Apply some clippy suggestions

schemars_derive/src/attr/doc.rs

@@ -52,7 +52,7 @@ fn get_doc(attrs: &[Attribute]) -> Option<String> {
         .iter()
         .flat_map(|a| a.split('\n'))
         .map(str::trim)
-        .skip_while(|s| *s == "")
+        .skip_while(|s| s.is_empty())
         .collect::<Vec<_>>();
 
     if let Some(&"") = lines.last() {
@@ -61,7 +61,7 @@ fn get_doc(attrs: &[Attribute]) -> Option<String> {
 
     // Added for backward-compatibility, but perhaps we shouldn't do this
     // https://github.com/rust-lang/rust/issues/32088
-    if lines.iter().all(|l| l.starts_with("*")) {
+    if lines.iter().all(|l| l.starts_with('*')) {
         for line in lines.iter_mut() {
             *line = line[1..].trim()
         }

schemars_derive/src/metadata.rs

@@ -105,7 +105,7 @@ impl<'a> SchemaMetadata<'a> {
     }
 }
 
-fn none_if_empty<'a>(s: &'a String) -> Option<&'a str> {
+fn none_if_empty(s: &String) -> Option<&str> {
     if s.is_empty() {
         None
     } else {

schemars_derive/src/schema_exprs.rs

@@ -98,7 +98,7 @@ fn expr_for_external_tagged_enum<'a>(
     }
 
     let mut schemas = Vec::new();
-    if unit_variants.len() > 0 {
+    if !unit_variants.is_empty() {
         schemas.push(unit_schema);
     }