[flake8-pyi] Fix false positive in __all__ (PYI053) (#26872)

Summary
--

This PR fixes a false positive on long strings in `__all__`. These
strings correspond to existing
exported symbols and are out of the stub author's control.

I initially hoped that we could use the existing
`Semantic::in_dunder_all_definition`, but this is
only set in `Checker::visit_exports` and used for adding global
references, so the flag is no longer
set when the rule runs in `Checker::visit_expr`.

I found another similar piece of existing code in
`Checker::handle_node_store` and factored that out
into a `Checker` helper method.

Test Plan
--

New mdtest

Author: ntBre

crates/ruff_linter/resources/mdtest/flake8-pyi/string-or-bytes-too-long.md

@@ -0,0 +1,16 @@
+# `string-or-bytes-too-long` (`PYI053`)
+
+```toml
+[lint]
+select = ["PYI053"]
+```
+
+## Long name in `__all__`
+
+Strings in `__all__` correspond to exported names and should be exempt from the rule.
+
+```pyi
+__all__ = [
+    "aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeef",
+]
+```

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -2792,34 +2792,7 @@ impl<'a> Checker<'a> {
             _ => {}
         }
 
-        let scope = self.semantic.current_scope();
-
-        if scope.kind.is_module()
-            && match parent {
-                Stmt::Assign(ast::StmtAssign { targets, .. }) => {
-                    if let Some(Expr::Name(ast::ExprName { id, .. })) = targets.first() {
-                        id == "__all__"
-                    } else {
-                        false
-                    }
-                }
-                Stmt::AugAssign(ast::StmtAugAssign { target, .. }) => {
-                    if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
-                        id == "__all__"
-                    } else {
-                        false
-                    }
-                }
-                Stmt::AnnAssign(ast::StmtAnnAssign { target, .. }) => {
-                    if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
-                        id == "__all__"
-                    } else {
-                        false
-                    }
-                }
-                _ => false,
-            }
-        {
+        if self.in_dunder_all_assignment(parent) {
             let (all_names, all_flags) = self.semantic.extract_dunder_all_names(parent);
 
             if all_flags.intersects(DunderAllFlags::INVALID_OBJECT) {
@@ -3285,6 +3258,41 @@ impl<'a> Checker<'a> {
 
         self.semantic.restore(snapshot);
     }
+
+    /// Report whether a module-level `__all__` assignment is being visited.
+    ///
+    /// This differs from [`SemanticModel::in_dunder_all_definition`], which is set only while
+    /// adding bindings for the entries in `__all__`.
+    pub(crate) fn in_dunder_all_assignment(&self, parent: &Stmt) -> bool {
+        if !self.semantic.current_scope().kind.is_module() {
+            return false;
+        }
+
+        match parent {
+            Stmt::Assign(ast::StmtAssign { targets, .. }) => {
+                if let Some(Expr::Name(ast::ExprName { id, .. })) = targets.first() {
+                    id == "__all__"
+                } else {
+                    false
+                }
+            }
+            Stmt::AugAssign(ast::StmtAugAssign { target, .. }) => {
+                if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
+                    id == "__all__"
+                } else {
+                    false
+                }
+            }
+            Stmt::AnnAssign(ast::StmtAnnAssign { target, .. }) => {
+                if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
+                    id == "__all__"
+                } else {
+                    false
+                }
+            }
+            _ => false,
+        }
+    }
 }
 
 struct ParsedAnnotationsCache<'a> {

crates/ruff_linter/src/rules/flake8_pyi/rules/string_or_bytes_too_long.rs

@@ -21,6 +21,9 @@ use crate::{AlwaysFixableViolation, Edit, Fix};
 /// checkers, the primary consumers of stub files. Replace very long constants
 /// with ellipses (`...`) to simplify the stub.
 ///
+/// The rule does not apply to long entries in `__all__`, which are assumed to
+/// be outside the stub author's control.
+///
 /// ## Example
 ///
 /// ```pyi
@@ -51,8 +54,10 @@ impl AlwaysFixableViolation for StringOrBytesTooLong {
 pub(crate) fn string_or_bytes_too_long(checker: &Checker, string: StringLike) {
     let semantic = checker.semantic();
 
+    let parent = semantic.current_statement();
+
     // Ignore docstrings.
-    if is_docstring_stmt(semantic.current_statement()) {
+    if is_docstring_stmt(parent) {
         return;
     }
 
@@ -64,6 +69,10 @@ pub(crate) fn string_or_bytes_too_long(checker: &Checker, string: StringLike) {
         return;
     }
 
+    if checker.in_dunder_all_assignment(parent) {
+        return;
+    }
+
     let length = match string {
         StringLike::String(ast::ExprStringLiteral { value, .. }) => value.chars().count(),
         StringLike::Bytes(ast::ExprBytesLiteral { value, .. }) => value.len(),