new lint: `bufreader_stdin` by JaafarTanoukhi · Pull Request #17029 · rust-lang/rust-clippy

JaafarTanoukhi · 2026-05-17T18:25:04Z

changelog: [bufreader_stdin]: new lint

rustbot · 2026-05-17T18:25:09Z

rustbot has assigned @llogiq.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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Owners of files modified in this PR: 7 candidates
7 candidates expanded to 7 candidates
Random selection from Jarcho, llogiq, samueltardieu

samueltardieu

Note that the suggestion cannot probably more than MaybeIncorrect, as the surrounding code might expect a BufReader, and not any type implementing BufRead.

The category should probably be upgraded to at least pedantic because of the risk of false positives (due to the type change).

View changes since this review

samueltardieu · 2026-06-02T14:00:13Z

+            && let TyKind::Path(ref qpath) = ty.kind
+            && let Some(did) = cx.qpath_res(qpath, ty.hir_id).opt_def_id()
+            && cx.tcx.def_path_str(did).ends_with("std::io::BufReader")


This is not the proper way to check for a given type. Look at the clippy_utils::paths module for that.

samueltardieu · 2026-06-02T14:02:15Z

+                    "using `BufReader::new` with `Stdin`",
+                    "instead of wrapping `Stdin` in `BufReader`, use the `lock` method directly",
+                    format!("{}.lock()", snippet(cx, arg.span, "..")),
+                    Applicability::MachineApplicable,


What if the expression doesn't come from a macro but part of it do? For example, BufReader::new(mac!(123)) where mac!(123) expands to std::io::stdin()? Please include examples involving such macros.

samueltardieu · 2026-06-02T14:02:42Z

+                    format!("{}.lock()", snippet(cx, arg.span, "..")),
+                    Applicability::MachineApplicable,
+                );
+            } else if cx.tcx.def_path_str(arg_did).ends_with("std::io::StdinLock") {


Ditto, use clippy_utils::paths if there is no diagnostic item. We'll add one later.

samueltardieu · 2026-06-02T14:03:27Z

        Box::new(|_| Box::new(unused_peekable::UnusedPeekable)),
        Box::new(|_| Box::new(bool_to_int_with_if::BoolToIntWithIf)),
        Box::new(|_| Box::new(box_default::BoxDefault)),
+        Box::new(move |_| Box::new(bufreader_stdin::BufreaderStdin::new())),


The syntax for registering lints has recently changed. Please rebase your PR and add your lint at the end of the lint list.

samueltardieu · 2026-06-02T14:05:08Z

+                    expr.span,
+                    "using `BufReader::new` with `Stdin`",
+                    "instead of wrapping `Stdin` in `BufReader`, use the `lock` method directly",
+                    format!("{}.lock()", snippet(cx, arg.span, "..")),


You want to check the applicability here, in case the source cannot be retrieved.

samueltardieu · 2026-06-02T14:13:15Z

+                    cx,
+                    BUFREADER_STDIN,
+                    expr.span,
+                    "using `BufReader::new` with `StdinLock`",


The error message could be clearer:

Suggested change

"using `BufReader::new` with `StdinLock`",

"wrapping already buffered `StdinLock` into a `BufReader`",

rustbot · 2026-06-02T14:17:07Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

DanBondarenko · 2026-07-17T11:01:09Z

@JaafarTanoukhi Hi! Are you planning to continue work on this? I'd love to pick this up. :)

JaafarTanoukhi · 2026-07-17T22:04:52Z

@DanBondarenko I was planning to get back to it sometime this week, but I don't mind if you'd like to take over.

DanBondarenko · 2026-07-18T13:52:46Z

@JaafarTanoukhi No worries, I'll leave it with you. Happy to help if anything comes up.

github-actions · 2026-07-22T10:02:02Z

No changes for f28a346

JaafarTanoukhi · 2026-07-22T10:03:42Z

@rustbot ready

samueltardieu · 2026-07-23T10:23:18Z

+                .ty_adt_def()
+                .map(rustc_middle::ty::AdtDef::did)
+        {
+            let snip = snippet(cx, arg.span, "..");


Use snippet_opt() instead, that would be cleaner than checking the output.

View changes since the review

samueltardieu · 2026-07-23T10:28:04Z

+            let (msg, help, sugg) = match arg_did_name {
+                Some(sym::Stdin) => (
+                    "wrapping already buffered `Stdin` into a `BufReader`",
+                    "instead of wrapping `Stdin` in `BufReader`, use the `lock` method directly",
+                    format!("{snip}.lock()"),
+                ),
+                Some(sym::StdinLock) => (
+                    "wrapping already buffered `StdinLock` into a `BufReader`",
+                    "instead of wrapping `StdinLock` in `BufReader`, use it directly",
+                    snip.to_string(),
+                ),
+                _ => return,
+            };


It would be better to make sure we will lint before using snippet() (or snippet_opt()) which require accessing the source. Here, we might have used snippet() while we are not yet sure we will lint. Also, the suggestion would be best built inside a span_lint_and_then() to ensure that the new string will be allocated only if the lint is actually emitted.

View changes since the review

samueltardieu · 2026-07-24T09:43:56Z

+                .typeck_results()
+                .expr_ty(arg)
+                .ty_adt_def()
+                .map(rustc_middle::ty::AdtDef::did)


Note: rustc_middle::ty is usually imported, not qualified.

View changes since the review

samueltardieu · 2026-07-24T09:52:09Z

+            && let TyKind::Path(ref qpath) = ty.kind
+            && let Some(did) = cx.qpath_res(qpath, ty.hir_id).opt_def_id()
+            && cx.tcx.is_diagnostic_item(sym::IoBufReader, did)
+            && let Some(arg_did) = cx
+                .typeck_results()
+                .expr_ty(arg)
+                .ty_adt_def()
+                .map(rustc_middle::ty::AdtDef::did)


This can be made shorter using clippy_utils::res traits.

Suggested change

&& let TyKind::Path(ref qpath) = ty.kind

&& let Some(did) = cx.qpath_res(qpath, ty.hir_id).opt_def_id()

&& cx.tcx.is_diagnostic_item(sym::IoBufReader, did)

&& let Some(arg_did) = cx

.typeck_results()

.expr_ty(arg)

.ty_adt_def()

.map(rustc_middle::ty::AdtDef::did)

&& ty.basic_res().is_diag_item(cx, sym::IoBufReader)

&& let Some(arg_did) = cx.typeck_results().expr_ty(arg).opt_def_id()

View changes since the review

samueltardieu · 2026-07-24T09:52:44Z

+        if let ExprKind::Call(func, [arg]) = expr.kind
+            && let ExprKind::Path(QPath::TypeRelative(ty, segment)) = func.kind
+            && segment.ident.name == sym::new
+            && let TyKind::Path(ref qpath) = ty.kind


Note: we usually write this as ty::Path, not TyKind::Path.

View changes since the review

rustbot · 2026-07-25T08:47:42Z

Some changes occurred in clippy_lints/src/doc

cc @notriddle

JaafarTanoukhi · 2026-07-25T10:16:24Z

@rustbot ready

samueltardieu

Could you please :

rename bufreader_stdin_macro.* into bufreader_stdin_unfixable.rs
squash all commits

I'll start the FCP.
@rustbot label lint-nominated

View changes since this review

rustbot · 2026-07-28T15:20:23Z

This lint has been nominated for inclusion.

A FCP topic has been created on Zulip.

samueltardieu · 2026-07-28T15:20:46Z

r? samueltardieu

JaafarTanoukhi · 2026-07-29T23:53:06Z

@rustbot ready

samueltardieu

Could you please rebase this PR?

View changes since this review

rustbot · 2026-10-07T08:18:35Z

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.

rustbot · 2026-10-11T14:49:39Z

☔ The latest upstream changes (possibly #13598) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned llogiq May 17, 2026

rustbot added needs-fcp PRs that add, remove, or rename lints and need an FCP S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels May 17, 2026

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JaafarTanoukhi force-pushed the new-lint-bufreader-stdin branch from fe83bad to 346b56a Compare July 18, 2026 18:21

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JaafarTanoukhi force-pushed the new-lint-bufreader-stdin branch from ac478a9 to 92e6186 Compare July 29, 2026 17:48

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New lint: bufreader_stdin

a24f598

JaafarTanoukhi force-pushed the new-lint-bufreader-stdin branch from 92e6186 to a24f598 Compare October 7, 2026 08:18

fix

f28a346

JaafarTanoukhi requested a review from samueltardieu October 7, 2026 10:17

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	"using `BufReader::new` with `StdinLock`",
	"wrapping already buffered `StdinLock` into a `BufReader`",

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