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

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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

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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

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Adjust UI tests for unit_bindings lint #112549

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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

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

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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

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

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This was referenced Sep 28, 2024

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

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

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jieyouxu mentioned this pull request Oct 21, 2024

Improve test coverage for unit_bindings lint #131999

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jieyouxu added the L-unit_bindings Lint: unit_bindings label Feb 11, 2025

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

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

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