Avoid printing continuations within import identifiers (#7744)

## Summary

It turns out that _some_ identifiers can contain newlines --
specifically, dot-delimited import identifiers, like:
```python
import foo\
    .bar
```

At present, we print all identifiers verbatim, which causes us to retain
the `\` in the formatted output. This also leads to violating some debug
assertions (see the linked issue, though that's a symptom of this
formatting failure).

This PR adds detection for import identifiers that contain newlines, and
formats them via `text` (slow) rather than `source_code_slice` (fast) in
those cases.

Closes #7734.

## Test Plan

`cargo test`

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/import.py

@@ -2,6 +2,13 @@
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as dfgsdfgsd, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd
 
+# Continuations.
+import foo\
+    .bar
+
+from foo\
+    .bar import baz
+
 # At the top-level, force one empty line after an import, but allow up to two empty
 # lines.
 import os

crates/ruff_python_formatter/src/other/alias.rs

@@ -1,6 +1,7 @@
 use ruff_formatter::write;
 use ruff_python_ast::Alias;
 
+use crate::other::identifier::DotDelimitedIdentifier;
 use crate::prelude::*;
 
 #[derive(Default)]
@@ -13,7 +14,7 @@ impl FormatNodeRule<Alias> for FormatAlias {
             name,
             asname,
         } = item;
-        name.format().fmt(f)?;
+        DotDelimitedIdentifier::new(name).fmt(f)?;
         if let Some(asname) = asname {
             write!(f, [space(), token("as"), space(), asname.format()])?;
         }

crates/ruff_python_formatter/src/other/identifier.rs

@@ -27,3 +27,43 @@ impl<'ast> IntoFormat<PyFormatContext<'ast>> for Identifier {
         FormatOwnedWithRule::new(self, FormatIdentifier)
     }
 }
+
+/// A formatter for a dot-delimited identifier, as seen in import statements:
+/// ```python
+/// import foo.bar
+/// ```
+///
+/// Dot-delimited identifiers can contain newlines via continuations (backslashes) after the
+/// dot-delimited segment, as in:
+/// ```python
+/// import foo\
+///    .bar
+/// ```
+///
+/// While identifiers can typically be formatted via verbatim source code slices, dot-delimited
+/// identifiers with newlines must be formatted via `text`. This struct implements both the fast
+/// and slow paths for such identifiers.
+#[derive(Debug)]
+pub(crate) struct DotDelimitedIdentifier<'a>(&'a Identifier);
+
+impl<'a> DotDelimitedIdentifier<'a> {
+    pub(crate) fn new(identifier: &'a Identifier) -> Self {
+        Self(identifier)
+    }
+}
+
+impl Format<PyFormatContext<'_>> for DotDelimitedIdentifier<'_> {
+    fn fmt(&self, f: &mut PyFormatter) -> FormatResult<()> {
+        // An import identifier can contain newlines by inserting continuations (backslashes) after
+        // a dot-delimited segment, as in:
+        // ```python
+        // import foo\
+        //    .bar
+        // ```
+        if memchr::memchr(b'\\', f.context().source()[self.0.range()].as_bytes()).is_some() {
+            text(self.0.as_str(), Some(self.0.start())).fmt(f)
+        } else {
+            source_text_slice(self.0.range()).fmt(f)
+        }
+    }
+}

crates/ruff_python_formatter/src/statement/stmt_import_from.rs

@@ -6,6 +6,7 @@ use ruff_text_size::Ranged;
 use crate::builders::{parenthesize_if_expands, PyFormatterExtensions, TrailingComma};
 use crate::comments::{SourceComment, SuppressionKind};
 use crate::expression::parentheses::parenthesized;
+use crate::other::identifier::DotDelimitedIdentifier;
 use crate::prelude::*;
 
 #[derive(Default)]
@@ -31,7 +32,7 @@ impl FormatNodeRule<StmtImportFrom> for FormatStmtImportFrom {
                     }
                     Ok(())
                 }),
-                module.as_ref().map(AsFormat::format),
+                module.as_ref().map(DotDelimitedIdentifier::new),
                 space(),
                 token("import"),
                 space(),

crates/ruff_python_formatter/tests/snapshots/format@statement__import.py.snap

@@ -8,6 +8,13 @@ from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjf
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa
 from a import aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as dfgsdfgsd, aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd
 
+# Continuations.
+import foo\
+    .bar
+
+from foo\
+    .bar import baz
+
 # At the top-level, force one empty line after an import, but allow up to two empty
 # lines.
 import os
@@ -98,6 +105,11 @@ from a import (
     aksjdhflsakhdflkjsadlfajkslhfdkjsaldajlahflashdfljahlfksajlhfajfjfsaahflakjslhdfkjalhdskjfa as sdkjflsdjlahlfd,
 )
 
+# Continuations.
+import foo.bar
+
+from foo.bar import baz
+
 # At the top-level, force one empty line after an import, but allow up to two empty
 # lines.
 import os