add section about implied bounds #1261

lcnr · 2022-09-05T05:39:16Z

cc https://github.com/rust-lang/rust/pull/100747/files#r962488178

MatthewPeterKelly

Thanks for adding this documentation to support rust-lang/rust#100747. I'll plan to hold off merging that PR until this one lands (so that I can add a good reference).

src/trait-bounds.md

MatthewPeterKelly · 2022-09-09T00:08:17Z

src/trait-bounds.md

+Only lifetime bounds are implied, trait bounds still have to be explicitly added.
+This behavior may change in the future however. The following example still causes an error:
+```rust,compile_fail
+use std::fmt::Debug;
+struct IsDebug<T: Debug>(T);
+// error[E0277]: `T` doesn't implement `Debug`
+fn doesnt_specify_t_debug<T>(x: IsDebug<T>) {}
+```


This provides helpful context - good idea to add it here.

MatthewPeterKelly

Looks good to me!

MatthewPeterKelly · 2022-09-09T21:09:49Z

(I don't have write access to the repo, so I think you'll need someone else to rubber stamp the PR).

MatthewPeterKelly · 2022-09-24T10:50:58Z

@Havvy - would you be able to do a quick review for this PR?

Havvy

Reading through this, I feel like the prose is unnecessarily vague. And in fact, it fails to answer two key questions:

What are all the places that lifetime bounds are inferred? As written, it looks like it is only for generics in functions. Are there others?
How are the bounds chosen by looking at the types of the function parameters and output? In the simplest case, as shown by the example, a top level reference to a generic type gives said generic type a lifetime bound of the lifetime of the reference. Are there more complicated cases? If so, what are they?

--

The examples are good.

src/trait-bounds.md

Havvy · 2022-10-11T22:44:41Z

src/trait-bounds.md

+This is inferred because the function signature contains the type `&'a T` which is
+only valid if `T: 'a` holds.
+
+Rust adds implied bounds for all inputs and outputs of functions. Inside of `requires_t_outlives_a`


When does it add implied bounds? You have a lot of good examples, but the actual rules for how the bounds are inferred are missing. I don't actually know the rules, so I can't be helpful here. If you don't either, I guess we can merge this and open an issue or we can find somebody who does know the rules. I do imagine it would be something like "For each parameter type and output type, if the type contains a reference to a generic type T (declared by the function item?), then the bounds with a lifetime 'a, then T: 'a is inferred."

I guess we can merge this and open an issue

let's do that then. I think explaining this would require the reference to have a section about well-formedness requirements and I don't have the time to write that one rn. Lets open an issue and i can try to look into that one at a later point.

Havvy

Couple nits.

src/trait-bounds.md

lcnr · 2022-10-26T07:06:36Z

done 👍

lcnr · 2023-08-10T12:58:12Z

👋 do you think we can go ahead and merge this?

Co-authored-by: Ryan Scheel <Ryan.havvy@gmail.com>

compile_fail is a dangerous thing to use since it doesn't validate which error is generated (it could be something like a typo).

ehuss

Sure, I think this can go ahead and get merged. I'd agree with Havvy's comments that this seems a little informal, but I can understand how being more precise would take a ton more work.

Resolved

Update books ## rust-lang/book 3 commits in 668c64760b5c7ea654facb4ba5fe9faddfda27cc..72187f5cd0beaaa9c6f584156bcd88f921871e83 2023-08-04 14:42:07 UTC to 2023-08-03 13:36:44 UTC - redirects: change link for the `#![no_std]` tutorial (rust-lang/book#3705) - [chpt10.2] - Small wording changes (rust-lang/book#3724) - Improve sentence (rust-lang/book#3725) ## rust-embedded/book 1 commits in 1e5556dd1b864109985d5871616ae6b9164bcead..99ad2847b865e96d8ae7b333d3ee96963557e621 2023-08-11 06:31:04 UTC to 2023-08-11 06:31:04 UTC - Fix a small typo in qemu.md (rust-embedded/book#359) ## rust-lang/nomicon 1 commits in 302b995bcb24b70fd883980fd174738c3a10b705..388750b081c0893c275044d37203f97709e058ba 2023-08-10 21:15:21 UTC to 2023-08-10 21:15:21 UTC - Document thiscall abi (rust-lang/nomicon#311) ## rust-lang/reference 10 commits in 1ea0178266b3f3f613b0fabdaf16a83961c99cdb..d43038932adeb16ada80e206d4c073d851298101 2023-08-12 19:07:28 UTC to 2023-07-16 20:12:46 UTC - Document thiscall abi (rust-lang/reference#1092) - add section about implied bounds (rust-lang/reference#1261) - Clearly specify the `instruction_set` effects (rust-lang/reference#1307) - Fix merge queue building twice. (rust-lang/reference#1383) - Clarify UB around immutability & mutation (rust-lang/reference#1385) - mention the extra const UB (rust-lang/reference#1273) - Operator expressions: make the note about division by zero clearer. (rust-lang/reference#1384) - Make unsafe keyword docs less confusing (rust-lang/reference#1379) - Say that division by zero for primitive types panics (rust-lang/reference#1382) - Add CI trigger for merge queues. (rust-lang/reference#1381) ## rust-lang/rust-by-example 3 commits in 8a87926a985ce32ca1fad1be4008ee161a0b91eb..07e0df2f006e59d171c6bf3cafa9d61dbeb520d8 2023-07-24 11:37:55 UTC to 2023-07-24 11:35:36 UTC - Added attribute unused_labels - fixed warning. (rust-lang/rust-by-example#1729) - more explanation about panic (rust-lang/rust-by-example#1728) - chore: add the portuguese version of this project to `readme.md` (rust-lang/rust-by-example#1727) ## rust-lang/rustc-dev-guide 31 commits in b5a12d95e32ae53791cc6ab44417774667ed2ac6..b123ab4754127d822ffb38349ce0fbf561f1b2fd 2023-08-14 08:34:59 UTC to 2023-07-11 06:02:34 UTC - fix: stabilize debugger_visualizer (rust-lang/rustc-dev-guide#1766) - feat(part-5-intro): make "Part 5" obvious (rust-lang/rustc-dev-guide#1753) - Update from `#[warn_]` to `#[warning]` diagnostic attributes (rust-lang/rustc-dev-guide#1765) - Add RPITIT documentation (rust-lang/rustc-dev-guide#1764) - fix(visitor.md): fix a type name in a code sample (rust-lang/rustc-dev-guide#1762) - fix(name-resolution): remove unnecessary closing paranthesis (rust-lang/rustc-dev-guide#1760) - fix(macro-expansion.md): fix the article `an` to `a` (rust-lang/rustc-dev-guide#1759) - fix(serialization.md): fix the name of a derive macro (rust-lang/rustc-dev-guide#1756) - fix(serialization.md): add a necessary plural suffix (rust-lang/rustc-dev-guide#1757) - fix(salsa.md): add punctuation to prevent confusion (rust-lang/rustc-dev-guide#1754) - fix(salsa.md): remove duplicate "To Be" verb (rust-lang/rustc-dev-guide#1755) - feat(fuzzing.md): make `halfempty` word a link (rust-lang/rustc-dev-guide#1750) - fix(about.md): use `a` instead of `an` (rust-lang/rustc-dev-guide#1751) - refactor(git.md): make git-scm links clickable (rust-lang/rustc-dev-guide#1747) - fix(walkthrough.md) add a comma operator to eliminate ambiguity (rust-lang/rustc-dev-guide#1749) - fix(git.md): remove a confusing end of sentence character (rust-lang/rustc-dev-guide#1748) - refactor(profiling/with_perf): remove a wrong to be verb (rust-lang/rustc-dev-guide#1746) - refactor(tests/headers): remove duplicate list item (rust-lang/rustc-dev-guide#1745) - refactor(test/headers.md): make the meaning more obvious (rust-lang/rustc-dev-guide#1744) - refactor(tests/ui): remove unnecessary duplicate word (rust-lang/rustc-dev-guide#1743) - refactor(compiletest): remove unnecessary duplicate word (rust-lang/rustc-dev-guide#1742) - generic_arguments.md: substs -> GenericArgs (rust-lang/rustc-dev-guide#1741) - fix(suggested): remove an unnecessary and confusing statement (rust-lang/rustc-dev-guide#1739) - fix(how-to-build-and-run): fix a typo ("fromer" -> "former") (rust-lang/rustc-dev-guide#1736) - fix(how-to-build-and-run): remove a wrong paragraph (rust-lang/rustc-dev-guide#1735) - coverage code has moved (rust-lang/rustc-dev-guide#1728) - linked issue is closed (rust-lang/rustc-dev-guide#1729) - remove duplicate reference in about-this-guide.md (rust-lang/rustc-dev-guide#1734) - Explain more in depth what early and late bound generic parameters are (rust-lang/rustc-dev-guide#1732) - add section for normalization with the new solver (rust-lang/rustc-dev-guide#1731) - Improve cleanup-crew.md with an example post (rust-lang/rustc-dev-guide#1730)

lcnr mentioned this pull request Sep 5, 2022

Add long description and test for E0311 rust-lang/rust#100747

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MatthewPeterKelly reviewed Sep 9, 2022

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lcnr mentioned this pull request Aug 10, 2023

Implied bounds / well-formedness of references treats contravariant lifetimes the same as covariant rust-lang/rust#106431

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lcnr and others added 9 commits August 10, 2023 13:45

add section about implied bounds

f193405

review

f01e75f

review

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SPAAAAAAAAAAACE

c0c23b8

Update src/trait-bounds.md

a707381

Co-authored-by: Ryan Scheel <Ryan.havvy@gmail.com>

parameters

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w

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24d44b5

Small editorial nits.

f24f128

compile_fail is a dangerous thing to use since it doesn't validate which error is generated (it could be something like a typo).

ehuss force-pushed the implied-bounds branch from f16d587 to f24f128 Compare August 10, 2023 20:46

ehuss approved these changes Aug 10, 2023

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ehuss added this pull request to the merge queue Aug 10, 2023

Merged via the queue into rust-lang:master with commit f7e6f04 Aug 10, 2023
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lcnr deleted the implied-bounds branch August 10, 2023 22:01

rustbot mentioned this pull request Aug 14, 2023

Update books rust-lang/rust#114815

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rustbot mentioned this pull request Aug 28, 2023

Update books rust-lang/rust#115321

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add section about implied bounds #1261

add section about implied bounds #1261

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add section about implied bounds #1261

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