chore(clippy): enable and fix many stricter lints (#3783)

* chore(clippy): enable and fix many stricter lints

These ensure that the codebase uses more idiomatic, performant, and
concise language constructions.

* chore: make non-Clippy compiler warnings also deny by default

packages/app-lib/src/util/fetch.rs

@@ -80,10 +80,9 @@ pub async fn fetch_advanced(
 ) -> crate::Result<Bytes> {
     let _permit = semaphore.0.acquire().await?;
 
-    let creds = if !header
+    let creds = if header
         .as_ref()
-        .map(|x| &*x.0.to_lowercase() == "authorization")
-        .unwrap_or(false)
+        .is_none_or(|x| &*x.0.to_lowercase() != "authorization")
         && (url.starts_with("https://cdn.modrinth.com")
             || url.starts_with(MODRINTH_API_URL)
             || url.starts_with(MODRINTH_API_URL_V3))

packages/app-lib/src/util/jre.rs

@@ -227,13 +227,11 @@ async fn get_all_jre_path() -> HashSet<PathBuf> {
     paths.unwrap_or_else(|_| HashSet::new())
 }
 
-#[cfg(target_os = "windows")]
-#[allow(dead_code)]
-pub const JAVA_BIN: &str = "javaw.exe";
-
-#[cfg(not(target_os = "windows"))]
-#[allow(dead_code)]
-pub const JAVA_BIN: &str = "java";
+pub const JAVA_BIN: &str = if cfg!(target_os = "windows") {
+    "javaw.exe"
+} else {
+    "java"
+};
 
 // For each example filepath in 'paths', perform check_java_at_filepath, checking each one concurrently
 // and returning a JavaVersion for every valid path that points to a java bin
@@ -249,7 +247,7 @@ pub async fn check_java_at_filepaths(
         .collect::<Vec<_>>()
         .await;
 
-    jres.into_iter().flat_map(|x| x.ok()).flatten().collect()
+    jres.into_iter().filter_map(|x| x.ok()).flatten().collect()
 }
 
 // For example filepath 'path', attempt to resolve it and get a Java version at this path