Create `unnecessary_send_constraint` lint for `&(dyn ... + Send)` #110961

john-h-k · 2023-04-28T19:19:19Z

Does not yet handle `Send` being only constraint (will still recommend removing it)

rustbot · 2023-04-28T19:19:29Z

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rustbot · 2023-04-28T19:19:32Z

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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library/core/src/cell.rs

john-h-k · 2023-04-28T19:23:55Z

Also, tests/ui/lint tests seem to have really inconsistent naming - when should I use hyphens vs underscores in file names?

john-h-k · 2023-04-28T19:27:44Z

Also, it appears no lint is generated for duplicate auto traits (like &(dyn Any + Any + Send)). Is there a reason for this? If not I can add that to as the logic is similar ish (in a separate PR)

compiler/rustc_lint/src/useless_send_constraint.rs

Co-authored-by: est31 <[email protected]>

rfcbot · 2023-06-27T19:06:48Z

🔔 This is now entering its final comment period, as per the review above. 🔔

compiler/rustc_lint/src/unnecessary_send_constraint.rs

RalfJung · 2023-06-27T19:46:02Z

compiler/rustc_lint/src/unnecessary_send_constraint.rs

+    /// References cannot be sent across threads unless they have a `Sync` bound, so constraining them to `Send` without `Sync` is unnecessary.
+    pub UNNECESSARY_SEND_CONSTRAINT,
+    Warn,
+    "constraining a reference to `Send` without `Sync` is unnecessary, consider removing it"


Shouldn't the recommendation be to replace Send by Sync? That seems more likely to be what the author intended.

See this comment: #110961 (comment)

In those situations, it stems from a Box<dyn Any + Send> to which a &dyn was made. It's easy to just copy Any + Send, but the Send part should be removed for shared references.

That seems like a fairly special case to me, I would not expect this to be common enough to warrant a general recommendation of dropping the Send. The more symmetric thing to do would be to have &dyn Any+Sync, to get a shared reference that can still be shared across thread boundaries. Of course then you'd need Box<dyn Any+Send+Sync> for the owned case.

So it should recommend replacing Send with Sync instead? just confirming before making the change

RalfJung · 2023-06-27T19:47:23Z

compiler/rustc_lint/messages.ftl

@@ -192,6 +192,12 @@ lint_redundant_semicolons =
        *[false] this semicolon
    }

+lint_unnecessary_send_constraint = constraining a reference to `Send` is meaningless
+    .suggestion = {$only_trait ->
+        [true] replace this with `std::any::Any`


Replacing by Any is a very strange suggestion. Why would type_id machinery be necessary? Can't the 'static bound cause a problem?

I wasn't sure what else to put, this is for the scenario where Send is the only trait specified (so removing it would be invalid)

compiler/rustc_lint/src/unnecessary_send_constraint.rs

rfcbot · 2023-07-07T19:08:42Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

nikomatsakis · 2023-07-18T18:21:05Z

It looks like we agreed to this. I'm going to drop nomination.

@rustbot labels -I-lang-nominated

Explicitly document how Send and Sync relate to references Some of these relations were already mentioned in the text, but that Send is implemented for &mut impl Send was not mentioned, neither did the docs list when &T is Sync. Inspired by the discussion in #110961. [Proof](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ed77bfc3c77ba664400ebc2734f500e6) based on `@lukas-code` 's [example](rust-lang/rust#110961 (comment)).

nikomatsakis · 2023-07-25T18:41:07Z

@rustbot author

The team role is done here, FCP is complete, we are now waiting on the author to address feedback though.

john-h-k · 2023-08-31T13:19:13Z

@rustbot ready

(still 1 part of the review I am unsure about so have asked for clarification, but other comments have been addressed)

steffahn · 2024-01-16T16:43:20Z

library/std/src/io/error.rs

-    pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)> {
+    pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Sync + 'static)> {


This is a mutable reference. For &mut … + Send, the Send makes sense, doesn’t it?

steffahn · 2024-01-16T16:51:29Z

library/std/src/io/error.rs

-    pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
+    pub fn get_ref(&self) -> Option<&(dyn error::Error + Sync + 'static)> {


dyn Error + Send + Sync has some API that dyn Error + Sync doesn’t have. Like an is method and a downcast_ref method.

Also, in any case, changing these methods is a breaking change the impact of which I’m not sure has been tested anywhere yet.

that dyn Error + Sync doesn’t have. Like an is method and a downcast_ref method.

dyn Error also implements is and downcast_* with exact the same signature.

dyn Error also implements is and downcast_* with exact the same signature.

Oops, my bad. We cannot use dyn Error's methods on dyn Error + Sync, though it seems odd to me.

Dylan-DPC · 2024-09-24T07:07:31Z

@john-h-k if you can rebase and resolve the conflicts, we can push this for a review

john-h-k added 4 commits April 28, 2023 14:48

Initial impl of lint_useless_send_constraint

efbdc5b

Does not yet handle `Send` being only constraint (will still recommend removing it)

Fixes

532368c

Trait test and edge case handling

6ed01d9

fmt

a00fd67

rustbot assigned davidtwco Apr 28, 2023

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john-h-k added 3 commits April 28, 2023 20:35

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john-h-k added 3 commits April 28, 2023 20:49

Compiling except for the actual lint

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Remove uses of send

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john-h-k mentioned this pull request Apr 29, 2023

Add lint_duplicate_trait for duplicate specified traits in a trait object #110991

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Create `unnecessary_send_constraint` lint for `&(dyn ... + Send)` #110961

Create `unnecessary_send_constraint` lint for `&(dyn ... + Send)` #110961

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rfcbot commented Jun 27, 2023

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Dylan-DPC commented Sep 24, 2024

		pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)> {
		pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Sync + 'static)> {

		pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
		pub fn get_ref(&self) -> Option<&(dyn error::Error + Sync + 'static)> {

Create unnecessary_send_constraint lint for &(dyn ... + Send) #110961

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Create unnecessary_send_constraint lint for &(dyn ... + Send) #110961

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Dylan-DPC commented Sep 24, 2024

Create `unnecessary_send_constraint` lint for `&(dyn ... + Send)` #110961

Create `unnecessary_send_constraint` lint for `&(dyn ... + Send)` #110961

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