype variant, and the fallback writes the opening the way serialize_struct would. Numeric ranges are unchanged (Numbers(0..5) inside an unwrapped variant already round-trips).

test_non_numeric_range_falls_back_to_struct_form in tests/601_support_for_number_ranges.rs checks the struct_names, explicit_struct_names and unwrap_variant_newtypes outputs and reads them back. It fails on master. A local differential check (not part of the PR) over 40 configurations (5 extension sets × struct_names × compact_structs × pretty/compact whitespace) on a struct holding Range<String>, RangeFrom<char> and newtype variants found the output with compact_ranges(true) identical to the output with compact_ranges(false), and all of them read back. cargo test, cargo test --all-features, cargo test --features integer128, cargo test --features indexmap and cargo fmt --all -- --check pass. cargo hack and the 1.64 MSRV toolchain were not run.

• I've included my change in CHANGELOG.md

[juntyr]

Thanks @ybai08!