Implement pylint import-outside-toplevel rule (C0415) (#5180)

## Summary

Implements pylint C0415 (import-outside-toplevel) — imports should be at
the top level of a file.

The great debate I had on this implementation is whether "top-level" is
one word or two (`toplevel` or `top_level`). I opted for 2 because that
seemed to be how it is used in the codebase but the rule string itself
uses one-word "toplevel." 🤷 I'd be happy to change it as desired.

I suppose this could be auto-fixed by moving the import to the
top-level, but it seems likely that the author's intent was to actually
import this dynamically, so I view the main point of this rule is to
force some sort of explanation, and auto-fixing might be annoying.

For reference, this is what "pylint" reports:
```
> pylint crates/ruff/resources/test/fixtures/pylint/import_outside_top_level.py
************* Module import_outside_top_level
...
crates/ruff/resources/test/fixtures/pylint/import_outside_top_level.py:4:4: C0415: Import outside toplevel (string) (import-outside-toplevel)
```

ruff would now report:

```
import_outside_top_level.py:4:5: PLC0415 `import` should be used only at the top level of a file
  |
3 | def import_outside_top_level():
4 |     import string # [import-outside-toplevel]
  |     ^^^^^^^^^^^^^ PLC0415
  |
```

Contributes to #970.

## Test Plan

Snapshot test.

crates/ruff_linter/resources/test/fixtures/pylint/import_outside_top_level.py

@@ -0,0 +1,22 @@
+from typing import TYPE_CHECKING
+
+# Verify that statements nested in conditionals (such as top-level type-checking blocks)
+# are still considered top-level
+if TYPE_CHECKING:
+    import string
+
+
+def import_in_function():
+    import symtable  # [import-outside-toplevel]
+    import os, sys  # [import-outside-toplevel]
+    import time as thyme  # [import-outside-toplevel]
+    import random as rand, socket as sock  # [import-outside-toplevel]
+    from collections import defaultdict # [import-outside-toplevel]
+    from math import sin as sign, cos as cosplay  # [import-outside-toplevel]
+
+
+class ClassWithImports:
+    import tokenize  # [import-outside-toplevel]
+
+    def __init__(self):
+        import trace  # [import-outside-toplevel]

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -530,6 +530,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::ModuleImportNotAtTopOfFile) {
                 pycodestyle::rules::module_import_not_at_top_of_file(checker, stmt);
             }
+            if checker.enabled(Rule::ImportOutsideTopLevel) {
+                pylint::rules::import_outside_top_level(checker, stmt);
+            }
             if checker.enabled(Rule::GlobalStatement) {
                 for name in names {
                     if let Some(asname) = name.asname.as_ref() {
@@ -706,6 +709,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::ModuleImportNotAtTopOfFile) {
                 pycodestyle::rules::module_import_not_at_top_of_file(checker, stmt);
             }
+            if checker.enabled(Rule::ImportOutsideTopLevel) {
+                pylint::rules::import_outside_top_level(checker, stmt);
+            }
             if checker.enabled(Rule::GlobalStatement) {
                 for name in names {
                     if let Some(asname) = name.asname.as_ref() {

crates/ruff_linter/src/codes.rs

@@ -211,6 +211,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "C0205") => (RuleGroup::Stable, rules::pylint::rules::SingleStringSlots),
         (Pylint, "C0208") => (RuleGroup::Stable, rules::pylint::rules::IterationOverSet),
         (Pylint, "C0414") => (RuleGroup::Stable, rules::pylint::rules::UselessImportAlias),
+        (Pylint, "C0415") => (RuleGroup::Preview, rules::pylint::rules::ImportOutsideTopLevel),
         (Pylint, "C2401") => (RuleGroup::Preview, rules::pylint::rules::NonAsciiName),
         (Pylint, "C2403") => (RuleGroup::Preview, rules::pylint::rules::NonAsciiImportName),
         #[allow(deprecated)]

crates/ruff_linter/src/rules/pylint/mod.rs

@@ -60,6 +60,7 @@ mod tests {
         Rule::GlobalVariableNotAssigned,
         Path::new("global_variable_not_assigned.py")
     )]
+    #[test_case(Rule::ImportOutsideTopLevel, Path::new("import_outside_top_level.py"))]
     #[test_case(Rule::ImportSelf, Path::new("import_self/module.py"))]
     #[test_case(Rule::InvalidAllFormat, Path::new("invalid_all_format.py"))]
     #[test_case(Rule::InvalidAllObject, Path::new("invalid_all_object.py"))]

crates/ruff_linter/src/rules/pylint/rules/import_outside_top_level.rs

@@ -0,0 +1,62 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::Stmt;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for `import` statements outside of a module's top-level scope, such
+/// as within a function or class definition.
+///
+/// ## Why is this bad?
+/// [PEP 8] recommends placing imports not only at the top-level of a module,
+/// but at the very top of the file, "just after any module comments and
+/// docstrings, and before module globals and constants."
+///
+/// `import` statements have effects that are global in scope; defining them at
+/// the top level has a number of benefits. For example, it makes it easier to
+/// identify the dependencies of a module, and ensures that any invalid imports
+/// are caught regardless of whether a specific function is called or class is
+/// instantiated.
+///
+/// An import statement would typically be placed within a function only to
+/// avoid a circular dependency, to defer a costly module load, or to avoid
+/// loading a dependency altogether in a certain runtime environment.
+///
+/// ## Example
+/// ```python
+/// def print_python_version():
+///     import platform
+///
+///     print(python.python_version())
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import platform
+///
+///
+/// def print_python_version():
+///     print(python.python_version())
+/// ```
+///
+/// [PEP 8]: https://peps.python.org/pep-0008/#imports
+#[violation]
+pub struct ImportOutsideTopLevel;
+
+impl Violation for ImportOutsideTopLevel {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`import` should be at the top-level of a file")
+    }
+}
+
+/// C0415
+pub(crate) fn import_outside_top_level(checker: &mut Checker, stmt: &Stmt) {
+    if !checker.semantic().current_scope().kind.is_module() {
+        checker
+            .diagnostics
+            .push(Diagnostic::new(ImportOutsideTopLevel, stmt.range()));
+    }
+}

crates/ruff_linter/src/rules/pylint/rules/mod.rs

@@ -18,6 +18,7 @@ pub(crate) use eq_without_hash::*;
 pub(crate) use global_at_module_level::*;
 pub(crate) use global_statement::*;
 pub(crate) use global_variable_not_assigned::*;
+pub(crate) use import_outside_top_level::*;
 pub(crate) use import_self::*;
 pub(crate) use invalid_all_format::*;
 pub(crate) use invalid_all_object::*;
@@ -88,6 +89,7 @@ mod eq_without_hash;
 mod global_at_module_level;
 mod global_statement;
 mod global_variable_not_assigned;
+mod import_outside_top_level;
 mod import_self;
 mod invalid_all_format;
 mod invalid_all_object;

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLC0415_import_outside_top_level.py.snap

@@ -0,0 +1,76 @@
+---
+source: crates/ruff_linter/src/rules/pylint/mod.rs
+---
+import_outside_top_level.py:10:5: PLC0415 `import` should be at the top-level of a file
+   |
+ 9 | def import_in_function():
+10 |     import symtable  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^ PLC0415
+11 |     import os, sys  # [import-outside-toplevel]
+12 |     import time as thyme  # [import-outside-toplevel]
+   |
+
+import_outside_top_level.py:11:5: PLC0415 `import` should be at the top-level of a file
+   |
+ 9 | def import_in_function():
+10 |     import symtable  # [import-outside-toplevel]
+11 |     import os, sys  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^ PLC0415
+12 |     import time as thyme  # [import-outside-toplevel]
+13 |     import random as rand, socket as sock  # [import-outside-toplevel]
+   |
+
+import_outside_top_level.py:12:5: PLC0415 `import` should be at the top-level of a file
+   |
+10 |     import symtable  # [import-outside-toplevel]
+11 |     import os, sys  # [import-outside-toplevel]
+12 |     import time as thyme  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^^^^^^ PLC0415
+13 |     import random as rand, socket as sock  # [import-outside-toplevel]
+14 |     from collections import defaultdict # [import-outside-toplevel]
+   |
+
+import_outside_top_level.py:13:5: PLC0415 `import` should be at the top-level of a file
+   |
+11 |     import os, sys  # [import-outside-toplevel]
+12 |     import time as thyme  # [import-outside-toplevel]
+13 |     import random as rand, socket as sock  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLC0415
+14 |     from collections import defaultdict # [import-outside-toplevel]
+15 |     from math import sin as sign, cos as cosplay  # [import-outside-toplevel]
+   |
+
+import_outside_top_level.py:14:5: PLC0415 `import` should be at the top-level of a file
+   |
+12 |     import time as thyme  # [import-outside-toplevel]
+13 |     import random as rand, socket as sock  # [import-outside-toplevel]
+14 |     from collections import defaultdict # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLC0415
+15 |     from math import sin as sign, cos as cosplay  # [import-outside-toplevel]
+   |
+
+import_outside_top_level.py:15:5: PLC0415 `import` should be at the top-level of a file
+   |
+13 |     import random as rand, socket as sock  # [import-outside-toplevel]
+14 |     from collections import defaultdict # [import-outside-toplevel]
+15 |     from math import sin as sign, cos as cosplay  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PLC0415
+   |
+
+import_outside_top_level.py:19:5: PLC0415 `import` should be at the top-level of a file
+   |
+18 | class ClassWithImports:
+19 |     import tokenize  # [import-outside-toplevel]
+   |     ^^^^^^^^^^^^^^^ PLC0415
+20 | 
+21 |     def __init__(self):
+   |
+
+import_outside_top_level.py:22:9: PLC0415 `import` should be at the top-level of a file
+   |
+21 |     def __init__(self):
+22 |         import trace  # [import-outside-toplevel]
+   |         ^^^^^^^^^^^^ PLC0415
+   |
+
+