New lint: `iter_missing_exact_size` by DanielEScherzer · Pull Request #17395 · rust-lang/rust-clippy

DanielEScherzer · 2026-07-10T02:47:38Z

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Followed lint naming conventions
Added passing UI tests (including committed .stderr file)
cargo test passes locally
Executed cargo dev update_lints
Added lint documentation
Run cargo dev fmt

Fixes #17393

changelog: [iter_missing_exact_size]: add lint

DanielEScherzer · 2026-07-10T02:49:42Z

Not exactly sure if the new name matches the guidelines, I didn't want the name to be too long, suggestions welcome

@rustbot label +A-lint

rustbot · 2026-07-10T02:49:45Z

Error: Unknown labels: A-lints

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

github-actions · 2026-07-10T02:58:01Z

Lintcheck changes for cfb5ade

Lint	Added	Removed	Changed
`clippy::iter_missing_exact_size`	6	0	0

This comment will be updated if you push new changes

DanielEScherzer · 2026-07-10T03:09:12Z

So the first round of CI said that I should add size_hint to the symbols in clippy_utils/src/sym.rs, now the second round says to remove that symbol as unused?

rust-clippy/tests/symbols-used.rs

Lines 41 to 44 in 9006d6c

    
           // Remove symbols used as `sym::*`. 
        
           let used_re = Regex::new(r"\bsym::(\w+)\b").unwrap(); 
        
           let rs_ext = OsStr::new("rs"); 
        
           for dir in ["clippy_lints", "clippy_lints_internal", "clippy_utils", "src"] {

assumes that the clippy symbols are used as sym, I was using use clippy_utils::sym as clippy_sym; and use rustc_span::sym;, I'll switch them

samueltardieu · 2026-07-10T17:19:45Z

So the first round of CI said that I should add size_hint to the symbols in clippy_utils/src/sym.rs, now the second round says to remove that symbol as unused?

rust-clippy/tests/symbols-used.rs

Lines 41 to 44 in 9006d6c

// Remove symbols used as `sym::*`.

let used_re = Regex::new(r"\bsym::(\w+)\b").unwrap();

let rs_ext = OsStr::new("rs");

for dir in ["clippy_lints", "clippy_lints_internal", "clippy_utils", "src"] {

assumes that the clippy symbols are used as sym, I was using use clippy_utils::sym as clippy_sym; and use rustc_span::sym;, I'll switch them

All symbols of rustc_span::sym are reexported by clippy_utils::sym, using the latter should be enough.

fpdotmonkey

My suggestion for the name would be "no-delegated-exact-size-iter"

View changes since this review

fpdotmonkey · 2026-07-10T22:02:32Z

+            span_lint_and_help(
+                cx,
+                ITER_MISSING_EXACT_SIZE,
+                lint_span,


Suggested change

lint_span,

item.span.into(),

Is there a reason to not have the label pointing to size_hint?

I believe I wrote this because they bound a simple expression to a variable and just used it just once. I would think that the size hint would be good as the help span. Something like "size_hint is based on an ExactSizeIterator here"

adjusted to have the size hint as a help span

DanielEScherzer · 2026-07-14T14:34:40Z

My suggestion for the name would be "no-delegated-exact-size-iter"

View changes since this review

Is it possible to have the name be part of what is decided in the FCP?

Gri-ffin · 2026-07-16T09:26:32Z

Is it possible to have the name be part of what is decided in the FCP?

Yes, feel free to join the discussion on Zulip (once the FCP starts).

I think this currently wouldn't fire if the user has

fn size_hint(&self) -> (usize, Option<usize>) {
    return self.range.size_hint();
}

If you can please add a test for this?

DanielEScherzer · 2026-07-16T17:43:55Z

Is it possible to have the name be part of what is decided in the FCP?

Yes, feel free to join the discussion on Zulip (once the FCP starts).

I think this currently wouldn't fire if the user has
fn size_hint(&self) -> (usize, Option<usize>) {
    return self.range.size_hint();
}
If you can please add a test for this?

I added support for function bodies where the only statement is a return (and optionally there might be a trailing expression that is dead code and is ignored)

rustbot · 2026-07-17T16:28:07Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

DanielEScherzer · 2026-07-17T16:28:55Z

Squashed and rebased
@Gri-ffin @fpdotmonkey @samueltardieu do you think this is ready for FCP?

Gri-ffin

@Gri-ffin @fpdotmonkey @samueltardieu do you think this is ready for FCP?

It's starting to look solid, but I'm not a maintainer, I'm just helping in the reviews, I'll leave that call to the maintainers.

View changes since this review

fpdotmonkey · 2026-07-20T10:32:05Z

Likewise, I am but a community reviewer. I think it looks good, though.

DanielEScherzer · 2026-07-20T15:49:55Z

Okay, @Gri-ffin do you have any more feedback? If not I'll start the FCP

Jarcho · 2026-07-20T16:30:03Z

@fpdotmonkey @Gri-ffin if you think the PR is ready then mark it as approved; that's what signals triagebot to assign a maintainer.

@DanielEScherzer The process for FCPs has a team member starting it. We could try and have it start once the the community review phase is done, but we'll need to see how the reviews play out first.

DanielEScherzer · 2026-07-20T16:33:15Z

@fpdotmonkey @Gri-ffin if you think the PR is ready then mark it as approved; that's what signals triagebot to assign a maintainer.

@DanielEScherzer The process for FCPs has a team member starting it. We could try and have it start once the the community review phase is done, but we'll need to see how the reviews play out first.

Oh, I thought I just needed to add lint-nominated label with triagebot, but I guess that isn't one of the labels that are listed under allow-unauthenticated, so nevermind

Gri-ffin

Other than these LGTM.

View changes since this review

Gri-ffin · 2026-07-20T17:14:40Z

+error: iterator can implement `ExactSizeIterator`
+  --> tests/ui/iter_missing_exact_size.rs:13:1
+   |
+LL | / impl Iterator for StringRepeater1 {
+LL | |
+LL | |     type Item = String;
+LL | |     fn next(&mut self) -> Option<Self::Item> {
+...  |
+LL | | }
+   | |_^
+   |
+help: this `size_hint()` implementation delegates to to the `size_hint()` of an `ExactSizeIterator`, so the overall iterator is likely to have an exact size
+  --> tests/ui/iter_missing_exact_size.rs:20:9
+   |
+LL |         self.range.size_hint()
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   = note: `-D clippy::iter-missing-exact-size` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::iter_missing_exact_size)]`


I personally find this confusing but it's nothing currently blocking.

which part is confusing?

IMO if I see this suggestion I would assume I need to modify the impl block, which isn't the case, personally I would prefer to only highlight size_hint, but it's only a personal opinion, it can be discussed later.

Ah, okay. But since the issue is with the overall implementation (impl Iterator with a delegating size_hint but without also implementing ExactSizeIterator) I think pointing to the overall impl block is still helpful

Gri-ffin

Thank you.

View changes since this review

DanielEScherzer · 2026-07-24T14:24:06Z

Likewise, I am but a community reviewer. I think it looks good, though.

Can you mark this as an approval in GitHub so that it counts as a community review and triggers the team review process?

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DanielEScherzer force-pushed the suggest-ExactSizeIterator branch from e4c0d8c to 2b8d2b7 Compare July 10, 2026 02:48

rustbot added the A-lint Area: New lints label Jul 10, 2026

DanielEScherzer force-pushed the suggest-ExactSizeIterator branch from 143523a to a9e1af0 Compare July 10, 2026 03:09

Gri-ffin reviewed Jul 10, 2026

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DanielEScherzer force-pushed the suggest-ExactSizeIterator branch from 2dc95c6 to c864c24 Compare July 13, 2026 20:03

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DanielEScherzer force-pushed the suggest-ExactSizeIterator branch from c864c24 to aaae638 Compare July 13, 2026 20:13

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New lint: iter_missing_exact_size

ca1028b

DanielEScherzer force-pushed the suggest-ExactSizeIterator branch from eb066f0 to ca1028b Compare July 17, 2026 16:28

Gri-ffin reviewed Jul 17, 2026

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Comment thread clippy_lints/src/iter_missing_exact_size.rs Outdated

Comment thread clippy_lints/src/iter_missing_exact_size.rs Outdated

Comment thread clippy_lints/src/iter_missing_exact_size.rs Outdated

Add symbol

db1ffc3

Gri-ffin reviewed Jul 20, 2026

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DanielEScherzer added 3 commits July 20, 2026 10:41

Comment cleanup

9947a81

filter_by_name_unhygienic()

c07b6b9

Tweak lint documentation

cfb5ade

Gri-ffin approved these changes Jul 20, 2026

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