Make --pretty work better on multi-line issues (#20056)

We can just print the first line for things where there's multiple lines
being the issue. Ideally, we would print the first line, last line, and
explicitly elide the lines in between. That's for a future change!
(maybe)

Fixes #18522.

NOTE: this is an aesthetic thing, there's no right formatting. So I
would be fine if this is closed because others think it looks worse.

Author: A5rocks

mypy/errors.py

@@ -1003,6 +1003,9 @@ def format_messages(
                     marker = "^"
                     if end_line == line and end_column > column:
                         marker = f'^{"~" * (end_column - column - 1)}'
+                    elif end_line != line:
+                        # just highlight the first line instead
+                        marker = f'^{"~" * (len(source_line_expanded) - column - 1)}'
                     a.append(" " * (DEFAULT_SOURCE_OFFSET + column) + marker)
         return a
 

test-data/unit/check-unreachable-code.test

@@ -1619,3 +1619,14 @@ reveal_type(bar().attr)  # N: Revealed type is "Never"
 1  # not unreachable
 reveal_type(foo().attr)  # N: Revealed type is "Never"
 1  # E: Statement is unreachable
+
+[case testUnreachableStatementPrettyHighlighting]
+# flags: --warn-unreachable --pretty
+def x() -> None:
+    assert False
+    if 5:
+        pass
+[out]
+main:4: error: Statement is unreachable
+        if 5:
+        ^~~~~

test-data/unit/daemon.test

@@ -122,7 +122,7 @@ Daemon stopped
 Daemon started
 foo.py:1: error: Function is missing a return type annotation
     def f():
-    ^
+    ^~~~~~~~
 foo.py:1: note: Use "-> None" if function does not return a value
 Found 1 error in 1 file (checked 1 source file)
 == Return code: 1