[refurb] Auto-fix annotated assignments (FURB101) (#21278)

## Summary

Fixed FURB101 (`read-whole-file`) to handle annotated assignments.
Previously, the rule would detect violations in code like `contents: str
= f.read()` but fail to generate a fix. Now it correctly generates fixes
that preserve type annotations (e.g., `contents: str =
Path("file.txt").read_text(encoding="utf-8")`).

Fixes #21274

## Problem Analysis

The FURB101 rule was only checking for `Stmt::Assign` statements when
determining whether a fix could be applied. When encountering annotated
assignments (`Stmt::AnnAssign`) like `contents: str = f.read()`, the
rule would:

1. Correctly detect the violation (the diagnostic was reported)
2. Fail to generate a fix because:
- The `visit_expr` method only matched `Stmt::Assign`, not
`Stmt::AnnAssign`
- The `generate_fix` function only accepted `Stmt::Assign` in its body
validation
   - The replacement code generation didn't account for type annotations

This occurred because Python's AST represents annotated assignments as a
different node type (`StmtAnnAssign`) with separate fields for the
target, annotation, and value, unlike regular assignments which use a
list of targets.

## Approach

The fix extends the rule to handle both assignment types:

1. **Updated `visit_expr` method**: Now matches both `Stmt::Assign` and
`Stmt::AnnAssign`, extracting:
   - Variable name from the target expression
   - Type annotation code (when present) using the code generator

2. **Updated `generate_fix` function**:
- Added `annotation: Option<String>` parameter to accept annotation code
- Updated body validation to accept both `Stmt::Assign` and
`Stmt::AnnAssign`
- Modified replacement code generation to preserve annotations: `{var}:
{annotation} = {binding}({filename_code}).{suggestion}`

3. **Added test case**: Added an annotated assignment test case to
verify the fix works correctly.

The implementation maintains backward compatibility with regular
assignments while adding support for annotated assignments, ensuring
type annotations are preserved in the generated fixes.

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/resources/test/fixtures/refurb/FURB101.py

@@ -125,3 +125,18 @@ with open(*filename, mode="r") as f:
 # `buffering`.
 with open(*filename, file="file.txt", mode="r") as f:
     x = f.read()
+
+# FURB101
+with open("file.txt", encoding="utf-8") as f:
+    contents: str = f.read()
+
+# FURB101 but no fix because it would remove the assignment to `x`
+with open("file.txt", encoding="utf-8") as f:
+    contents, x = f.read(), 2
+
+# FURB101 but no fix because it would remove the `process_contents` call
+with open("file.txt", encoding="utf-8") as f:
+    contents = process_contents(f.read())
+
+with open("file.txt", encoding="utf-8") as f:
+    contents: str = process_contents(f.read())

crates/ruff_linter/src/rules/refurb/rules/read_whole_file.rs

@@ -125,20 +125,8 @@ impl<'a> Visitor<'a> for ReadMatcher<'a, '_> {
                     open.item.range(),
                 );
 
-                let target = match self.with_stmt.body.first() {
-                    Some(Stmt::Assign(assign))
-                        if assign.value.range().contains_range(expr.range()) =>
-                    {
-                        match assign.targets.first() {
-                            Some(Expr::Name(name)) => Some(name.id.as_str()),
-                            _ => None,
-                        }
-                    }
-                    _ => None,
-                };
-
                 if let Some(fix) =
-                    generate_fix(self.checker, &open, target, self.with_stmt, &suggestion)
+                    generate_fix(self.checker, &open, expr, self.with_stmt, &suggestion)
                 {
                     diagnostic.set_fix(fix);
                 }
@@ -190,15 +178,16 @@ fn make_suggestion(open: &FileOpen<'_>, generator: Generator) -> String {
 fn generate_fix(
     checker: &Checker,
     open: &FileOpen,
-    target: Option<&str>,
+    expr: &Expr,
     with_stmt: &ast::StmtWith,
     suggestion: &str,
 ) -> Option<Fix> {
-    if !(with_stmt.items.len() == 1 && matches!(with_stmt.body.as_slice(), [Stmt::Assign(_)])) {
+    if with_stmt.items.len() != 1 {
         return None;
     }
 
     let locator = checker.locator();
+
     let filename_code = locator.slice(open.filename.range());
 
     let (import_edit, binding) = checker
@@ -210,9 +199,39 @@ fn generate_fix(
         )
         .ok()?;
 
-    let replacement = match target {
-        Some(var) => format!("{var} = {binding}({filename_code}).{suggestion}"),
-        None => format!("{binding}({filename_code}).{suggestion}"),
+    // Only replace context managers with a single assignment or annotated assignment in the body.
+    // The assignment's RHS must also be the same as the `read` call in `expr`, otherwise this fix
+    // would remove the rest of the expression.
+    let replacement = match with_stmt.body.as_slice() {
+        [Stmt::Assign(ast::StmtAssign { targets, value, .. })] if value.range() == expr.range() => {
+            match targets.as_slice() {
+                [Expr::Name(name)] => {
+                    format!(
+                        "{name} = {binding}({filename_code}).{suggestion}",
+                        name = name.id
+                    )
+                }
+                _ => return None,
+            }
+        }
+        [
+            Stmt::AnnAssign(ast::StmtAnnAssign {
+                target,
+                annotation,
+                value: Some(value),
+                ..
+            }),
+        ] if value.range() == expr.range() => match target.as_ref() {
+            Expr::Name(name) => {
+                format!(
+                    "{var}: {ann} = {binding}({filename_code}).{suggestion}",
+                    var = name.id,
+                    ann = locator.slice(annotation.range())
+                )
+            }
+            _ => return None,
+        },
+        _ => return None,
     };
 
     let applicability = if checker.comment_ranges().intersects(with_stmt.range()) {

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB101_FURB101.py.snap

@@ -189,3 +189,58 @@ FURB101 `open` and `read` should be replaced by `Path("file.txt").read_text()`
 51 |     # the user reads the whole file and that bit they can replace.
    |
 help: Replace with `Path("file.txt").read_text()`
+
+FURB101 [*] `open` and `read` should be replaced by `Path("file.txt").read_text(encoding="utf-8")`
+   --> FURB101.py:130:6
+    |
+129 | # FURB101
+130 | with open("file.txt", encoding="utf-8") as f:
+    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+131 |     contents: str = f.read()
+    |
+help: Replace with `Path("file.txt").read_text(encoding="utf-8")`
+1   + import pathlib
+2   | def foo():
+3   |     ...
+4   | 
+--------------------------------------------------------------------------------
+128 |     x = f.read()
+129 | 
+130 | # FURB101
+    - with open("file.txt", encoding="utf-8") as f:
+    -     contents: str = f.read()
+131 + contents: str = pathlib.Path("file.txt").read_text(encoding="utf-8")
+132 | 
+133 | # FURB101 but no fix because it would remove the assignment to `x`
+134 | with open("file.txt", encoding="utf-8") as f:
+
+FURB101 `open` and `read` should be replaced by `Path("file.txt").read_text(encoding="utf-8")`
+   --> FURB101.py:134:6
+    |
+133 | # FURB101 but no fix because it would remove the assignment to `x`
+134 | with open("file.txt", encoding="utf-8") as f:
+    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+135 |     contents, x = f.read(), 2
+    |
+help: Replace with `Path("file.txt").read_text(encoding="utf-8")`
+
+FURB101 `open` and `read` should be replaced by `Path("file.txt").read_text(encoding="utf-8")`
+   --> FURB101.py:138:6
+    |
+137 | # FURB101 but no fix because it would remove the `process_contents` call
+138 | with open("file.txt", encoding="utf-8") as f:
+    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+139 |     contents = process_contents(f.read())
+    |
+help: Replace with `Path("file.txt").read_text(encoding="utf-8")`
+
+FURB101 `open` and `read` should be replaced by `Path("file.txt").read_text(encoding="utf-8")`
+   --> FURB101.py:141:6
+    |
+139 |     contents = process_contents(f.read())
+140 |
+141 | with open("file.txt", encoding="utf-8") as f:
+    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+142 |     contents: str = process_contents(f.read())
+    |
+help: Replace with `Path("file.txt").read_text(encoding="utf-8")`