Float parsing can fail on valid float literals

[hanna-kruppe]

That is, it returns Err(..) on inputs with lots of digits and extreme exponents — it doesn't (shouldn't) panic.

This is a limitation of the current code that has been known since that code was originally written, but is quite nontrivial to fix, so I didn't get around to it yet. Basically one would have to implement an algorithm similar to glib's strtod and replace the current slow paths (Algorithm M and possibly Algorithm R) with it. If someone wants to have a crack at it, go ahead, just give me a heads up so we don't duplicate work. See also #27307 for some discussion.

This bug doesn't affect reasonably short (17-ish decimal digits) representations of finite (even subnormal) floats, and neither does it affect most inputs that are rounded down to zero or rounded up to infinity. The only problem are very small exponents with so many integer digits that the number can't just be rounded down to zero and can't be represented with our custom 1280-bit bignums. One example, for f64, is 1234567890123456789012345678901234567890e-340 (which is a finite and normal float, about 1.23456789e-301). This would be annoying on its own, but it also trips up constant evaluation in the compiler (see #31109).

---

Edit 2021-07-03: added test case:

fn main() {
    let _original: f64 = 1234567890123456789012345678901234567890e-340;
    let _c21: f64 = 2.225073858507201136057409796709131975934819546351645648023426109724822222021076945516529523908135087914149158913039621106870086438694594645527657207407820621743379988141063267329253552286881372149012981122451451889849057222307285255133155755015914397476397983411801999323962548289017107081850690630666655994938275772572015763062690663332647565300009245888316433037779791869612049497390377829704905051080609940730262937128958950003583799967207254304360284078895771796150945516748243471030702609144621572289880258182545180325707018860872113128079512233426288368622321503775666622503982534335974568884423900265498198385487948292206894721689831099698365846814022854243330660339850886445804001034933970427567186443383770486037861622771738545623065874679014086723327636718749999999999999999999999999999999999999e-308;
    let _c22: f64 = 2.22507385850720113605740979670913197593481954635164564802342610972482222202107694551652952390813508791414915891303962110687008643869459464552765720740782062174337998814106326732925355228688137214901298112245145188984905722230728525513315575501591439747639798341180199932396254828901710708185069063066665599493827577257201576306269066333264756530000924588831643303777979186961204949739037782970490505108060994073026293712895895000358379996720725430436028407889577179615094551674824347103070260914462157228988025818254518032570701886087211312807951223342628836862232150377566662250398253433597456888442390026549819838548794829220689472168983109969836584681402285424333066033985088644580400103493397042756718644338377048603786162277173854562306587467901408672332763671875e-308;
    let _c23: f64 = 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000222507385850720138309023271733240406421921598046233183055332741688720443481391819585428315901251102056406733973103581100515243416155346010885601238537771882113077799353200233047961014744258363607192156504694250373420837525080665061665815894872049117996859163964850063590877011830487479978088775374994945158045160505091539985658247081864511353793580499211598108576605199243335211435239014879569960959128889160299264151106346631339366347758651302937176204732563178148566435087212282863764204484681140761391147706280168985324411002416144742161856716615054015428508471675290190316132277889672970737312333408698898317506783884692609277397797285865965494109136909540613646756870239867831529068098461721092462539672851562500000000000000001;
    let _c25: f64 = 179769313486231580793728971405303415079934132710037826936173778980444968292764750946649017977587207096330286416692887910946555547851940402630657488671505820681908902000708383676273854845817711531764475730270069855571366959622842914819860834936475292719074168444365510704342711559699508093042880177904174497791.9999999999999999999999999999999999999999999999999999999999999999999999;
    let _c29: f64 = 2.47032822920623272e-324;
    let _c37: f64 = 6.631236871469758276785396630275967243399099947355303144249971758736286630139265439618068200788048744105960420552601852889715006376325666595539603330361800519107591783233358492337208057849499360899425128640718856616503093444922854759159988160304439909868291973931426625698663157749836252274523485312442358651207051292453083278116143932569727918709786004497872322193856150225415211997283078496319412124640111777216148110752815101775295719811974338451936095907419622417538473679495148632480391435931767981122396703443803335529756003353209830071832230689201383015598792184172909927924176339315507402234836120730914783168400715462440053817592702766213559042115986763819482654128770595766806872783349146967171293949598850675682115696218943412532098591327667236328125E-316;
    let _c38: f64 = 3.237883913302901289588352412501532174863037669423108059901297049552301970670676565786835742587799557860615776559838283435514391084153169252689190564396459577394618038928365305143463955100356696665629202017331344031730044369360205258345803431471660032699580731300954848363975548690010751530018881758184174569652173110473696022749934638425380623369774736560008997404060967498028389191878963968575439222206416981462690113342524002724385941651051293552601421155333430225237291523843322331326138431477823591142408800030775170625915670728657003151953664260769822494937951845801530895238439819708403389937873241463484205608000027270531106827387907791444918534771598750162812548862768493201518991668028251730299953143924168545708663913273994694463908672332763671875E-319;
    let _c39: f64 = 6.953355807847677105972805215521891690222119817145950754416205607980030131549636688806115726399441880065386399864028691275539539414652831584795668560082999889551357784961446896042113198284213107935110217162654939802416034676213829409720583759540476786936413816541621287843248433202369209916612249676005573022703244799714622116542188837770376022371172079559125853382801396219552418839469770514904192657627060319372847562301074140442660237844114174497210955449896389180395827191602886654488182452409583981389442783377001505462015745017848754574668342161759496661766020028752888783387074850773192997102997936619876226688096314989645766000479009083731736585750335262099860150896718774401964796827166283225641992040747894382698751809812609536720628966577351093292236328125E-310;
    let _c40: f64 = 3.339068557571188581835713701280943911923401916998521771655656997328440314559615318168849149074662609099998113009465566426808170378434065722991659642619467706034884424989741080790766778456332168200464651593995817371782125010668346652995912233993254584461125868481633343674905074271064409763090708017856584019776878812425312008812326260363035474811532236853359905334625575404216060622858633280744301892470300555678734689978476870369853549413277156622170245846166991655321535529623870646888786637528995592800436177901746286272273374471701452991433047257863864601424252024791567368195056077320885329384322332391564645264143400798619665040608077549162173963649264049738362290606875883456826586710961041737908872035803481241600376705491726170293986797332763671875E-319;
    let _c64: f64 = 2.4703282292062327208828439643411068618252990130716238221279284125033775363510437593264991818081799618989828234772285886546332835517796989819938739800539093906315035659515570226392290858392449105184435931802849936536152500319370457678249219365623669863658480757001585769269903706311928279558551332927834338409351978015531246597263579574622766465272827220056374006485499977096599470454020828166226237857393450736339007967761930577506740176324673600968951340535537458516661134223766678604162159680461914467291840300530057530849048765391711386591646239524912623653881879636239373280423891018672348497668235089863388587925628302755995657524455507255189313690836254779186948667994968324049705821028513185451396213837722826145437693412532098591327667236328124999e-324;
    let _c65: f64 = 2.4703282292062327208828439643411068618252990130716238221279284125033775363510437593264991818081799618989828234772285886546332835517796989819938739800539093906315035659515570226392290858392449105184435931802849936536152500319370457678249219365623669863658480757001585769269903706311928279558551332927834338409351978015531246597263579574622766465272827220056374006485499977096599470454020828166226237857393450736339007967761930577506740176324673600968951340535537458516661134223766678604162159680461914467291840300530057530849048765391711386591646239524912623653881879636239373280423891018672348497668235089863388587925628302755995657524455507255189313690836254779186948667994968324049705821028513185451396213837722826145437693412532098591327667236328125e-324;
    let _c66: f64 = 2.4703282292062327208828439643411068618252990130716238221279284125033775363510437593264991818081799618989828234772285886546332835517796989819938739800539093906315035659515570226392290858392449105184435931802849936536152500319370457678249219365623669863658480757001585769269903706311928279558551332927834338409351978015531246597263579574622766465272827220056374006485499977096599470454020828166226237857393450736339007967761930577506740176324673600968951340535537458516661134223766678604162159680461914467291840300530057530849048765391711386591646239524912623653881879636239373280423891018672348497668235089863388587925628302755995657524455507255189313690836254779186948667994968324049705821028513185451396213837722826145437693412532098591327667236328125001e-324;
    let _c67: f64 = 7.4109846876186981626485318930233205854758970392148714663837852375101326090531312779794975454245398856969484704316857659638998506553390969459816219401617281718945106978546710679176872575177347315553307795408549809608457500958111373034747658096871009590975442271004757307809711118935784838675653998783503015228055934046593739791790738723868299395818481660169122019456499931289798411362062484498678713572180352209017023903285791732520220528974020802906854021606612375549983402671300035812486479041385743401875520901590172592547146296175134159774938718574737870961645638908718119841271673056017045493004705269590165763776884908267986972573366521765567941072508764337560846003984904972149117463085539556354188641513168478436313080237596295773983001708984374999e-324;
    let _c68: f64 = 7.4109846876186981626485318930233205854758970392148714663837852375101326090531312779794975454245398856969484704316857659638998506553390969459816219401617281718945106978546710679176872575177347315553307795408549809608457500958111373034747658096871009590975442271004757307809711118935784838675653998783503015228055934046593739791790738723868299395818481660169122019456499931289798411362062484498678713572180352209017023903285791732520220528974020802906854021606612375549983402671300035812486479041385743401875520901590172592547146296175134159774938718574737870961645638908718119841271673056017045493004705269590165763776884908267986972573366521765567941072508764337560846003984904972149117463085539556354188641513168478436313080237596295773983001708984375e-324;
    let _c69: f64 = 7.4109846876186981626485318930233205854758970392148714663837852375101326090531312779794975454245398856969484704316857659638998506553390969459816219401617281718945106978546710679176872575177347315553307795408549809608457500958111373034747658096871009590975442271004757307809711118935784838675653998783503015228055934046593739791790738723868299395818481660169122019456499931289798411362062484498678713572180352209017023903285791732520220528974020802906854021606612375549983402671300035812486479041385743401875520901590172592547146296175134159774938718574737870961645638908718119841271673056017045493004705269590165763776884908267986972573366521765567941072508764337560846003984904972149117463085539556354188641513168478436313080237596295773983001708984375001e-324;
    let _c76: f64 = 94393431193180696942841837085033647913224148539854e-358;
    let _c79: f64 = 104308485241983990666713401708072175773165034278685682646111762292409330928739751702404658197872319129036519947435319418387839758990478549477777586673075945844895981012024387992135617064532141489278815239849108105951619997829153633535314849999674266169258928940692239684771590065027025835804863585454872499320500023126142553932654370362024104462255244034053203998964360882487378334860197725139151265590832887433736189468858614521708567646743455601905935595381852723723645799866672558576993978025033590728687206296379801363024094048327273913079612469982585674824156000783167963081616214710691759864332339239688734656548790656486646106983450809073750535624894296242072010195710276073042036425579852459556183541199012652571123898996574563824424330960027873516082763671875e-1075;
}

[pnkfelix]

cc me

[arielb1]

can't we just have a slowpath that uses 3072-bit bignums or something?

I mean, for numbers with a very long number of digits, we could:

• find one of the (2, or 1 if exactly equal) halfway-floating-point-numbers closest to our string. This can be done by truncating the number to 20 or so digits, converting that to a float, and taking the halfway-point above that.
• compare our string with that halfway-float and use that to pick which float to round it to.

Certainly, at the worst-case, we do need to compare our string with an halfway-float, After all, 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000247032822920623272088284396434110686182529901307162382212792841250337753635104375932649918180817996189898282347722858865463328355177969898199387398005390939063150356595155702263922908583924491051844359318028499365361525003193704576782492193656236698636584807570015857692699037063119282795585513329278343384093519780155312465972635795746227664652728272200563740064854999770965994704540208281662262378573934507363390079677619305775067401763246736009689513405355374585166611342237666786041621596804619144672918403005300575308490487653917113865916462395249126236538818796362393732804238910186723484976682350898633885879256283027559956575244555072551893136908362547791869486679949683240497058210285131854513962138377228261454376934125320985913276672363281249 is zero and 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000247032822920623272088284396434110686182529901307162382212792841250337753635104375932649918180817996189898282347722858865463328355177969898199387398005390939063150356595155702263922908583924491051844359318028499365361525003193704576782492193656236698636584807570015857692699037063119282795585513329278343384093519780155312465972635795746227664652728272200563740064854999770965994704540208281662262378573934507363390079677619305775067401763246736009689513405355374585166611342237666786041621596804619144672918403005300575308490487653917113865916462395249126236538818796362393732804238910186723484976682350898633885879256283027559956575244555072551893136908362547791869486679949683240497058210285131854513962138377228261454376934125320985913276672363281251 is the smallest denormal.

[hanna-kruppe]

@arielb1 How would you actually implement the second step? Keep in mind that this is in libcore, so we can't use float formatting because that would allocate. (Maybe you could use the underlying functions that are also in libcore, but this probably complicates everything significantly. However I have to admit that I don't even know what the interface of those functions looks like, so maybe it's not that bad.)

[arielb1]

I actually read the paper, and its Algorithm R is a variant of what I've came up with. I guess we just need to enlarge our bignums to sufficiently-many bits and use sticky bits+rounding (we don't really need 1075 digits = 3571 bits because we can skip the powers-of-2).

[ahrvoje]

I've gathered a collection of String to Double test cases proved to be problematic in various projects during the past few decades:
https://github.com/ahrvoje/numerics/blob/master/strtod/strtod_tests.toml

Rust 1.14.0 (e8a0123 2016-12-16) for Windows fails on 17 of 81 conversion tests (C21, C22, C23, C25, C29, C37, C38, C39, C40, C64, C65, C66, C67, C68, C69, C76, C79), with the following being the shortest one (C29):

fn main() {
    println!("{}", 2.47032822920623272e-324);
}

error[E0080]: constant evaluation error
 --> src\main.rs:2:20
  |
2 |     println!("{}", 2.47032822920623272e-324);
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^ unimplemented constant expression: could not evaluate float literal (see issue #31407)

[hanna-kruppe]

@ahrvoje That's a very nice collection of test cases, thanks for sharing!

[nagisa]

This is a regression from 1.7.0 to 1.8.0. It broke my crate (lol).

[anatol]

@nagisa's crate is math.rs.