Avoid infinite loop between I002 and PYI025

crates/ruff/tests/cli/lint.rs

@@ -2871,6 +2871,48 @@ fn flake8_import_convention_unused_aliased_import_no_conflict() {
     );
 }
 
+// https://github.com/astral-sh/ruff/issues/20891
+#[test]
+fn required_import_set_conflicts_with_pyi025() {
+    assert_cmd_snapshot!(
+        Command::new(get_cargo_bin(BIN_NAME))
+            .args(STDIN_BASE_OPTIONS)
+            .arg("--config")
+            .arg(r#"lint.isort.required-imports = ["from collections.abc import Set"]"#)
+            .args(["--select", "I002,PYI025"])
+            .arg("-")
+            .pass_stdin("1")
+    );
+}
+
+// https://github.com/astral-sh/ruff/issues/20891
+#[test]
+fn required_import_set_aliased_as_abstract_set_no_conflict() {
+    assert_cmd_snapshot!(
+        Command::new(get_cargo_bin(BIN_NAME))
+            .args(STDIN_BASE_OPTIONS)
+            .arg("--config")
+            .arg(r#"lint.isort.required-imports = ["from collections.abc import Set as AbstractSet"]"#)
+            .args(["--select", "I002,PYI025"])
+            .arg("-")
+            .pass_stdin("1")
+    );
+}
+
+// https://github.com/astral-sh/ruff/issues/20891
+#[test]
+fn required_import_set_without_pyi025_no_conflict() {
+    assert_cmd_snapshot!(
+        Command::new(get_cargo_bin(BIN_NAME))
+            .args(STDIN_BASE_OPTIONS)
+            .arg("--config")
+            .arg(r#"lint.isort.required-imports = ["from collections.abc import Set"]"#)
+            .args(["--select", "I002"])
+            .arg("-")
+            .pass_stdin("1")
+    );
+}
+
 // https://github.com/astral-sh/ruff/issues/19842
 #[test]
 fn pyupgrade_up026_respects_isort_required_import_fix() {

crates/ruff/tests/cli/snapshots/cli__lint__required_import_set_aliased_as_abstract_set_no_conflict.snap

@@ -0,0 +1,24 @@
+---
+source: crates/ruff/tests/cli/lint.rs
+info:
+  program: ruff
+  args:
+    - check
+    - "--no-cache"
+    - "--output-format"
+    - concise
+    - "--config"
+    - "lint.isort.required-imports = [\"from collections.abc import Set as AbstractSet\"]"
+    - "--select"
+    - "I002,PYI025"
+    - "-"
+  stdin: "1"
+---
+success: false
+exit_code: 1
+----- stdout -----
+-:1:1: I002 [*] Missing required import: `from collections.abc import Set as AbstractSet`
+Found 1 error.
+[*] 1 fixable with the `--fix` option.
+
+----- stderr -----

crates/ruff/tests/cli/snapshots/cli__lint__required_import_set_conflicts_with_pyi025.snap

@@ -0,0 +1,25 @@
+---
+source: crates/ruff/tests/cli/lint.rs
+info:
+  program: ruff
+  args:
+    - check
+    - "--no-cache"
+    - "--output-format"
+    - concise
+    - "--config"
+    - "lint.isort.required-imports = [\"from collections.abc import Set\"]"
+    - "--select"
+    - "I002,PYI025"
+    - "-"
+  stdin: "1"
+---
+success: false
+exit_code: 2
+----- stdout -----
+
+----- stderr -----
+ruff failed
+  Cause: Required import `from collections.abc import Set` specified in `lint.isort.required-imports` (I002) conflicts with `unaliased-collections-abc-set-import` (PYI025), which requires this import to be aliased as `AbstractSet`.
+
+Help: Either alias the required import (`from collections.abc import Set as AbstractSet`), or disable PYI025.

crates/ruff/tests/cli/snapshots/cli__lint__required_import_set_without_pyi025_no_conflict.snap

@@ -0,0 +1,24 @@
+---
+source: crates/ruff/tests/cli/lint.rs
+info:
+  program: ruff
+  args:
+    - check
+    - "--no-cache"
+    - "--output-format"
+    - concise
+    - "--config"
+    - "lint.isort.required-imports = [\"from collections.abc import Set\"]"
+    - "--select"
+    - I002
+    - "-"
+  stdin: "1"
+---
+success: false
+exit_code: 1
+----- stdout -----
+-:1:1: I002 [*] Missing required import: `from collections.abc import Set`
+Found 1 error.
+[*] 1 fixable with the `--fix` option.
+
+----- stderr -----

crates/ruff_workspace/src/configuration.rs

@@ -253,6 +253,7 @@ impl Configuration {
             .unwrap_or_default();
 
         conflicting_import_settings(&isort, &flake8_import_conventions)?;
+        conflicting_required_import_pyi025(&isort, &rules)?;
 
         let future_annotations = lint.future_annotations.unwrap_or_default();
 
@@ -1679,6 +1680,40 @@ fn conflicting_import_settings(
     Ok(())
 }
 
+/// Detect conflicts between I002 (missing-required-import) and PYI025
+/// (unaliased-collections-abc-set-import).
+///
+/// If `required-imports` includes `from collections.abc import Set` (without
+/// aliasing it as `AbstractSet`) and PYI025 is enabled, the configuration is
+/// contradictory: I002 requires the unaliased import, while PYI025 forbids it.
+fn conflicting_required_import_pyi025(
+    isort: &isort::settings::Settings,
+    rules: &RuleTable,
+) -> Result<()> {
+    if !rules.enabled(Rule::UnaliasedCollectionsAbcSetImport) {
+        return Ok(());
+    }
+
+    for required_import in &isort.required_imports {
+        let qualified_name = required_import.qualified_name();
+        if qualified_name.segments() == ["collections", "abc", "Set"]
+            && required_import.bound_name() != "AbstractSet"
+        {
+            return Err(anyhow!(
+                "Required import `from collections.abc import Set` specified in \
+                `lint.isort.required-imports` (I002) conflicts with \
+                `unaliased-collections-abc-set-import` (PYI025), which requires \
+                this import to be aliased as `AbstractSet`.\n\n\
+                Help: Either alias the required import \
+                (`from collections.abc import Set as AbstractSet`), \
+                or disable PYI025."
+            ));
+        }
+    }
+
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use std::str::FromStr;