Add `explicit_default_arguments` lint by vogelbirb · Pull Request #16123 · rust-lang/rust-clippy

vogelbirb · 2025-11-21T01:52:38Z

This lint checks if the generic arguments of a type alias are set to the default.

fixes #14848
r? @Jarcho
Relevant Zulip threads: https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/How.20to.20access.20generic.20params.20of.20a.20.60DefId.60.3F/with/547384320/, https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/Is.20there.20an.20easier.20way.20to.20walk.20through.20the.20predicates.3F/with/560255667

changelog: new lint: [`explicit_default_arguments`]

…lint

github-actions · 2025-12-29T06:20:51Z

Lintcheck changes for 90ff88e

Lint	Added	Removed	Changed
`clippy::explicit_default_arguments`	9	0	0

This comment will be updated if you push new changes

vogelbirb · 2025-12-29T07:03:34Z

Do you think the function walk_ty_recursive and some of its helper functions are worth moving to clippy_utils? I'm not too sure how that works.

vogelbirb · 2026-01-22T23:10:25Z

Hi, @Jarcho. I just wanted to follow up on this PR, it's ready for review and has been waiting for a few weeks now. I took into account your advice on Zulip. It would be great if you could take a look at it. Thanks.

Most notably, `ExprKind::Path` is still a work in progress. Also added a check for equality constraints in `impl Trait` and trait objects. Unlike consts, statics were missing so a check for them was added.

rustbot · 2026-03-23T23:04:35Z

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.

pitaj · 2026-03-24T02:49:46Z

r? clippy

rustbot · 2026-03-28T18:01:05Z

☔ The latest upstream changes (possibly #16687) made this pull request unmergeable. Please resolve the merge conflicts.

ada4a

First pass over the code. A lot of the comments are stylistic, apologies for that -- it just makes the rest of the code easier to review^^

View changes since this review

ada4a · 2026-04-03T16:23:36Z

+    let TyKind::Path(
+        qpath @ QPath::Resolved(
+            _,
+            Path {
+                segments:
+                    [
+                        ..,
+                        PathSegment {
+                            args: Some(hir_ty_generics),
+                            ..
+                        },
+                    ],
+                ..
+            },
+        ),
+    ) = hir_ty.kind


the indentation is terrifying.. maybe use a let-chain instead?

Suggested change

let TyKind::Path(

qpath @ QPath::Resolved(

_,

Path {

segments:

[

..,

PathSegment {

args: Some(hir_ty_generics),

..

},

],

..

},

),

) = hir_ty.kind

let hir_ty_generics = if let TyKind::Path(ref qpath) = hir_ty.kind

&& let QPath::Resolved(_, path) = qpath

&& let Some(last_segment) = path.segments.last()

&& let Some(hir_ty_generics) = last_segment.args

{

hir_ty_generics

} else {

return;

};

ada4a · 2026-04-03T16:25:12Z

+            redundant_tys_spans.push(redundant_ty.span());
+        }
+    }
+    let no_generics_hir_ty_snippet = snippet_opt(cx, hir_ty.span.until(hir_ty_generics.span_ext)).unwrap();


unwrapping in clippy is generally not a good idea... it's preferable to use snippet, since it accepts a fallback string in case source code is not available (which can happen)

ada4a · 2026-04-03T16:28:17Z

+                if i > 0 {
+                    generics_string.push_str(", ");
+                }
+                generics_string.push_str(&snippet_opt(cx, arg.span()).unwrap());


see previous comment

ada4a · 2026-04-03T16:31:38Z

+    let (defaults, aliased_to_alias) = match_generics(cx, aliased_ty_args.as_slice(), alias_ty_params.as_slice());
+
+    let mut redundant_tys_spans = Vec::new();
+    for (i, generic_arg) in resolved_ty_args.iter().enumerate() {


could replace with .into_iter() to avoid copying below

ada4a · 2026-04-03T16:33:52Z

+            hir_ty.span,
+            format!(
+                "redudant usage of default argument `{}`",
+                snippet_opt(cx, redundant_ty_span).unwrap()


please use snippet

ada4a · 2026-04-03T16:48:46Z

+        if expr.span.from_expansion() {
+            return;
+        }
+        let ty = cx.typeck_results().expr_ty(expr);


does this one get used anywhere?

ada4a · 2026-04-03T16:50:09Z

+                    check_typair(cx, (cx.typeck_results().node_type(ty.hir_id), *ty));
+                    for ty_pair in path_generic_args(iter::once(segment))
+                        .into_iter()
+                        .map(|hir_ty| (cx.typeck_results().node_type(hir_ty.hir_id), hir_ty))
+                    {
+                        check_typair(cx, ty_pair);
+                    }
+                },
+                QPath::Resolved(ty, path) => {
+                    if let Some(ty) = ty {
+                        check_typair(cx, (cx.typeck_results().node_type(ty.hir_id), *ty));
+                    }
+                    for ty_pair in path_generic_args(path.segments)
+                        .into_iter()
+                        .map(|hir_ty| (cx.typeck_results().node_type(hir_ty.hir_id), hir_ty))
+                    {
+                        check_typair(cx, ty_pair);
+                    }
+                },
+                _ => {},
+            },
+            ExprKind::Closure(Closure {
+                fn_decl: FnDecl { inputs, output, .. },
+                ..
+            }) => {
+                for input in *inputs {
+                    check_typair(cx, (cx.typeck_results().node_type(input.hir_id), *input));
+                }
+                if let FnRetTy::Return(ty) = *output {
+                    check_typair(cx, (cx.typeck_results().node_type(ty.hir_id), *ty));
+                }
+            },
+            ExprKind::Cast(_, &ty)
+            | ExprKind::Type(_, &ty)
+            | ExprKind::Let(&LetExpr { ty: Some(&ty), .. })
+            | ExprKind::UnsafeBinderCast(_, _, Some(&ty))
+            | ExprKind::OffsetOf(&ty, _) => check_typair(cx, (cx.typeck_results().expr_ty(expr), ty)),


you might want to do let typeck = cx.typeck_results(); to save yourself some characters across this match

ada4a · 2026-04-03T16:54:19Z

+error: redudant usage of default argument `i32`
+  --> tests/ui/explicit_default_arguments.rs:28:28
+   |
+LL | fn multi_all_defaults() -> Multi<i32, String> {
+   |                            ^^^^^^^^^^^^^^^^^^ help: try: `Multi`
+
+error: redudant usage of default argument `String`
+  --> tests/ui/explicit_default_arguments.rs:28:28
+   |
+LL | fn multi_all_defaults() -> Multi<i32, String> {
+   |                            ^^^^^^^^^^^^^^^^^^ help: try: `Multi`


these two should ideally be one lint

you don't need to repeat the name of the argument -- highlight its span instead

Suggested change

error: redudant usage of default argument `i32`

--> tests/ui/explicit_default_arguments.rs:28:28

|

LL | fn multi_all_defaults() -> Multi<i32, String> {

| ^^^^^^^^^^^^^^^^^^ help: try: `Multi`

error: redudant usage of default argument `String`

--> tests/ui/explicit_default_arguments.rs:28:28

|

LL | fn multi_all_defaults() -> Multi<i32, String> {

| ^^^^^^^^^^^^^^^^^^ help: try: `Multi`

error: redundant usage of default arguments

--> tests/ui/explicit_default_arguments.rs:28:28

|

LL | fn multi_all_defaults() -> Multi<i32, String> {

| ^^^ ^^^^^^

|

help: consider removing them

|

LL - fn multi_all_defaults() -> Multi<i32, String> {

LL + fn multi_all_defaults() -> Multi {

|

ada4a · 2026-04-03T16:56:29Z

+            hir_ty.hir_id,
+            hir_ty.span,
+            format!(
+                "redudant usage of default argument `{}`",


Suggested change

"redudant usage of default argument `{}`",

"redundant usage of default argument `{}`",

ada4a · 2026-04-03T17:00:17Z

+                snippet_opt(cx, redundant_ty_span).unwrap()
+            ),
+            |diag| {
+                diag.span_suggestion(hir_ty.span, "try", &sugg_text_snippet, Applicability::MachineApplicable);


Consider replacing hir_ty_generics.span_ext instead. That way, you will no longer need to include no_generics_hir_ty_snippet in sugg_text_snippet. As an added bonus, the suggestion should have a nicer diff (only the generics will be highlighted)

rustbot · 2026-04-03T17:02:06Z

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

vogelbirb · 2026-04-06T17:32:38Z

Thanks for the thorough review. There's a lot to address, so I'll get to it as soon as I have the time.

rustbot added the needs-fcp PRs that add, remove, or rename lints and need an FCP label Nov 21, 2025

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gauravagerwala added a commit to gauravagerwala/rust-clippy that referenced this pull request Dec 7, 2025

Update design for PR rust-lang#16123: Add explicit_default_arguments …

31a1946

…lint

vogelbirb force-pushed the master branch from 2400ee2 to 0f7af0c Compare December 22, 2025 00:12

vogelbirb force-pushed the master branch from 03c6251 to 2eae875 Compare December 29, 2025 05:37

rustbot assigned Jarcho Dec 29, 2025

vogelbirb marked this pull request as ready for review December 29, 2025 06:51

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties label Dec 29, 2025

vogelbirb changed the title ~~Add explicit_default_arguments lint~~ Add explicit_default_arguments lint Dec 29, 2025

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vogelbirb force-pushed the master branch from 48a6cbe to b6b208b Compare January 17, 2026 18:00

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rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jan 21, 2026

vogelbirb force-pushed the master branch from db1cc35 to 065354d Compare January 21, 2026 15:11

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 Jan 21, 2026

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BennoLossin mentioned this pull request Mar 19, 2026

lint idea: useless_default_generic_parameters #14848

Open

ojeda mentioned this pull request Mar 19, 2026

Clippy wanted features & bugfixes Rust-for-Linux/linux#349

Open

76 tasks

Matheus L. P. added 4 commits March 23, 2026 18:04

Implement explicit_default_arguments lint

c3ac37b

feat: working with fn arguments

9d05682

feat: lint structs & refactor more

4ecd83b

feat: lint all kind of items

fc84fc1

Matheus L. P. added 9 commits March 23, 2026 18:04

feat: walk through some types recursively

acf894c

feat: finish recusive type walker

fe4337c

fix: check GATs in impl Traits

287520b

feat: check some exprs and pats

69ff8a9

Most notably, `ExprKind::Path` is still a work in progress. Also added a check for equality constraints in `impl Trait` and trait objects. Unlike consts, statics were missing so a check for them was added.

feat: check closures

0067036

test(explicit_default_arguments): rewrite

e27eafa

refactor: fix Clippy suggestions

d5df4b0

fix: update lint declaration

01cf7dd

fix: update lint example

9a939cf

vogelbirb force-pushed the master branch from 065354d to 9a939cf Compare March 23, 2026 23:04

fix: correct type signature

90ff88e

rustbot assigned Centri3 and unassigned Jarcho Mar 24, 2026

ada4a requested changes Apr 3, 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 Apr 3, 2026

	"redudant usage of default argument `{}`",
	"redundant usage of default argument `{}`",

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