[unconventional-import-alias] Fix infinite loop between ICN001 and I002 (ICN001) (#15480)

## Summary

This fixes the infinite loop reported in #14389 by raising an error to
the user about conflicting ICN001 (`unconventional-import-alias`) and
I002 (`missing-required-import`) configuration options.

## Test Plan

Added a CLI integration test reproducing the old behavior and then
confirming the fix.

Closes #14389

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff/tests/lint.rs

@@ -2160,3 +2160,17 @@ fn flake8_import_convention_invalid_aliases_config_module_name() -> Result<()> {
         "#);});
     Ok(())
 }
+
+#[test]
+fn flake8_import_convention_unused_aliased_import() {
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .args(STDIN_BASE_OPTIONS)
+        .arg("--config")
+        .arg(r#"lint.isort.required-imports = ["import pandas"]"#)
+        .args(["--select", "I002,ICN001,F401"])
+        .args(["--stdin-filename", "test.py"])
+        .arg("--unsafe-fixes")
+        .arg("--fix")
+        .arg("-")
+        .pass_stdin("1"));
+}

crates/ruff/tests/snapshots/lint__flake8_import_convention_unused_aliased_import.snap

@@ -0,0 +1,31 @@
+---
+source: crates/ruff/tests/lint.rs
+info:
+  program: ruff
+  args:
+    - check
+    - "--no-cache"
+    - "--output-format"
+    - concise
+    - "--config"
+    - "lint.isort.required-imports = [\"import pandas\"]"
+    - "--select"
+    - "I002,ICN001,F401"
+    - "--stdin-filename"
+    - test.py
+    - "--unsafe-fixes"
+    - "--fix"
+    - "-"
+  stdin: "1"
+snapshot_kind: text
+---
+success: false
+exit_code: 2
+----- stdout -----
+
+----- stderr -----
+ruff failed
+  Cause: Required import specified in `lint.isort.required-imports` (I002) conflicts with the required import alias specified in either `lint.flake8-import-conventions.aliases` or `lint.flake8-import-conventions.extend-aliases` (ICN001):
+    - `pandas` -> `pd`
+
+Help: Remove the required import or alias from your configuration.

crates/ruff_python_semantic/src/imports.rs

@@ -57,7 +57,7 @@ impl NameImport {
     }
 
     /// Returns the [`QualifiedName`] of the imported name (e.g., given `from foo import bar as baz`, returns `["foo", "bar"]`).
-    fn qualified_name(&self) -> QualifiedName {
+    pub fn qualified_name(&self) -> QualifiedName {
         match self {
             NameImport::Import(import) => QualifiedName::user_defined(&import.name.name),
             NameImport::ImportFrom(import_from) => collect_import_from_member(

crates/ruff_workspace/src/configuration.rs

@@ -25,7 +25,7 @@ use ruff_linter::line_width::{IndentWidth, LineLength};
 use ruff_linter::registry::RuleNamespace;
 use ruff_linter::registry::{Rule, RuleSet, INCOMPATIBLE_CODES};
 use ruff_linter::rule_selector::{PreviewOptions, Specificity};
-use ruff_linter::rules::pycodestyle;
+use ruff_linter::rules::{flake8_import_conventions, isort, pycodestyle};
 use ruff_linter::settings::fix_safety_table::FixSafetyTable;
 use ruff_linter::settings::rule_table::RuleTable;
 use ruff_linter::settings::types::{
@@ -232,6 +232,21 @@ impl Configuration {
 
         let line_length = self.line_length.unwrap_or_default();
 
+        let rules = lint.as_rule_table(lint_preview)?;
+
+        // LinterSettings validation
+        let isort = lint
+            .isort
+            .map(IsortOptions::try_into_settings)
+            .transpose()?
+            .unwrap_or_default();
+        let flake8_import_conventions = lint
+            .flake8_import_conventions
+            .map(Flake8ImportConventionsOptions::into_settings)
+            .unwrap_or_default();
+
+        conflicting_import_settings(&isort, &flake8_import_conventions)?;
+
         Ok(Settings {
             cache_dir: self
                 .cache_dir
@@ -259,7 +274,7 @@ impl Configuration {
 
             #[allow(deprecated)]
             linter: LinterSettings {
-                rules: lint.as_rule_table(lint_preview)?,
+                rules,
                 exclude: FilePatternSet::try_from_iter(lint.exclude.unwrap_or_default())?,
                 extension: self.extension.unwrap_or_default(),
                 preview: lint_preview,
@@ -341,10 +356,7 @@ impl Configuration {
                     .flake8_implicit_str_concat
                     .map(Flake8ImplicitStrConcatOptions::into_settings)
                     .unwrap_or_default(),
-                flake8_import_conventions: lint
-                    .flake8_import_conventions
-                    .map(Flake8ImportConventionsOptions::into_settings)
-                    .unwrap_or_default(),
+                flake8_import_conventions,
                 flake8_pytest_style: lint
                     .flake8_pytest_style
                     .map(Flake8PytestStyleOptions::try_into_settings)
@@ -374,11 +386,7 @@ impl Configuration {
                     .flake8_gettext
                     .map(Flake8GetTextOptions::into_settings)
                     .unwrap_or_default(),
-                isort: lint
-                    .isort
-                    .map(IsortOptions::try_into_settings)
-                    .transpose()?
-                    .unwrap_or_default(),
+                isort,
                 mccabe: lint
                     .mccabe
                     .map(McCabeOptions::into_settings)
@@ -1553,6 +1561,34 @@ fn warn_about_deprecated_top_level_lint_options(
     );
 }
 
+/// Detect conflicts between I002 (missing-required-import) and ICN001 (unconventional-import-alias)
+fn conflicting_import_settings(
+    isort: &isort::settings::Settings,
+    flake8_import_conventions: &flake8_import_conventions::settings::Settings,
+) -> Result<()> {
+    use std::fmt::Write;
+    let mut err_body = String::new();
+    for required_import in &isort.required_imports {
+        let name = required_import.qualified_name().to_string();
+        if let Some(alias) = flake8_import_conventions.aliases.get(&name) {
+            writeln!(err_body, "    - `{name}` -> `{alias}`").unwrap();
+        }
+    }
+
+    if !err_body.is_empty() {
+        return Err(anyhow!(
+            "Required import specified in `lint.isort.required-imports` (I002) \
+            conflicts with the required import alias specified in either \
+            `lint.flake8-import-conventions.aliases` or \
+            `lint.flake8-import-conventions.extend-aliases` (ICN001):\
+                \n{err_body}\n\
+            Help: Remove the required import or alias from your configuration."
+        ));
+    }
+
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use std::str::FromStr;