[fastapi] Fix false positives for path parameters that FastAPI doesn't recognize (FAST003) (#20687)

## Summary

Fixes #20680

---------

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

crates/ruff_linter/resources/test/fixtures/fastapi/FAST003.py

@@ -227,3 +227,32 @@ async def read_thing(query: str):
 @app.get("/things/{ thing_id : str }")
 async def read_thing(query: str):
     return {"query": query}
+
+
+# https://github.com/astral-sh/ruff/issues/20680
+# These should NOT trigger FAST003 because FastAPI doesn't recognize them as path parameters
+
+# Non-ASCII characters in parameter name
+@app.get("/f1/{用户身份}")
+async def f1():
+    return locals()
+
+# Space in parameter name  
+@app.get("/f2/{x: str}")
+async def f2():
+    return locals()
+
+# Non-ASCII converter
+@app.get("/f3/{complex_number:ℂ}")
+async def f3():
+    return locals()
+
+# Mixed non-ASCII characters
+@app.get("/f4/{用户_id}")
+async def f4():
+    return locals()
+
+# Space in parameter name with converter
+@app.get("/f5/{param: int}")
+async def f5():
+    return locals()

crates/ruff_linter/src/rules/fastapi/rules/fastapi_unused_path_parameter.rs

@@ -1,12 +1,12 @@
-use std::iter::Peekable;
 use std::ops::Range;
-use std::str::CharIndices;
+use std::sync::LazyLock;
+
+use regex::{CaptureMatches, Regex};
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast as ast;
 use ruff_python_ast::{Arguments, Expr, ExprCall, ExprSubscript, Parameter, ParameterWithDefault};
 use ruff_python_semantic::{BindingKind, Modules, ScopeKind, SemanticModel};
-use ruff_python_stdlib::identifiers::is_identifier;
 use ruff_text_size::{Ranged, TextSize};
 
 use crate::Fix;
@@ -165,11 +165,6 @@ pub(crate) fn fastapi_unused_path_parameter(
 
     // Check if any of the path parameters are not in the function signature.
     for (path_param, range) in path_params {
-        // Ignore invalid identifiers (e.g., `user-id`, as opposed to `user_id`)
-        if !is_identifier(path_param) {
-            continue;
-        }
-
         // If the path parameter is already in the function or the dependency signature,
         // we don't need to do anything.
         if named_args.contains(&path_param) {
@@ -461,15 +456,19 @@ fn parameter_alias<'a>(parameter: &'a Parameter, semantic: &SemanticModel) -> Op
 /// the parameter name. For example, `/{x}` is a valid parameter, but `/{ x }` is treated literally.
 #[derive(Debug)]
 struct PathParamIterator<'a> {
-    input: &'a str,
-    chars: Peekable<CharIndices<'a>>,
+    inner: CaptureMatches<'a, 'a>,
 }
 
 impl<'a> PathParamIterator<'a> {
     fn new(input: &'a str) -> Self {
-        PathParamIterator {
-            input,
-            chars: input.char_indices().peekable(),
+        /// Matches the Starlette pattern for path parameters with optional converters from
+        /// <https://github.com/Kludex/starlette/blob/e18637c68e36d112b1983bc0c8b663681e6a4c50/starlette/routing.py#L121>
+        static FASTAPI_PATH_PARAM_REGEX: LazyLock<Regex> = LazyLock::new(|| {
+            Regex::new(r"\{([a-zA-Z_][a-zA-Z0-9_]*)(?::[a-zA-Z_][a-zA-Z0-9_]*)?\}").unwrap()
+        });
+
+        Self {
+            inner: FASTAPI_PATH_PARAM_REGEX.captures_iter(input),
         }
     }
 }
@@ -478,19 +477,10 @@ impl<'a> Iterator for PathParamIterator<'a> {
     type Item = (&'a str, Range<usize>);
 
     fn next(&mut self) -> Option<Self::Item> {
-        while let Some((start, c)) = self.chars.next() {
-            if c == '{' {
-                if let Some((end, _)) = self.chars.by_ref().find(|&(_, ch)| ch == '}') {
-                    let param_content = &self.input[start + 1..end];
-                    // We ignore text after a colon, since those are path converters
-                    // See also: https://fastapi.tiangolo.com/tutorial/path-params/?h=path#path-convertor
-                    let param_name_end = param_content.find(':').unwrap_or(param_content.len());
-                    let param_name = &param_content[..param_name_end];
-
-                    return Some((param_name, start..end + 1));
-                }
-            }
-        }
-        None
+        self.inner
+            .next()
+            // Extract the first capture group (the path parameter), but return the range of the
+            // whole match (everything in braces and including the braces themselves).
+            .and_then(|capture| Some((capture.get(1)?.as_str(), capture.get(0)?.range())))
     }
 }