[PULL 05/30] rust: qemu_api_macros: make pattern matching more readable and efficient

[Paolo Bonzini]

"let ... else" is useful when visiting syntax trees; it avoids multiple
levels of indentation and places the error close to the pattern.

While at it, use "ref" to avoid moving the syntax tree objects.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/qemu-api-macros/src/lib.rs | 84 +++++++++++++++------------------
 1 file changed, 37 insertions(+), 47 deletions(-)

diff --git a/rust/qemu-api-macros/src/lib.rs b/rust/qemu-api-macros/src/lib.rs
index eda0d46d122..0bffd2691b4 100644
--- a/rust/qemu-api-macros/src/lib.rs
+++ b/rust/qemu-api-macros/src/lib.rs
@@ -16,50 +16,41 @@ fn get_fields<'a>(
     input: &'a DeriveInput,
     msg: &str,
 ) -> Result<&'a Punctuated<Field, Comma>, MacroError> {
-    if let Data::Struct(s) = &input.data {
-        if let Fields::Named(fs) = &s.fields {
-            Ok(&fs.named)
-        } else {
-            Err(MacroError::Message(
-                format!("Named fields required for {}", msg),
-                input.ident.span(),
-            ))
-        }
-    } else {
-        Err(MacroError::Message(
+    let Data::Struct(ref s) = &input.data else {
+        return Err(MacroError::Message(
             format!("Struct required for {}", msg),
             input.ident.span(),
-        ))
-    }
+        ));
+    };
+    let Fields::Named(ref fs) = &s.fields else {
+        return Err(MacroError::Message(
+            format!("Named fields required for {}", msg),
+            input.ident.span(),
+        ));
+    };
+    Ok(&fs.named)
 }
 
 fn get_unnamed_field<'a>(input: &'a DeriveInput, msg: &str) -> Result<&'a 
Field, MacroError> {
-    if let Data::Struct(s) = &input.data {
-        let unnamed = match &s.fields {
-            Fields::Unnamed(FieldsUnnamed {
-                unnamed: ref fields,
-                ..
-            }) => fields,
-            _ => {
-                return Err(MacroError::Message(
-                    format!("Tuple struct required for {}", msg),
-                    s.fields.span(),
-                ))
-            }
-        };
-        if unnamed.len() != 1 {
-            return Err(MacroError::Message(
-                format!("A single field is required for {}", msg),
-                s.fields.span(),
-            ));
-        }
-        Ok(&unnamed[0])
-    } else {
-        Err(MacroError::Message(
+    let Data::Struct(ref s) = &input.data else {
+        return Err(MacroError::Message(
             format!("Struct required for {}", msg),
             input.ident.span(),
-        ))
+        ));
+    };
+    let Fields::Unnamed(FieldsUnnamed { ref unnamed, .. }) = &s.fields else {
+        return Err(MacroError::Message(
+            format!("Tuple struct required for {}", msg),
+            s.fields.span(),
+        ));
+    };
+    if unnamed.len() != 1 {
+        return Err(MacroError::Message(
+            format!("A single field is required for {}", msg),
+            s.fields.span(),
+        ));
     }
+    Ok(&unnamed[0])
 }
 
 fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), MacroError> {
@@ -210,20 +201,19 @@ fn get_repr_uN(input: &DeriveInput, msg: &str) -> 
Result<Path, MacroError> {
 }
 
 fn get_variants(input: &DeriveInput) -> Result<&Punctuated<Variant, Comma>, 
MacroError> {
-    if let Data::Enum(e) = &input.data {
-        if let Some(v) = e.variants.iter().find(|v| v.fields != Fields::Unit) {
-            return Err(MacroError::Message(
-                "Cannot derive TryInto for enum with non-unit 
variants.".to_string(),
-                v.fields.span(),
-            ));
-        }
-        Ok(&e.variants)
-    } else {
-        Err(MacroError::Message(
+    let Data::Enum(ref e) = &input.data else {
+        return Err(MacroError::Message(
             "Cannot derive TryInto for union or struct.".to_string(),
             input.ident.span(),
-        ))
+        ));
+    };
+    if let Some(v) = e.variants.iter().find(|v| v.fields != Fields::Unit) {
+        return Err(MacroError::Message(
+            "Cannot derive TryInto for enum with non-unit 
variants.".to_string(),
+            v.fields.span(),
+        ));
     }
+    Ok(&e.variants)
 }
 
 #[rustfmt::skip::macros(quote)]
-- 
2.49.0