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.

rustbot · 2026-07-18T17:57:21Z

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.

github-actions · 2026-07-22T10:02:02Z

Lintcheck changes for 0a80617

Lint	Added	Removed	Changed
`clippy::multiple_crate_versions`	0	0	3

This comment will be updated if you push new changes

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 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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samueltardieu requested changes Jun 2, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jun 2, 2026

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rustbot added the has-merge-commits PR has merge commits, merge with caution. label Jul 18, 2026

JaafarTanoukhi added 5 commits July 18, 2026 21:01

new lint: bufreader_stdin

0b48a45

fixed change log

6ff18aa

ran cargo dev fmt

fc21fd7

removed extra space in test

75d8e79

made sure test and output match

346b56a

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

rustbot removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) has-merge-commits PR has merge commits, merge with caution. labels Jul 18, 2026

JaafarTanoukhi added 2 commits July 22, 2026 02:15

addressed PR comments

e0e5252

optimized get_diagnostic_item usage

0a80617

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Jul 22, 2026

JaafarTanoukhi requested a review from samueltardieu July 23, 2026 09:34

samueltardieu requested changes Jul 24, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jul 24, 2026

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

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