Upgrading from 0.1 to 0.2

Verbora 0.2.0 is not source-compatible with 0.1.0. Bumping a dependency from "0.1" to "0.2" will produce compile errors in most programs, and in a few places it will produce different answers without producing a compile error at all. This page is for fixing both.

Read the second half even if the first half fixes your build. The changes that do not break compilation are the dangerous ones — a different ranking, a different tag, a different sentiment score. They are listed under Changes that do not break the build.

What happened

0.1.0 was a port. Its behaviour was defined by agreement with the implementation it had been ported from: the fixtures recorded that implementation's output, the documentation explained where a rule came from rather than what it does, and several tables existed only because something else shipped them.

0.2.0 finishes the migration to a Rust-native specification. Every behaviour is now defined by a published standard or by an explicit Verbora contract, and the tests assert the contract rather than the ancestry. Six rules were applied across all nineteen crates, and between them they account for most of the churn:

1. No sentinels. Absence is Option::None, never a magic value carved out of a numeric range.
2. No NaN escapes. It poisons comparison and sorting silently.
3. No panics outside preconditions the type system cannot express. Invalid states are made unrepresentable by Result-returning constructors instead.
4. No function silently rewrites its input. Case folding, trimming and normalisation are the caller's explicit choice.
5. The crate root is the entire public surface. Modules are private and everything is re-exported, so each item has exactly one path.
6. The text unit is stated and justified per crate, never inherited.

Rule 5 alone breaks every use verbora_distance::levenshtein::…-style import in existing code. Rules 1–3 are why so many return types moved.

Fix the build first

Every module path is gone

This is the single most common error, and the fix is mechanical.

// 0.1
use verbora_distance::levenshtein::{Options, levenshtein};
use verbora_spellcheck::edits::edits;
use verbora_ngrams::text::ngrams_str;
use verbora_core::stopwords::StopWords;

Import from the crate root instead. Every public item has exactly one path now, and it is verbora_<crate>::Item. If the root does not export it, it is no longer public — see the per-crate tables below for what replaced it.

The rename table for everything that survived

These items still exist, under a different name or a different signature. This is the list to scan first, because each row is a small edit rather than a redesign.

Crate	0.1	0.2
verbora-distance	damerau_levenshtein(a, b, &Options) → f64	damerau_levenshtein(a, b) → usize
	levenshtein(a, b, &Options) → f64	levenshtein(a, b) → usize
	Options { restricted: true, .. }	osa(a, b) → usize — a separate function
	Options { insertion_cost, .. }	LevenshteinCosts::new(…) / OsaCosts::new(…) / DamerauCosts::new(…), each returning Result<_, CostError>, passed to *_weighted
	jaro_winkler(a, b, &Options)	jaro_winkler(a, b)
	hamming(a, b, ignore_case) → i64, -1 for incomparable	hamming(a, b) → Option<usize>
	hamming_checked, INCOMPARABLE	gone — hamming is the checked form
	SearchResult { substring: String, distance: f64, offset: isize }	SearchResult<'t, D> with substring() -> &'t str, distance() -> D, range() -> Range<usize> (bytes)
	StringMetric, Levenshtein, DamerauLevenshtein, JaroWinkler, Dice, Hamming	gone — call the free functions
verbora-core	StopWords::english()	StopWords::for_language(StopWordLanguage::En)
	StopWords::remove(&mut self, w) → ()	→ bool
	StopWords::remove_all(…) → ()	→ usize
	Token, collapse_whitespace, is_whitespace, trim_edge_empties	gone
	DoubleKeyPhonetic::process_double → (String, String)	→ (String, Option<String>)
verbora-trie	add_string, add_strings	insert, insert_all
	get_size()	node_count() — and len() is new, and counts words
	is_case_sensitive() → bool	case_handling() → CaseHandling
	with_case_sensitivity(bool)	with_case_handling(CaseHandling)
	find_matches_on_path, iter_matches_on_path, MatchesOnPath	prefix_matches, iter_prefix_matches, PrefixMatches
	find_prefix → (Option<Cow>, Cow)	longest_prefix → PrefixSplit { word, rest }
	find_prefix_lengths → (Option<usize>, usize)	longest_prefix_lengths → PrefixSplitLengths { word, rest }
verbora-transliterators	transliterate, transliterate_into	transliterate_ja, transliterate_ja_into, transliterate_ja_normalized
	Phase	gone — the pipeline is internal
verbora-normalizers	normalize, normalize_token (English contraction expansion: "I'D" → ["I", "would"])	gone — this crate is Unicode normalization and diacritic folding only
	normalize_ja, normalize_no, normalize_sv	gone — nfkc covers most of normalize_ja, but not all of it: width folding and composite symbols (㍼54年㋃㏪ → 昭和54年4月11日) match, while iteration marks (時々刻々 → 時時刻刻) and the small-tsu rewrite before the n-row (まっなか → まんなか) are not applied — nfkc returns those unchanged, so treat it as a replacement only if you did not rely on those two; normalize_no and normalize_sv existed for the Norwegian and Swedish stemmers, which is exactly where the umlaut-folding defect lived, and they are no longer public
verbora-inflectors	pluralize → Result<String, EmptyToken>	→ String (total; the empty token has no inflected form and comes back unchanged)
	CountInflector, CountInflectorFr	OrdinalInflector, OrdinalInflectorFr (the French one takes a Gender)
	CaseMode::{Lower, Capitalize, Upper}	CaseMode::{Preserve, Title, Upper}
	restore_case(token)	CaseMode::of(token)
	PatternError	RuleError (#[non_exhaustive])
	Rule::apply → Option<String>	→ Option<Cow<'t, str>>
verbora-spellcheck	Spellcheck::get_corrections → Vec<String>	corrections → Vec<Correction<'_>>, or correction_words → Vec<String>
	par_get_corrections_batch	par_corrections_batch → Vec<Vec<Correction<'_>>>
	frequency → Option<f64>	→ Option<u32>
	frequencies → (&str, f64)	→ (&str, u32)
	edits, edits_utf16, edits_with_max_distance*, Edits, EditUnit, ALPHABET, sort_by_frequency	gone — candidate generation is internal
	Spellcheck::trie()	gone
	DeletionIndex::neighbors → DeletionNeighbors	→ Result<DeletionNeighbors, DistanceBeyondIndex>
verbora-phonetics	DoubleMetaphone::process → (String, String)	→ DoubleMetaphoneCode, with primary(), alternate() -> Option<&str>, into_parts()
	SoundEx::process_with, Metaphone::process_with, *_utf16, try_process	gone — one key, one call, no max_length, no PhoneticError
	SoundExDM	gone. 0.1 shipped two Daitch–Mokotoff implementations; DaitchMokotoff is the one that remains. They were separate code, so re-check your keys rather than assuming a rename
	DaitchMokotoffCode	DaitchMokotoff::codes → Vec<String>
	phoneticize_tokens_with, tokenize_and_phoneticize_with	folded into phoneticize_tokens / tokenize_and_phoneticize
verbora-sentiment	SentimentAnalyzer::new(lang: &str, stemmer, kind: &str)	SentimentAnalyzer::with_stemmer(Language, VocabularyKind, stemmer)
	without_stemmer(&str, &str)	without_stemmer(Language, VocabularyKind)
	get_sentiment → f64	→ Option<f64> (None when no word scored)
	Score::value → f64 (sum / count, so NaN when nothing scored)	Score::mean → Option<f64> — None is the empty case; Score::over is Option<f64> for the same reason
	Polarity (an enum of Number/Text)	a struct: value() -> f64, as_written() -> Option<&'static str>
	Error	UnsupportedPair / UnknownName
	Vocabulary::shared_for	Vocabulary::shared(kind, Language)
	Language::from_pattern, as_str	Language::from_code, code (plus FromStr)
verbora-language	f32 confidences	Confidence, built with Confidence::new(f32) -> Option<Self>
	PhoneticRecommendation::SoundExDaitchMokotoff	PhoneticRecommendation::DaitchMokotoff (plus new Cologne and BeiderMorse variants)
	LanguageDetection { candidates } (public field)	candidates(), len(), is_empty(), single(…)
verbora-util	CyclicDependency	Cycle
	VertexKey, Vertex	VertexId
	Bag, StorageBackend, FileBackend, StoragePlugin, StorageType	gone
	Language (stop words)	still verbora_util::Language, now a re-export of verbora_core::StopWordLanguage
verbora-wordnet	Pos	PartOfSpeech
	Find, Source, Probe, Probes, FilePair, IndexHit, IndexRecord, DataRecord, PointerRef	gone or replaced by Synset, SynsetRef, IndexEntry, WordRef, Gloss
verbora-stemmers	verbora_stemmers::Token re-export	gone with verbora_core::Token
	stopwords::{add_all, contains, remove, …}(Language, …)	inherent methods on Language: Language::De.contains(w), .add_all(…), .reset()
	TokenizeAndStem::is_word_char	gone (HYPHEN_JOINS_LETTERS is the remaining knob)

The crates that were rewritten rather than renamed

For these, there is no row-by-row mapping worth printing, because almost nothing survived under a recognisable shape. Read the feature page and the rustdoc, and plan on rewriting the call site.

Crate	What is there now
verbora-tokenizers	Three tokenizers grounded in UAX #29 — WordTokenizer, SegmentTokenizer, SentenceTokenizer. The sixteen AggressiveTokenizer* language variants, TreebankWordTokenizer, RegexpTokenizer, WordPunctTokenizer, CaseTokenizer, OrthographyTokenizer, TokenizerJa and Utf16Token are all gone: their rules could not be traced to any standard.
verbora-ngrams	ngrams(seq, n: NonZeroUsize) over a slice, Padded for boundary symbols, char_ngrams(text, n) for scalar windows. The *_str, *_with_stats, *_zh, bigrams/trigrams/multrigrams families, NGramStats, ngram_key and the process-global set_tokenizer are all gone. Word n-grams are now the composition you write: tokenize, then ngrams over the token slice.
verbora-tagger	BrillTagger, Lexicon, RuleSet, Corpus, Trainer, Evaluation, Tag, TaggedToken. BrillPosTagger, BrillPosTrainer, BrillPosTester, TransformationRule, RuleTemplate, Predicate, TaggerError and the rest of the 0.1 surface are gone.
verbora-tfidf	TfIdf with add_document(&str) -> usize, add_terms, tfidf(query, index) -> Option<f64>, rank(query) -> Vec<DocumentScore>, and an owned Analyzer holding the tokenizer, case folding and stop-word list. The dynamic DocKey/DynValue/JsonValue layer, Interner, TermId, Terms, Encoding, TfIdfError and the process-global tokenizer and stop-word setters are gone. to_json now returns Result<String, ExportError>; from_json returns Result<Self, RestoreError>.
verbora-classifiers	BayesClassifier, LogisticRegressionClassifier, MaxEntClassifier over a reworked training and persistence surface (TrainingReport, TrainingStep, StopReason, ModelDefect, stamped model files). Context, Feature, FeatureSet, GenerateFeatures, GISScaler, MECorpus, MESentence and the rest are gone. Maximum entropy is now an implementation of Generalised Iterative Scaling rather than a reproduction; in 0.1 Sample::new rejected every non-empty argument, so the feature did not work at all.
verbora-analyzers	analyze(&[TaggedWord]) -> SentenceAnalysis, with Role, TagClass, Terminator, SentenceType and ImpliedSubject. The mutable SentenceAnalyzer with its part()/type_of() staging, TaggedSentence, Punct, Field, SenType and TypeError are gone.
verbora-wordnet	WordNet over Synset, SynsetRef, Sense, Pointer, PointerSymbol, Gloss, IndexEntry, PrebuiltIndex and four Storage modes.

SenType did not simply get renamed. 0.1's SenType had five variants including Unknown and Command. 0.2's SentenceType has four — Declarative, Interrogative, Imperative, Exclamative — and absence is Option::None rather than an Unknown variant. A match ported across mechanically will compile and mean something different.

The one break that is not in your code

Every fix above is something the compiler points at inside a .rs file. This one fails earlier, in Cargo.toml, before anything is compiled:

# 0.1
verbora-core = { version = "0.1", features = ["serde"] }
# 0.2 — drop the features key entirely
verbora-core = "0.2"

verbora-core declared a serde feature that nothing in the crate ever used: no item derived or implemented a serde trait, and no dependent enabled it. It is gone, and no crate in 0.2.0 has a serde feature. Cargo reports this as the package 'verbora-core' does not have the feature 'serde', which does not name a version and so does not obviously point here.

The crates that do serialize — verbora-tfidf and verbora-classifiers — always used serde as a plain dependency rather than a feature, so their JSON persistence is unchanged.

Every other Cargo feature carried over: parallel on the fourteen crates that had it, and language-detection / fast-language-detection on verbora-language.

#[non_exhaustive], once and properly

35 public enums are #[non_exhaustive] in 0.2.0, up from 6 in 0.1.0. A downstream match over any of them now needs a wildcard arm, or it fails to compile with E0004: non-exhaustive patterns.

// 0.1: compiled, because the enum was closed.
match err {
    CostError::NotFinite { .. } => …,
    CostError::Negative { .. } => …,
    CostError::TranspositionBelowThreshold { .. } => …,
}

// 0.2: add the arm.
match err {
    CostError::NotFinite { .. } => …,
    CostError::Negative { .. } => …,
    CostError::TranspositionBelowThreshold { .. } => …,
    _ => …,
}

The 21 error enums are the ones you are most likely to be matching on:

verbora_classifiers::{ClassifierError, LoadError, MaxEntError, ModelDefect, RestoreError, StampError} · verbora_distance::CostError · verbora_inflectors::RuleError · verbora_tagger::{CorpusParseError, LexiconError, LiteralError, RuleParseError} · verbora_tfidf::{ExportError, RestoreError, StampError} · verbora_tokenizers::AbbreviationError · verbora_util::{GraphError, PathError} · verbora_wordnet::{Error, ParseSenseError, RecordError}

The other 14 are the data enums those errors carry, or that a caller matches alongside them: verbora_core::StopWordLanguage · verbora_classifiers::TrainingEvent · verbora_distance::Operation · verbora_language::{Language, PhoneticRecommendation, Script, StrategyBasis, TransliterationAdvice} · verbora_sentiment::{Language, VocabularyKind} · verbora_tagger::{Condition, Language, Template} · verbora_util::AbbreviationLanguage.

Why now, and why the payloads too

Pre-1.0 is the window where this costs a wildcard arm rather than a major version. An error enum that is closed cannot gain a variant without a breaking release, which in practice means either shipping a breaking release to describe a newly-distinguished failure or folding it into an existing variant and losing the distinction. Sealing them here buys the freedom to name failures precisely later, at the price of one arm today.

The 14 data enums are marked for the same reason, one level down. Sealing an error type while leaving the enum it carries closed gives the freedom straight back: a newly-distinguished failure almost always needs a new payload value to describe it — a new Operation, a new Language, a new Template — and adding one to a closed enum is the breaking change the outer seal was meant to avoid. They are marked together deliberately.

Three things this does not change:

• if let and matches! are unaffected unless they were exhaustive.
• You can still construct these enums' variants from your own crate. The attribute is on the enum, not on its variants, so it constrains matching only: CostError::Negative { operation, value } still compiles downstream.
• A match that already had a _ arm, or that binds the whole value, compiles unchanged.

Changes that do not break the build

These compile after the mechanical fixes above and then return something different. Each one is a defect fix — 0.1's answer was wrong — but "wrong" is not the same as "not what your snapshot test asserts".

Ordering

Correction ranking is a documented total order now, and re-sorting the results cannot disagree with it. In 0.1, Spellcheck::get_corrections returned Vec<String> ranked by a comparator that reproduced another runtime's sort semantics over f64 frequencies. In 0.2 the ranking is distance ascending, then frequency descending, then word ascending, and it is written out as Correction's own Ord — hand-written rather than derived, because a derived Ord compares fields in declaration order and would put word first. Neighbor's order is distance ascending, then word ascending, on the same reasoning. Code that collected 0.1's results and re-sorted them gets a different order.

use verbora_spellcheck::Spellcheck;

// Repeats are frequencies: "the" occurs three times, "he" twice, "she" once.
let sc = Spellcheck::new(["the", "the", "the", "he", "he", "she", "th"]);

// Distance first, then frequency descending: the exact match leads, and
// "the" outranks "she" at the same distance because it is more common.
assert_eq!(sc.correction_words("he", 1), ["he", "the", "she"]);

let best = sc.best_correction("he", 1).expect("a correction exists");
assert_eq!((best.word, best.distance, best.frequency), ("he", 0, 2));

Frequencies are u32 rather than f64 for the same reason the ranking moved. A count is an integer, and 0.1's f64 frequencies were not merely imprecise: for twelve specific words the stored value was NaN, which a comparator silently reorders everything around. Spellcheck::frequency now returns Option<u32>, and frequencies() yields (&str, u32); there is no NaN left in this crate to guard against.

LogisticRegressionClassifier had a related defect: fit built its target columns in the feature map's enumeration order while classifications() reported weights in insertion order. Those orders differ exactly when a label looks like an integer, so a document trained as "2" classified as "1", and vice versa, confidently. Retrain any model with integer-like labels.

The unit of measurement

verbora-distance and verbora-stemmers counted UTF-16 code units in 0.1 and count Unicode scalar values in 0.2. verbora-trie counts scalars per node. Below U+10000 the two readings coincide; above it they do not.

use verbora_trie::Trie;

let mut t = Trie::new();
t.insert("a👍");

// 0.1's `get_size()` reported 4 here: root, 'a', and the two halves of the
// surrogate pair. One scalar is now one node, whatever plane it lives in.
assert_eq!(t.node_count(), 3);
assert_eq!(t.len(), 1); // and `len()` counts stored words, which is new

Stemmer output changes for input containing astral characters, for the same reason.

SearchResult carried the same unit problem and a sentinel besides: 0.1's offset was a UTF-16 code unit index into the target, signed, and genuinely -1 when the backtrace exited through column 0. 0.2 returns range() -> Range<usize> in bytes, derived from the borrowed substring, so &target[found.range()] == found.substring() for every input and there is no negative case to handle:

use verbora_distance::levenshtein_search;

let target = "Zürich, Berlin, Wien";
let found = levenshtein_search("Berlin", target);

// "Zürich, " is eight characters but nine bytes, because "ü" takes two.
assert_eq!(found.range(), 9..15);
assert_eq!(&target[found.range()], found.substring());
assert_eq!(found.distance(), 0);

Behavioural fixes that change results

Each was found during the migration, each is now pinned by a test that fails without the fix, and each will move an output your program may be asserting on.

Crate	What changed	What it means for you
verbora-sentiment	The tokenizer's UAX #29 boundaries split hyphenated lexicon keys, making 2,313 entries unreachable, several of them sign-inverted — "non-approved" scored +1 against a stored polarity of -2. Multi-token span matching now reaches them, and the phrase keys single-token lookup never could.	Scores move, sometimes by a sign. Re-baseline any threshold tuned on 0.1.
verbora-stemmers	Swedish and Norwegian folded a/o umlauts before consulting stop-word lists spelled with them, so 116 of 428 Swedish entries could never match. In those languages those are distinct letters.	Swedish and Norwegian stop-word filtering removes more than it used to.
verbora-stemmers	The German stemmer's character gate was a byte-for-byte copy of the Spanish one: it admitted a/e/i/n/o/u accents and omitted a/o/u umlauts and eszett.	German stems change.
verbora-stemmers	The Dutch stop-word list spelled an entry with a trailing space, so the pronoun je was never filtered.	One more Dutch stop word is filtered.
verbora-tagger	292 keys of the bundled English lexicon carried corpus markup rather than tokens — 256 with a backslash, 36 with an embedded Penn tag — and 199 of them had no correctly-spelled counterpart, so those tokens fell through to the capitalised default. tag_of("Asia/Pacific") returned NNP; it now returns JJ.	Tags change on tokens the broken keys covered. The bundled English lexicon now holds 92,538 keys.
verbora-tagger	Tag::new("*") is refused (Err(LiteralError::Wildcard)), because * printed as the wildcard and reparsed as TagPattern::Any — a rule that rewrote one tag became one that rewrote every tag, across the documented persistence path. Corpus::parse_brown inherits this as CorpusParseError::WildcardTag.	A corpus or rule file containing * as a literal tag is now an error instead of silently corrupting the rule set.
verbora-tagger	Eleven bundled Dutch rules could never fire, ten of them naming a sentence-boundary marker that is not a tag in this tagger. They are gone, so the advertised count is the count that can fire: RuleSet::bundled(Language::Dutch) now reports 273.	The Dutch rule count changes; no rule that ever fired was removed.
verbora-tagger	Template::instantiate pushed one Condition per position inspected rather than one per distinct condition, so a window template double- (or triple-) counted a corrected token, defeating the trainer's min_score guard.	Trained rule sets differ. Retrain.
verbora-transliterators	Vowel lengthening consumed the next scalar without asking whether it began a longer key, so ハロウィン came out harōin and スウェーデン came out sūēden. Six keys collide this way after any of seventy morae.	Romanisations change for the affected syllables.
verbora-language	fold_cyrillic folded two blocks while the script router feeds it a third, so 100 uppercase letters went unfolded — including Ґ, one of four Ukrainian-versus-Russian discriminators. The same text in different case gave a different answer.	Cyrillic detection is now case-invariant, and uppercase Ukrainian text carries its signal. The Cyrillic model was retrained against the corrected extractor.
verbora-phonetics	Four Beider–Morse Italian rules carried U+FFFD where accented vowels belonged, so real Italian input had the vowel deleted from its encoding.	Beider–Morse encodings change for the affected Italian inputs.
verbora-distance	jaro(x, x) returned 0.0 for single-unit inputs while jaro_winkler returned 1.0 for the same pair; dice_coefficient("", "") returned NaN. Search could return text absent from the target, because a UTF-16 slice could split a surrogate pair and from_utf16_lossy substituted U+FFFD.	Those cases now return the documented answers, and SearchResult::substring() is a borrow of the target, so it cannot be text the target does not contain.
verbora-tfidf	The between-documents invariant was restored in finish, which unwinding skips, so a panicking caller iterator left the counter dirty and the next document reported a present term as Some(0) — documented to mean absent.	Corpora built through a panicking tokenizer were wrong; they are not now.

Resource behaviour

Not a correctness change, but it will change how your program behaves under load:

• verbora-spellcheck's deletion generation was cubic in word length. An 800-scalar token cost 4.0 GB of peak RSS; it now costs 32.4 MB. k = 0, documented as a membership test, no longer builds an index at all. DeletionIndex was re-keyed onto a u64 hash for the same reason — cubic to quadratic in word length. The index itself stays quadratic in the longest word: that is the symmetric-delete structure's own price, now documented.
• verbora-trie's insert_all fed size_hint().0 straight to Vec::reserve, so an iterator that lies — which size_hint explicitly permits — could abort with a capacity overflow or reserve tens of gigabytes. The hint is now clamped.
• verbora-wordnet's Resident and LazyResident allocated by file metadata with no ceiling. The dictionary file is caller-supplied, so its size was always an input; the three preloading storage modes now share the ceiling Indexed already had. (Storage::Pread preloads nothing and never had the problem.)
• MaxEntClassifier::restore's JSON parser descended one stack frame per nesting level with no bound, so a model file of 20,000 nested brackets overflowed the stack — which aborts rather than unwinding, so no caller could catch it. It is bounded at 128 levels.

What this page does not tell you

No performance figure on this site has been re-measured against 0.2.0. The benchmark campaign for this release has not been run. Every timing published here and on the benchmark pages was produced against 0.1.0-era code and is marked pending; the kernels underneath several of them have been replaced since. Treat every number as historical, and measure your own workload rather than reasoning from a ratio on this site. The resource figures in the section above are peak-RSS measurements recorded with the fix, not throughput.

It also does not cover the places where only the justification moved. Several claims in 0.1's documentation were wrong about code that is unchanged, and several fixtures were re-derived from the publications they should always have come from without any value moving:

• An empty phonetic key does not imply the input had no recognised letter (Metaphone on "y", Cologne on "h", MatchRatingApproach on any single vowel), and DaitchMokotoff does advance by byte length past a matched pattern, which consumes one character too many for four two-byte keys.
• A shortest-path tie-break follows relaxation order, not insertion order, and 149 French inflector entries described as load-bearing are reached by no rule.
• Cologne's 170 expected values were re-derived from Postel (1969) and Match Rating's from NBS Special Publication 500-2 (1977). Where that paper reads its two comparison passes as consecutive, Verbora interleaves them — a difference of 31,044 ratings in 1,129,284 pairs, now stated as a Verbora decision rather than inherited. The implementation did not change; the claim about it did.

If you were relying on one of those claims rather than on the code, your program was already doing something else.

If you are stuck

1. Check the crate's rustdoc first. The crate root is the whole public surface now, so cargo doc --open -p verbora-<crate> shows you everything in one list.
2. Check the crate's README.md. Every crate has one as of 0.2.0, it is that crate's crates.io landing page, and its examples are compiled as doctests.
3. Check the feature page for the subsystem — linked from Features — for what the replacement is for, not just what it is called.
4. Report a documentation error as a bug. If this page sent you somewhere that does not exist, that is worth an issue: the repository.

Next

• Installation — the version pins and the crate table.
• Your first program — the current API, four ways.
• Choosing the right API — what to reach for now.