fix crash issue when using shadowfile with pretty #17853 (#17894)

- Fix crash issue when using <code>--pretty</code> with
<code>--shadow-file</code>

Example Scenario:
<code>a.py</code>

```python
b: bytes
```

<code>b.py</code>

```python
a: int = ""
b: bytes = 1
```
<code>output</code>
```
$ mypy a.py --pretty --shadow-file a.py b.py
a.py:1: error: Incompatible types in assignment (expression has type "str",
variable has type "int")
    a: int = ""
             ^~
a.py:2: error: Incompatible types in assignment (expression has type "int",
variable has type "bytes")
    b: bytes = 1
               ^
```

Fixes #17853

Author: changhoetyng

mypy/errors.py

@@ -922,9 +922,25 @@ def file_messages(self, path: str, formatter: ErrorFormatter | None = None) -> l
         self.flushed_files.add(path)
         source_lines = None
         if self.options.pretty and self.read_source:
-            source_lines = self.read_source(path)
+            # Find shadow file mapping and read source lines if a shadow file exists for the given path.
+            # If shadow file mapping is not found, read source lines
+            mapped_path = self.find_shadow_file_mapping(path)
+            if mapped_path:
+                source_lines = self.read_source(mapped_path)
+            else:
+                source_lines = self.read_source(path)
         return self.format_messages(error_tuples, source_lines)
 
+    def find_shadow_file_mapping(self, path: str) -> str | None:
+        """Return the shadow file path for a given source file path or None."""
+        if self.options.shadow_file is None:
+            return None
+
+        for i in self.options.shadow_file:
+            if i[0] == path:
+                return i[1]
+        return None
+
     def new_messages(self) -> list[str]:
         """Return a string list of new error messages.
 

test-data/unit/cmdline.test

@@ -914,6 +914,23 @@ s4.py:2: error: Incompatible return value type (got "int", expected "str")
 s3.py:2: error: Incompatible return value type (got "List[int]", expected "int")
 s1.py:2: error: Incompatible return value type (got "int", expected "str")
 
+[case testShadowFileWithPretty]
+# cmd: mypy a.py --pretty --shadow-file a.py b.py
+[file a.py]
+b: bytes
+[file b.py]
+a: int = ""
+b: bytes = 1
+[out]
+a.py:1: error: Incompatible types in assignment (expression has type "str",
+variable has type "int")
+    a: int = ""
+             ^~
+a.py:2: error: Incompatible types in assignment (expression has type "int",
+variable has type "bytes")
+    b: bytes = 1
+               ^
+
 [case testConfigWarnUnusedSection1]
 # cmd: mypy foo.py quux.py spam/eggs.py
 [file mypy.ini]