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detect redundant nested match #13798

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detect redundant nested match #13798

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22 changes: 22 additions & 0 deletions clippy_lints/src/matches/collapsible_match.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -108,6 +108,28 @@ fn check_arm<'tcx>(
"the outer pattern can be modified to include the inner pattern",
);
});
} else if let Some(inner) = IfLetOrMatch::parse(cx, inner_expr)
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@lapla-cogito lapla-cogito Dec 9, 2024 •
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The conditional branch added this time could have been written by nesting ifs together with the conditional branch above, but I have not done so for the sake of readability (the former condition is long).

&& let IfLetOrMatch::Match(inner_scrutinee, inner_arms, ..) = inner
&& outer_pat.span.eq_ctxt(inner_scrutinee.span)
&& let Some(inner_binding_id) = path_to_local(peel_ref_operators(cx, inner_scrutinee))
&& let Some(outer_binding_id) = path_to_local(peel_ref_operators(cx, outer_then_body))
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Isn't this pattern here unreachable, considering that outer_then_body will be the match expression and never a path to a local variable? The test that was added already gets a warning today and the linked issue doesn't have a warning with this change, so it would be nice to see a test case for what this changes/one that didn't warn before

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I think it would be better if this lint rewrote the nested match into a simply simpler match... I will update this explanation with code examples later.

// check if the inner match can be collapsed into the outer match
&& inner_binding_id == outer_binding_id
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@lapla-cogito lapla-cogito Dec 9, 2024 •
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checks if the outer match and inner match refer to the same binding

&& outer_guard.is_none_or(|e| !is_local_used(cx, e, inner_binding_id))
&& !inner_arms
.iter()
.any(|arm| is_local_used(cx, arm.body, inner_binding_id))
{
let msg = "this match can be collapsed into the outer match";
span_lint_and_then(cx, COLLAPSIBLE_MATCH, inner_expr.span, msg, |diag| {
let mut help_span = MultiSpan::from_spans(vec![outer_pat.span, inner_scrutinee.span]);
help_span.push_span_label(outer_pat.span, "outer pattern");
help_span.push_span_label(inner_scrutinee.span, "inner pattern");
diag.span_help(
help_span,
"the outer pattern can be modified to include the inner pattern",
);
});
}
}

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12 changes: 12 additions & 0 deletions tests/ui/collapsible_match.rs
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Expand Up @@ -112,6 +112,18 @@ fn lint_cases(opt_opt: Option<Option<u32>>, res_opt: Result<Option<u32>, String>
},
None => return,
}

match res_opt {
Ok(val) => match res_opt {
Ok(val) => match val {
//~^ ERROR: this `match` can be collapsed into the outer `match`
Some(n) => foo(n),
_ => return,
},
_ => return,
},
_ => return,
}
}

fn negative_cases(res_opt: Result<Option<u32>, String>, res_res: Result<Result<u32, String>, String>) {
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34 changes: 27 additions & 7 deletions tests/ui/collapsible_match.stderr
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Expand Up @@ -191,7 +191,27 @@ LL | Some(n) => foo(n),
| ^^^^^^^ with this pattern

error: this `match` can be collapsed into the outer `match`
--> tests/ui/collapsible_match.rs:252:22
--> tests/ui/collapsible_match.rs:118:24
|
LL | Ok(val) => match val {
| ________________________^
LL | |
LL | | Some(n) => foo(n),
LL | | _ => return,
LL | | },
| |_____________^
|
help: the outer pattern can be modified to include the inner pattern
--> tests/ui/collapsible_match.rs:118:16
|
LL | Ok(val) => match val {
| ^^^ replace this binding
LL |
LL | Some(n) => foo(n),
| ^^^^^^^ with this pattern

error: this `match` can be collapsed into the outer `match`
--> tests/ui/collapsible_match.rs:264:22
|
LL | Some(val) => match val {
| ______________________^
Expand All @@ -201,15 +221,15 @@ LL | | },
| |_________^
|
help: the outer pattern can be modified to include the inner pattern
--> tests/ui/collapsible_match.rs:252:14
--> tests/ui/collapsible_match.rs:264:14
|
LL | Some(val) => match val {
| ^^^ replace this binding
LL | E::A(val) | E::B(val) => foo(val),
| ^^^^^^^^^^^^^^^^^^^^^ with this pattern

error: this `if let` can be collapsed into the outer `if let`
--> tests/ui/collapsible_match.rs:283:9
--> tests/ui/collapsible_match.rs:295:9
|
LL | / if let Some(u) = a {
LL | |
Expand All @@ -218,15 +238,15 @@ LL | | }
| |_________^
|
help: the outer pattern can be modified to include the inner pattern
--> tests/ui/collapsible_match.rs:282:27
--> tests/ui/collapsible_match.rs:294:27
|
LL | if let Issue9647::A { a, .. } = x {
| ^ replace this binding
LL | if let Some(u) = a {
| ^^^^^^^ with this pattern, prefixed by `a`:

error: this `if let` can be collapsed into the outer `if let`
--> tests/ui/collapsible_match.rs:292:9
--> tests/ui/collapsible_match.rs:304:9
|
LL | / if let Some(u) = a {
LL | |
Expand All @@ -235,12 +255,12 @@ LL | | }
| |_________^
|
help: the outer pattern can be modified to include the inner pattern
--> tests/ui/collapsible_match.rs:291:35
--> tests/ui/collapsible_match.rs:303:35
|
LL | if let Issue9647::A { a: Some(a), .. } = x {
| ^ replace this binding
LL | if let Some(u) = a {
| ^^^^^^^ with this pattern

error: aborting due to 13 previous errors
error: aborting due to 14 previous errors

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