Add allow-by-default lint for unit bindings #112380

jieyouxu · 2023-06-07T09:34:11Z

Example

#![warn(unit_bindings)]

macro_rules! owo {
    () => {
        let whats_this = ();
    }
}

fn main() {
    // No warning if user explicitly wrote `()` on either side.
    let expr = ();
    let () = expr;
    let _ = ();

    let _ = expr; //~ WARN binding has unit type
    let pat = expr; //~ WARN binding has unit type
    let _pat = expr; //~ WARN binding has unit type

    // No warning for let bindings with unit type in macro expansions.
    owo!();

    // No warning if user explicitly annotates the unit type on the binding.
    let pat: () = expr;
}

outputs

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:17:5
   |
LL |     let _ = expr;
   |     ^^^^-^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`
   |
note: the lint level is defined here
  --> $DIR/unit-bindings.rs:3:9
   |
LL | #![warn(unit_bindings)]
   |         ^^^^^^^^^^^^^

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:18:5
   |
LL |     let pat = expr;
   |     ^^^^---^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`

warning: binding has unit type `()`
  --> $DIR/unit-bindings.rs:19:5
   |
LL |     let _pat = expr;
   |     ^^^^----^^^^^^^^
   |         |
   |         this pattern is inferred to be the unit type `()`

warning: 3 warnings emitted

This lint is not triggered if any of the following conditions are met:

The user explicitly annotates the binding with the () type.
The binding is from a macro expansion.
The user explicitly wrote let () = init;
The user explicitly wrote let pat = ();. This is allowed for local lifetimes.

Known Issue

It is known that this lint can trigger on some proc-macro generated code whose span returns false for Span::from_expansion because e.g. the proc-macro simply forwards user code spans, and otherwise don't have distinguishing syntax context compared to non-macro-generated code.

Closes #71432.

rustbot · 2023-06-07T09:34:19Z

r? @compiler-errors

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

est31 · 2023-06-07T13:40:00Z

It's more a lint for bindings of unit type, right? Maybe it should be renamed to unit_bindings? Or are there hypothetically other instances where the lint could warn?

I'm wondering because clippy has a let and return lint, something which I am hitting often because I like to make the pattern and don't really like the lint. Still, it would be good to instantly know the difference between the lint this PR is adding and clippy::let_and_return.

jieyouxu · 2023-06-07T13:45:47Z

It's more a lint for bindings of unit type, right? Maybe it should be renamed to unit_bindings? Or are there hypothetically other instances where the lint could warn?

Yep, I can rename it to unit_binding, it's called useless_binding now because I couldn't come up with a better name for it. I would also appreciate any feedback on the lint wording / note wording, because I don't think they're very useful (heh) right now.

compiler/rustc_lint_defs/src/lib.rs

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

WaffleLapkin

The implementation looks good.

WaffleLapkin · 2023-06-12T07:22:07Z

T-lang requested a crater run, can you add a temporary commit that makes this lint deny-by-default, so that we can make a crater run and see how much crates have triggered this?

jieyouxu · 2023-06-12T07:45:13Z

T-lang requested a crater run, can you add a temporary commit that makes this lint deny-by-default, so that we can make a crater run and see how much crates have triggered this?

I will make a temporary commit, but I think crater supports -D unit_bindings (cf. https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Allowing.20a.20lint.20on.20diesel.20for.20bors.20try/near/328933796). Something like #107880 (comment).

EDIT: there are several UI tests I will need to fix.

danielhenrymantilla · 2023-06-12T10:07:59Z

Related to this change, the following limitation:

issue: let_unit_value should be allowed when I explicitly write "let () = ..."
PR in progress: Don't lint let_unit_value when () is explicit

As in, while I agree that implicit bindings of () have a rather high likelihood of being an oversight, and thus, a mistake/logic bug (which makes the lint, and this PR, very useful!), there are more rare but nonetheless legitimate cases where a () value is bound, but in a properly annotated way to acknowledge this fact (either by using the () pattern, or a : () type annotation). In such cases, this lint firing is a false-positive, and thus, an annoyance. So this is something to be aware of ⚠️

I think it will just be a matter of waiting for rust-lang/rust-clippy#10844 to be merged, and we will be good to go! 🙂

jieyouxu · 2023-06-12T11:29:35Z

Opened #112549 to adjust multiple UI tests to help reduce the amount of test failures.

jieyouxu · 2023-06-12T11:39:35Z

I updated the lint to not fire in the following cases:

The user explicitly wrote let () = init;.
The user explicitly wrote let pat = ();.

jieyouxu · 2023-06-12T11:55:12Z

Maybe the unit_bindings lint should also not fire when the binding is let _?

…, r=Nilstrieb Adjust UI tests for `unit_bindings` lint - Explicitly annotate `let x: () = expr;` where `x` has unit type, or remove the unit binding to leave only `expr;` instead. - Use `let () = init;` or `let pat = ();` where appropriate. - Fix disjoint-capture-in-same-closure test which wasn't actually testing a closure: `tests/ui/closures/2229_closure_analysis/run_pass/disjoint-capture-in-same-closure.rs`. Note that unfortunately there's *a lot* of UI tests, there are a couple of places where I may have left something like `let (): ()` (this is not needed but is left over from an ealier version of the lint) which is bad style. This PR is to help with the `unit_bindings` lint at rust-lang#112380.

WaffleLapkin · 2023-06-13T15:07:10Z

@jieyouxu I think it makes sense to lint for let _, unsure if there are any cases where you'd want to ignore the unit like that (i.e. without asserting the type like let () = or let _: () = do)

WaffleLapkin · 2023-06-13T15:08:35Z

I will make a temporary commit, but I think crater supports -D unit_bindings

Oh, in this case I think you can drop the commit and we can start a perf run with the flag

jieyouxu · 2023-06-13T15:37:15Z

Oh, in this case I think you can drop the commit and we can start a perf run with the flag

IIUC this is going to still have to wait on #112549 and a few more UI test adjustments, since I think it won't successfully build due to the UI tests.

bors · 2023-11-19T01:00:21Z

⌛ Testing commit f0520fa with merge 16ee2f7...

…ffleLapkin Add allow-by-default lint for unit bindings ### Example ```rust #![warn(unit_bindings)] macro_rules! owo { () => { let whats_this = (); } } fn main() { // No warning if user explicitly wrote `()` on either side. let expr = (); let () = expr; let _ = (); let _ = expr; //~ WARN binding has unit type let pat = expr; //~ WARN binding has unit type let _pat = expr; //~ WARN binding has unit type // No warning for let bindings with unit type in macro expansions. owo!(); // No warning if user explicitly annotates the unit type on the binding. let pat: () = expr; } ``` outputs ``` warning: binding has unit type `()` --> $DIR/unit-bindings.rs:17:5 | LL | let _ = expr; | ^^^^-^^^^^^^^ | | | this pattern is inferred to be the unit type `()` | note: the lint level is defined here --> $DIR/unit-bindings.rs:3:9 | LL | #![warn(unit_bindings)] | ^^^^^^^^^^^^^ warning: binding has unit type `()` --> $DIR/unit-bindings.rs:18:5 | LL | let pat = expr; | ^^^^---^^^^^^^^ | | | this pattern is inferred to be the unit type `()` warning: binding has unit type `()` --> $DIR/unit-bindings.rs:19:5 | LL | let _pat = expr; | ^^^^----^^^^^^^^ | | | this pattern is inferred to be the unit type `()` warning: 3 warnings emitted ``` This lint is not triggered if any of the following conditions are met: - The user explicitly annotates the binding with the `()` type. - The binding is from a macro expansion. - The user explicitly wrote `let () = init;` - The user explicitly wrote `let pat = ();`. This is allowed for local lifetimes. ### Known Issue It is known that this lint can trigger on some proc-macro generated code whose span returns false for `Span::from_expansion` because e.g. the proc-macro simply forwards user code spans, and otherwise don't have distinguishing syntax context compared to non-macro-generated code. For those kind of proc-macros, I believe the correct way to fix them is to instead emit identifers with span like `Span::mixed_site().located_at(user_span)`. Closes rust-lang#71432.

bors · 2023-11-19T01:04:14Z

💔 Test failed - checks-actions

est31 · 2023-11-19T20:03:02Z

the build failure is legit, likely needs a rebase plus x fmt (rustfmt now formats let chains). @rustbot author

This lint is not triggered if any of the following conditions are met: - The user explicitly annotates the binding with the `()` type. - The binding is from a macro expansion. - The user explicitly wrote `let () = init;` - The user explicitly wrote `let pat = ();`. This is allowed for local lifetimes.

WaffleLapkin · 2023-11-22T12:00:31Z

@bors r+

bors · 2023-11-22T12:00:34Z

📌 Commit 8da09ae has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2023-11-22T14:03:20Z

⌛ Testing commit 8da09ae with merge 73bc121...

bors · 2023-11-22T16:13:12Z

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 73bc121 to master...

rust-timer · 2023-11-23T01:02:59Z

Finished benchmarking commit (73bc121): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.0%, 2.5%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.0%, 2.5%]	3

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 676.864s -> 675.017s (-0.27%)
Artifact size: 313.80 MiB -> 313.79 MiB (-0.00%)

Pkgsrc changes: * Adapt checksums and patches. Upstream chnages: Version 1.76.0 (2024-02-08) ========================== Language -------- - [Document Rust ABI compatibility between various types] (rust-lang/rust#115476) - [Also: guarantee that char and u32 are ABI-compatible] (rust-lang/rust#118032) - [Warn against ambiguous wide pointer comparisons] (rust-lang/rust#117758) Compiler -------- - [Lint pinned `#[must_use]` pointers (in particular, `Box<T>` where `T` is `#[must_use]`) in `unused_must_use`.] (rust-lang/rust#118054) - [Soundness fix: fix computing the offset of an unsized field in a packed struct] (rust-lang/rust#118540) - [Soundness fix: fix dynamic size/align computation logic for packed types with dyn Trait tail] (rust-lang/rust#118538) - [Add `$message_type` field to distinguish json diagnostic outputs] (rust-lang/rust#115691) - [Enable Rust to use the EHCont security feature of Windows] (rust-lang/rust#118013) - [Add tier 3 {x86_64,i686}-win7-windows-msvc targets] (rust-lang/rust#118150) - [Add tier 3 aarch64-apple-watchos target] (rust-lang/rust#119074) - [Add tier 3 arm64e-apple-ios & arm64e-apple-darwin targets] (rust-lang/rust#115526) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries --------- - [Add a column number to `dbg!()`] (rust-lang/rust#114962) - [Add `std::hash::{DefaultHasher, RandomState}` exports] (rust-lang/rust#115694) - [Fix rounding issue with exponents in fmt] (rust-lang/rust#116301) - [Add T: ?Sized to `RwLockReadGuard` and `RwLockWriteGuard`'s Debug impls.] (rust-lang/rust#117138) - [Windows: Allow `File::create` to work on hidden files] (rust-lang/rust#116438) Stabilized APIs --------------- - [`Arc::unwrap_or_clone`] (https://doc.rust-lang.org/stable/std/sync/struct.Arc.html#method.unwrap_or_clone) - [`Rc::unwrap_or_clone`] (https://doc.rust-lang.org/stable/std/rc/struct.Rc.html#method.unwrap_or_clone) - [`Result::inspect`] (https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.inspect) - [`Result::inspect_err`] (https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.inspect_err) - [`Option::inspect`] (https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.inspect) - [`type_name_of_val`] (https://doc.rust-lang.org/stable/std/any/fn.type_name_of_val.html) - [`std::hash::{DefaultHasher, RandomState}`] (https://doc.rust-lang.org/stable/std/hash/index.html#structs) These were previously available only through `std::collections::hash_map`. - [`ptr::{from_ref, from_mut}`] (https://doc.rust-lang.org/stable/std/ptr/fn.from_ref.html) - [`ptr::addr_eq`](https://doc.rust-lang.org/stable/std/ptr/fn.addr_eq.html) Cargo ----- See [Cargo release notes] (https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-176-2024-02-08). Rustdoc ------- - [Don't merge cfg and doc(cfg) attributes for re-exports] (rust-lang/rust#113091) - [rustdoc: allow resizing the sidebar / hiding the top bar] (rust-lang/rust#115660) - [rustdoc-search: add support for traits and associated types] (rust-lang/rust#116085) - [rustdoc: Add highlighting for comments in items declaration] (rust-lang/rust#117869) Compatibility Notes ------------------- - [Add allow-by-default lint for unit bindings] (rust-lang/rust#112380) This is expected to be upgraded to a warning by default in a future Rust release. Some macros emit bindings with type `()` with user-provided spans, which means that this lint will warn for user code. - [Remove x86_64-sun-solaris target.] (rust-lang/rust#118091) - [Remove asmjs-unknown-emscripten target] (rust-lang/rust#117338) - [Report errors in jobserver inherited through environment variables] (rust-lang/rust#113730) This [may warn](rust-lang/rust#120515) on benign problems too. - [Update the minimum external LLVM to 16.] (rust-lang/rust#117947) - [Improve `print_tts`](rust-lang/rust#114571) This change can break some naive manual parsing of token trees in proc macro code which expect a particular structure after `.to_string()`, rather than just arbitrary Rust code. - [Make `IMPLIED_BOUNDS_ENTAILMENT` into a hard error from a lint] (rust-lang/rust#117984) - [Vec's allocation behavior was changed when collecting some iterators] (rust-lang/rust#110353) Allocation behavior is currently not specified, nevertheless changes can be surprising. See [`impl FromIterator for Vec`] (https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#impl-FromIterator%3CT%3E-for-Vec%3CT%3E) for more details. - [Properly reject `default` on free const items] (rust-lang/rust#117818)

…pkin Improve test coverage for `unit_bindings` lint Follow-up to rust-lang#112380, apparently at the time I didn't add much of any test coverage outside of just "generally works as intended on the test suites and in the crater run". r? compiler

Rollup merge of rust-lang#131999 - jieyouxu:unit-bindings, r=WaffleLapkin Improve test coverage for `unit_bindings` lint Follow-up to rust-lang#112380, apparently at the time I didn't add much of any test coverage outside of just "generally works as intended on the test suites and in the crater run". r? compiler

rustbot assigned compiler-errors Jun 7, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 7, 2023

jieyouxu force-pushed the useless-bindings-lint branch from c374f0e to 595d1bc Compare June 7, 2023 13:51

jieyouxu changed the title ~~Add allow-by-default lint for useless bindings~~ Add allow-by-default lint for unit bindings Jun 7, 2023

jieyouxu force-pushed the useless-bindings-lint branch from 595d1bc to ceb30c6 Compare June 7, 2023 13:55

WaffleLapkin reviewed Jun 8, 2023

View reviewed changes

compiler/rustc_lint_defs/src/lib.rs Outdated Show resolved Hide resolved

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs Outdated Show resolved Hide resolved

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs Outdated Show resolved Hide resolved

WaffleLapkin self-assigned this Jun 8, 2023

jieyouxu force-pushed the useless-bindings-lint branch from ceb30c6 to 9bde17a Compare June 9, 2023 03:09

WaffleLapkin approved these changes Jun 12, 2023

View reviewed changes

jieyouxu force-pushed the useless-bindings-lint branch from 9bde17a to eeadf63 Compare June 12, 2023 07:47

This comment has been minimized.

Sign in to view

This comment was marked as outdated.

Sign in to view

jieyouxu mentioned this pull request Jun 12, 2023

Adjust UI tests for unit_bindings lint #112549

Merged

jieyouxu force-pushed the useless-bindings-lint branch from eeadf63 to 4ea39f2 Compare June 12, 2023 11:38

This comment has been minimized.

Sign in to view

jieyouxu force-pushed the useless-bindings-lint branch from 4ea39f2 to 0ea2618 Compare June 13, 2023 16:19

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Nov 19, 2023

This comment has been minimized.

Sign in to view

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Nov 19, 2023

rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Nov 19, 2023

jieyouxu force-pushed the useless-bindings-lint branch from f0520fa to 8da09ae Compare November 20, 2023 03:47

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 Nov 22, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 22, 2023

bors merged commit 73bc121 into rust-lang:master Nov 22, 2023
12 checks passed

rustbot added this to the 1.76.0 milestone Nov 22, 2023

jieyouxu deleted the useless-bindings-lint branch March 15, 2024 14:45

jieyouxu mentioned this pull request Aug 19, 2024

Rescope temp lifetime in if-let into IfElse with migration lint #107251

Merged

This was referenced Sep 28, 2024

Don't warn on proc macro generated code in needless_return rust-lang/rust-clippy#13464

Merged

proc-macro span syntax context and hygiene is underspecified and underdocumented #130995

Open

jieyouxu mentioned this pull request Oct 21, 2024

Improve test coverage for unit_bindings lint #131999

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Add allow-by-default lint for unit bindings #112380

Add allow-by-default lint for unit bindings #112380

jieyouxu commented Jun 7, 2023 •

edited

Loading

rustbot commented Jun 7, 2023

est31 commented Jun 7, 2023

jieyouxu commented Jun 7, 2023 •

edited

Loading

WaffleLapkin left a comment

WaffleLapkin commented Jun 12, 2023

jieyouxu commented Jun 12, 2023 •

edited

Loading

This comment has been minimized.

This comment was marked as outdated.

danielhenrymantilla commented Jun 12, 2023

jieyouxu commented Jun 12, 2023

jieyouxu commented Jun 12, 2023

This comment has been minimized.

jieyouxu commented Jun 12, 2023

WaffleLapkin commented Jun 13, 2023

WaffleLapkin commented Jun 13, 2023

jieyouxu commented Jun 13, 2023

bors commented Nov 19, 2023

This comment has been minimized.

bors commented Nov 19, 2023

est31 commented Nov 19, 2023

WaffleLapkin commented Nov 22, 2023

bors commented Nov 22, 2023

bors commented Nov 22, 2023

bors commented Nov 22, 2023

rust-timer commented Nov 23, 2023

Add allow-by-default lint for unit bindings #112380

Add allow-by-default lint for unit bindings #112380

Conversation

jieyouxu commented Jun 7, 2023 • edited Loading

Example

Known Issue

rustbot commented Jun 7, 2023

est31 commented Jun 7, 2023

jieyouxu commented Jun 7, 2023 • edited Loading

WaffleLapkin left a comment

Choose a reason for hiding this comment

WaffleLapkin commented Jun 12, 2023

jieyouxu commented Jun 12, 2023 • edited Loading

This comment has been minimized.

This comment was marked as outdated.

danielhenrymantilla commented Jun 12, 2023

jieyouxu commented Jun 12, 2023

jieyouxu commented Jun 12, 2023

This comment has been minimized.

jieyouxu commented Jun 12, 2023

WaffleLapkin commented Jun 13, 2023

WaffleLapkin commented Jun 13, 2023

jieyouxu commented Jun 13, 2023

bors commented Nov 19, 2023

This comment has been minimized.

bors commented Nov 19, 2023

est31 commented Nov 19, 2023

WaffleLapkin commented Nov 22, 2023

bors commented Nov 22, 2023

bors commented Nov 22, 2023

bors commented Nov 22, 2023

rust-timer commented Nov 23, 2023

Overall result: no relevant changes - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

jieyouxu commented Jun 7, 2023 •

edited

Loading

jieyouxu commented Jun 7, 2023 •

edited

Loading

jieyouxu commented Jun 12, 2023 •

edited

Loading