solc-json-interface: do not unconditionally skip serialization of custom keys (#337)

The data structure can be used to build the JSON input for `resolc` too.
In that case serializing of provided custom options should not be
dismissed.

Makes the memory settings struct more modular as a drive-by.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/solc-json-interface/src/standard_json/input/settings/polkavm/memory.rs

@@ -2,20 +2,35 @@
 
 use serde::{Deserialize, Serialize};
 
+pub const DEFAULT_HEAP_SIZE: u32 = 64 * 1024;
+pub const DEFAULT_STACK_SIZE: u32 = 32 * 1024;
+
 /// The PolkaVM memory configuration.
 #[derive(Clone, Copy, Debug, Deserialize, Serialize)]
 pub struct MemoryConfig {
     /// The emulated EVM linear heap memory size in bytes.
-    pub heap_size: u32,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub heap_size: Option<u32>,
     /// The PVM stack size in bytes.
-    pub stack_size: u32,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub stack_size: Option<u32>,
+}
+
+impl MemoryConfig {
+    /// A shorthand constructor.
+    pub fn new(heap_size: Option<u32>, stack_size: Option<u32>) -> Self {
+        Self {
+            heap_size,
+            stack_size,
+        }
+    }
 }
 
 impl Default for MemoryConfig {
     fn default() -> Self {
         Self {
-            heap_size: 64 * 1024,
-            stack_size: 32 * 1024,
+            heap_size: Some(DEFAULT_HEAP_SIZE),
+            stack_size: Some(DEFAULT_STACK_SIZE),
         }
     }
 }

crates/solc-json-interface/src/standard_json/input/settings/polkavm/mod.rs

@@ -11,16 +11,18 @@ pub mod memory;
 #[derive(Clone, Copy, Default, Debug, Serialize, Deserialize)]
 pub struct PolkaVM {
     /// The PolkaVM target machine memory configuration settings.
-    pub memory_config: MemoryConfig,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub memory_config: Option<MemoryConfig>,
     /// Instruct LLVM to emit debug information.
-    pub debug_information: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub debug_information: Option<bool>,
 }
 
 impl PolkaVM {
     pub fn new(memory_config: Option<MemoryConfig>, debug_information: bool) -> Self {
         Self {
-            memory_config: memory_config.unwrap_or_default(),
-            debug_information,
+            memory_config,
+            debug_information: Some(debug_information),
         }
     }
 }