Consider unterminated f-strings in FStringRanges (#8154)

## Summary

This PR removes the `debug_assertion` in the `Indexer` to allow
unterminated f-strings. This is mainly a fix in the development build
which now matches the release build.

The fix is simple: remove the `debug_assertion` which means that the
there could be `FStringStart` and possibly `FStringMiddle` tokens
without a corresponding f-string range in the `Indexer`. This means that
the code requesting for the f-string index need to account for the
`None` case, making the code safer.

This also updates the code which queries the `FStringRanges` to account
for the `None` case. This will happen when the `FStringStart` /
`FStringMiddle` tokens are present but the `FStringEnd` token isn't
which means that the `Indexer` won't contain the range for that
f-string.

## Test Plan

`cargo test`

Taking the following code as an example:

```python
f"{123}
```

This only emits a `FStringStart` token, but no `FStringMiddle` or
`FStringEnd` tokens.

And,

```python
f"\.png${
```

This emits a `FStringStart` and `FStringMiddle` token, but no
`FStringEnd` token.

fixes: #8065

Author: dhruvmanila

crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/implicit.rs

@@ -110,18 +110,27 @@ pub(crate) fn implicit(
     {
         let (a_range, b_range) = match (a_tok, b_tok) {
             (Tok::String { .. }, Tok::String { .. }) => (*a_range, *b_range),
-            (Tok::String { .. }, Tok::FStringStart) => (
-                *a_range,
-                indexer.fstring_ranges().innermost(b_range.start()).unwrap(),
-            ),
-            (Tok::FStringEnd, Tok::String { .. }) => (
-                indexer.fstring_ranges().innermost(a_range.start()).unwrap(),
-                *b_range,
-            ),
-            (Tok::FStringEnd, Tok::FStringStart) => (
-                indexer.fstring_ranges().innermost(a_range.start()).unwrap(),
-                indexer.fstring_ranges().innermost(b_range.start()).unwrap(),
-            ),
+            (Tok::String { .. }, Tok::FStringStart) => {
+                match indexer.fstring_ranges().innermost(b_range.start()) {
+                    Some(b_range) => (*a_range, b_range),
+                    None => continue,
+                }
+            }
+            (Tok::FStringEnd, Tok::String { .. }) => {
+                match indexer.fstring_ranges().innermost(a_range.start()) {
+                    Some(a_range) => (a_range, *b_range),
+                    None => continue,
+                }
+            }
+            (Tok::FStringEnd, Tok::FStringStart) => {
+                match (
+                    indexer.fstring_ranges().innermost(a_range.start()),
+                    indexer.fstring_ranges().innermost(b_range.start()),
+                ) {
+                    (Some(a_range), Some(b_range)) => (a_range, b_range),
+                    _ => continue,
+                }
+            }
             _ => continue,
         };
 

crates/ruff_linter/src/rules/pycodestyle/rules/invalid_escape_sequence.rs

@@ -78,18 +78,21 @@ pub(crate) fn invalid_escape_sequence(
     token: &Tok,
     token_range: TextRange,
 ) {
-    let token_source_code = match token {
+    let (token_source_code, string_start_location) = match token {
         Tok::FStringMiddle { value, is_raw } => {
             if *is_raw {
                 return;
             }
-            value.as_str()
+            let Some(range) = indexer.fstring_ranges().innermost(token_range.start()) else {
+                return;
+            };
+            (value.as_str(), range.start())
         }
         Tok::String { kind, .. } => {
             if kind.is_raw() {
                 return;
             }
-            locator.slice(token_range)
+            (locator.slice(token_range), token_range.start())
         }
         _ => return,
     };
@@ -206,17 +209,6 @@ pub(crate) fn invalid_escape_sequence(
             invalid_escape_sequence.push(diagnostic);
         }
     } else {
-        let tok_start = if token.is_f_string_middle() {
-            // SAFETY: If this is a `FStringMiddle` token, then the indexer
-            // must have the f-string range.
-            indexer
-                .fstring_ranges()
-                .innermost(token_range.start())
-                .unwrap()
-                .start()
-        } else {
-            token_range.start()
-        };
         // Turn into raw string.
         for invalid_escape_char in &invalid_escape_chars {
             let diagnostic = Diagnostic::new(
@@ -231,8 +223,8 @@ pub(crate) fn invalid_escape_sequence(
                 // `assert`, etc.) and the string. For example, `return"foo"` is valid, but
                 // `returnr"foo"` is not.
                 Fix::safe_edit(Edit::insertion(
-                    pad_start("r".to_string(), tok_start, locator),
-                    tok_start,
+                    pad_start("r".to_string(), string_start_location, locator),
+                    string_start_location,
                 )),
             );
             invalid_escape_sequence.push(diagnostic);

crates/ruff_python_index/src/fstring_ranges.rs

@@ -5,8 +5,11 @@ use ruff_text_size::{TextRange, TextSize};
 
 /// Stores the ranges of all f-strings in a file sorted by [`TextRange::start`].
 /// There can be multiple overlapping ranges for nested f-strings.
+///
+/// Note that the ranges for all unterminated f-strings are not stored.
 #[derive(Debug)]
 pub struct FStringRanges {
+    // Mapping from the f-string start location to its range.
     raw: BTreeMap<TextSize, TextRange>,
 }
 
@@ -89,7 +92,6 @@ impl FStringRangesBuilder {
     }
 
     pub(crate) fn finish(self) -> FStringRanges {
-        debug_assert!(self.start_locations.is_empty());
         FStringRanges { raw: self.raw }
     }
 }