Lament the invincibility of the Turbofish #53562

varkor · 2018-08-21T13:47:42Z

Here a test case is added to ensure that any others attempting to drive the Turbofish to extinction have second thoughts. Previously the entire test suite would succeed if generic arguments were accepted without disambiguation, making for confusing and heartbreaking circumstances.

rust-highfive · 2018-08-21T13:47:53Z

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

Centril · 2018-08-21T14:02:18Z

I must say: "oh noes".
Never has there been a sadder day.
Yet for all our woes,
There is great melancholy here, I must say.
The Great Turbofish is here to stay.

radix · 2018-08-21T15:36:26Z

src/test/ui/bastion-of-the-turbofish.rs

+// in permitting generic arguments to be provided without the consent of the
+// Great Turbofish. Should you be so naïve as to try to revolt against its
+// mighty clutches, here shall its wrath be indomitably displayed. This
+// program must pass for all eternity, fundamentally at odds with a impetuous


I'm not sure if there's some sort of poetic device being used here, but to my untrained eye, "a impetuous" seems like an error.

llogiq · 2018-08-21T16:03:52Z

A turbofish, often lamented
and shunned, even called "demented",
has the last laugh this time
so with song and with rhyme
it is hereby rust-astic cemented.

nagisa · 2018-08-21T16:27:22Z

@bors r+ rollup

bors · 2018-08-21T16:27:23Z

📌 Commit 7f4442c5105f4950cf4c19d37cf39fed5347ee0d has been approved by nagisa

wycats · 2018-08-21T17:31:31Z

@varkor While I definitely agree that this makes it "not a slam dunk", I don't think it means that there is no hope. The case in question is very obscure, would be very unlikely to compile both ways in any real scenario, is easily linted (in the case of ambiguity). It is therefore possible that we could decide to make the change anyway (with a long enough time horizon, and with enough practical surveying), and I wouldn't want someone interested in doing the (harder) work of a transition plan to find this issue and decide that there was no point in even attempting it.

varkor · 2018-08-21T21:15:06Z

@wycats: I personally would love to see a transition plan, though I think the constraints are difficult. The test case here is just to make sure that if someone does intend to tackle the problem, they're aware of this particular ambiguity. I think there's enough context to make clear the point of the test (the origin should be easily located with a quick search).

withoutboats · 2018-08-21T21:22:16Z

Wouldn't propose any change to this PR, but I agree with @wycats that this isn't necessarily the hard blocker. I would enumerate these constraints on getting rid of the turbofish:

How will we migrate these ambiguous edge cases?
How does it impact our grammatical complexity (both formally and informally)? How much more difficult is it to implement a Rust parser?
How does the implementation impact compile times?

None of these seem impossible to overcome, but possibly difficult.

eddyb · 2018-08-22T01:45:54Z

We can "just" make this test fail in Rust 2018, right? Riiight?

ts826848 · 2018-08-22T13:55:03Z

src/test/ui/bastion-of-the-turbofish.rs

+// Bastion of the Turbofish
+// ------------------------
+// Beware travellers, lest you venture into waters callous and unforgiving,
+// where hope must abandoned, ere it is cruelly torn from you. For here stands


// where hope must *be* abandoned?

Nicely caught, thanks!

varkor · 2018-08-22T14:02:03Z

@bors r=nagisa rollup

bors · 2018-08-22T14:02:04Z

📌 Commit 3d5fef6 has been approved by nagisa

KamilaBorowska · 2018-08-22T14:31:16Z

Something worth nothing is that in C# 2.0, a breaking change was introduced to change the way a<b,c>(d) is parsed. I can see this being feasible to do in Rust as well.

…nagisa Lament the invincibility of the Turbofish Here a test case is added to ensure that any others attempting to drive the Turbofish to extinction have second thoughts. Previously the [entire test suite would succeed](rust-lang#53511) if generic arguments were accepted without disambiguation, making for [confusing and heartbreaking circumstances](rust-lang/rfcs#2527).

mark-i-m · 2018-08-22T19:52:29Z

@varkor Perhaps the test should include a link to this issue for context?

@ghost

Rollup of 10 pull requests Successful merges: - #53418 (Mark some suggestions as MachineApplicable) - #53431 (Moved some feature gate ui tests to correct location) - #53442 (Update version of rls-data used with save-analysis) - #53504 (Set applicability for more suggestions.) - #53541 (Fix missing impl trait display as ret type) - #53544 (Point at the trait argument when using unboxed closure) - #53558 (Normalize source line and column numbers.) - #53562 (Lament the invincibility of the Turbofish) - #53574 (Suggest direct raw-pointer dereference) - #53585 (Remove super old comment on function that parses items) Failed merges: - #53472 (Use FxHash{Map,Set} instead of the default Hash{Map,Set} everywhere in rustc.) - #53563 (use String::new() instead of String::from(""), "".to_string(), "".to_owned() or "".into()) r? @ghost

Centril · 2018-08-22T20:09:09Z

My world is filled with unbounded sorrow.
I feel there is no tomorrow.
All that I had ever wished for,
Burnt down to its core.
But perhaps someone left open the door,
Could there be a solution in store?
For Turbofish to be fixed...
Or will that evermore be nixed?
The opinions seem to be mixed.

varkor · 2018-08-22T20:17:47Z

@mark-i-m: I added one, but slightly too late for the rollup. We can make the change as a follow-up if need be (though given how much attention has been drawn to it now, I imagine it won't be hard to find if anyone stumbles upon it in the test suite 😄).

bors · 2018-08-22T22:17:20Z

☔ The latest upstream changes (presumably #53607) made this pull request unmergeable. Please resolve the merge conflicts.

varkor · 2018-08-22T22:26:39Z

Are things going to go terribly wrong if I re-approve this after it's been merged as a rollup, with the context clarification? Let's see.

@bors r=nagisa rollup

bors · 2018-08-22T22:26:40Z

📌 Commit b188c2a has been approved by nagisa

…nagisa Lament the invincibility of the Turbofish Here a test case is added to ensure that any others attempting to drive the Turbofish to extinction have second thoughts. Previously the [entire test suite would succeed](rust-lang#53511) if generic arguments were accepted without disambiguation, making for [confusing and heartbreaking circumstances](rust-lang/rfcs#2527).

Rollup of 16 pull requests Successful merges: - #53311 (Window Mutex: Document that we properly initialize the SRWLock) - #53503 (Discourage overuse of mem::forget) - #53545 (Fix #50865: ICE on impl-trait returning functions reaching private items) - #53559 (add macro check for lint) - #53562 (Lament the invincibility of the Turbofish) - #53563 (use String::new() instead of String::from(""), "".to_string(), "".to_owned() or "".into()) - #53592 (docs: minor stylistic changes to str/string docs) - #53594 (Update RELEASES.md to include clippy-preview) - #53600 (Fix a grammatical mistake in "expected generic arguments" errors) - #53614 (update nomicon and book) - #53617 (tidy: Stop requiring a license header) - #53618 (Add missing fmt examples) - #53636 (Prefer `.nth(n)` over `.skip(n).next()`.) - #53644 (Use SmallVec for SmallCStr) - #53664 (Remove unnecessary closure in rustc_mir/build/mod.rs) - #53666 (Added rustc_codegen_llvm to compiler documentation.)

rust-highfive assigned nikomatsakis Aug 21, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 21, 2018

varkor mentioned this pull request Aug 21, 2018

Undisambiguated generic arguments in expressions rust-lang/rfcs#2527

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radix reviewed Aug 21, 2018

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varkor force-pushed the bastion-of-the-turbofish branch from 6b664fa to 865b76a Compare August 21, 2018 15:41

KamilaBorowska approved these changes Aug 21, 2018

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varkor force-pushed the bastion-of-the-turbofish branch from 865b76a to 7f4442c Compare August 21, 2018 16:26

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 21, 2018

ts826848 reviewed Aug 22, 2018

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varkor force-pushed the bastion-of-the-turbofish branch from 7f4442c to 3d5fef6 Compare August 22, 2018 14:01

GuillaumeGomez mentioned this pull request Aug 22, 2018

Rollup of 10 pull requests #53607

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varkor force-pushed the bastion-of-the-turbofish branch from 3d5fef6 to 8e90807 Compare August 22, 2018 20:03

bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 22, 2018

Lament the invincibility of the Turbofish

b188c2a

varkor force-pushed the bastion-of-the-turbofish branch from 8e90807 to b188c2a Compare August 22, 2018 22:25

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 22, 2018

Mark-Simulacrum mentioned this pull request Aug 22, 2018

Rollup of 9 pull requests #53620

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kennytm mentioned this pull request Aug 24, 2018

Rollup of 16 pull requests #53662

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bors merged commit b188c2a into rust-lang:master Aug 24, 2018

varkor deleted the bastion-of-the-turbofish branch August 24, 2018 22:06

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Lament the invincibility of the Turbofish #53562

Lament the invincibility of the Turbofish #53562

varkor commented Aug 21, 2018

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Lament the invincibility of the Turbofish #53562

Lament the invincibility of the Turbofish #53562

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