Add more must_use attributes (#2846)

… so people get a warning when they accidentally add a semicolon after the response expression in a handler function.

Also update changelogs of axum-core, axum-extra.

axum-core/src/response/into_response.rs

@@ -111,6 +111,7 @@ use std::{
 /// ```
 pub trait IntoResponse {
     /// Create a response.
+    #[must_use]
     fn into_response(self) -> Response;
 }
 

axum-core/src/response/into_response_parts.rs

@@ -105,21 +105,25 @@ pub struct ResponseParts {
 
 impl ResponseParts {
     /// Gets a reference to the response headers.
+    #[must_use]
     pub fn headers(&self) -> &HeaderMap {
         self.res.headers()
     }
 
     /// Gets a mutable reference to the response headers.
+    #[must_use]
     pub fn headers_mut(&mut self) -> &mut HeaderMap {
         self.res.headers_mut()
     }
 
     /// Gets a reference to the response extensions.
+    #[must_use]
     pub fn extensions(&self) -> &Extensions {
         self.res.extensions()
     }
 
     /// Gets a mutable reference to the response extensions.
+    #[must_use]
     pub fn extensions_mut(&mut self) -> &mut Extensions {
         self.res.extensions_mut()
     }

axum-core/src/response/mod.rs

@@ -117,6 +117,7 @@ where
 ///
 /// See [`Result`] for more details.
 #[derive(Debug)]
+#[must_use]
 pub struct ErrorResponse(Response);
 
 impl<T> From<T> for ErrorResponse