Avoid infinite loop between I002 and PYI025 (#23352)

## Summary

Fixes #20891.

When `lint.isort.required-imports` includes `from collections.abc import
Set` (unaliased) and PYI025 is enabled, the two rules conflict: I002
inserts the unaliased import, while PYI025 demands it be aliased as
`AbstractSet`. This causes an infinite autofix loop ("Failed to converge
after 100 iterations").

Rather than patching the import-insertion logic at the rule level, this
PR rejects the contradictory configuration at startup in
`Configuration::into_settings()`, following the existing pattern
established by `conflicting_import_settings()`. The error message
explains the conflict and suggests two resolutions: alias the required
import as `AbstractSet`, or disable PYI025.

### Prior art

PR #21115 took a different approach (modifying `add_required_imports.rs`
and `imports.rs` to relax alias matching). As noted in review, the
better fix is to "reject this configuration entirely at an earlier
stage" since it doesn't make sense to simultaneously require an
unaliased import and forbid it. This PR implements that suggestion
directly.

## Test plan

- 3 unit tests: conflicting config → error, aliased as `AbstractSet` →
ok, PYI025 not enabled → ok
- Reproduction case: `ruff check --config ruff.toml --unsafe-fixes
--fix` now emits a clear configuration error instead of entering the
infinite loop
- `cargo test -p ruff_workspace` — all 26 tests pass

Author: kar-ganap

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

@@ -254,6 +254,7 @@ 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();
 
@@ -1693,6 +1694,40 @@ 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;