[Rust] updated some function to "const" (#1098)

Co-authored-by: Michael Ward <mward@drw.com>

Author: mward

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/LibRsDef.java

@@ -189,7 +189,7 @@ static void generateReadBuf(final Appendable writer, final ByteOrder byteOrder)
         indent(writer, 0, "#[allow(dead_code)]\n");
         indent(writer, 0, "impl<%s> %s<%s> {\n", BUF_LIFETIME, READ_BUF_TYPE, BUF_LIFETIME);
         indent(writer, 1, "#[inline]\n");
-        indent(writer, 1, "pub fn new(data: &%s [u8]) -> Self {\n", BUF_LIFETIME);
+        indent(writer, 1, "pub const fn new(data: &%s [u8]) -> Self {\n", BUF_LIFETIME);
         indent(writer, 2, "Self { data }\n");
         indent(writer, 1, "}\n\n");
 
@@ -205,7 +205,7 @@ static void generateReadBuf(final Appendable writer, final ByteOrder byteOrder)
         uniquePrimitiveTypes.remove("u8");
         indent(writer, 0, "\n");
         indent(writer, 1, "#[inline]\n");
-        indent(writer, 1, "pub fn get_u8_at(&self, index: usize) -> u8 {\n");
+        indent(writer, 1, "pub const fn get_u8_at(&self, index: usize) -> u8 {\n");
         indent(writer, 2, "self.data[index]\n");
         indent(writer, 1, "}\n");
 
@@ -238,7 +238,7 @@ static void generateWriteBuf(final Writer writer, final ByteOrder byteOrder) thr
         indent(writer, 0, "}\n");
 
         indent(writer, 0, "impl<%s> %s<%s> {\n", BUF_LIFETIME, WRITE_BUF_TYPE, BUF_LIFETIME);
-        indent(writer, 1, "pub fn new(data: &%s mut [u8]) -> Self {\n", BUF_LIFETIME);
+        indent(writer, 1, "pub const fn new(data: &%s mut [u8]) -> Self {\n", BUF_LIFETIME);
         indent(writer, 2, "Self { data }\n");
         indent(writer, 1, "}\n\n");
 
@@ -255,7 +255,7 @@ static void generateWriteBuf(final Writer writer, final ByteOrder byteOrder) thr
 
         uniquePrimitiveTypes.remove("u8");
         indent(writer, 1, "#[inline]\n");
-        indent(writer, 1, "pub fn put_u8_at(&mut self, index: usize, value: u8) {\n");
+        indent(writer, 1, "pub const fn put_u8_at(&mut self, index: usize, value: u8) {\n");
         indent(writer, 2, "self.data[index] = value;\n");
         indent(writer, 1, "}\n\n");
 

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/MessageCoderDef.java

@@ -83,7 +83,7 @@ void generate(
         appendWrapFn(sb);
 
         indent(sb, 2, "#[inline]\n");
-        indent(sb, 2, "pub fn encoded_length(&self) -> usize {\n");
+        indent(sb, 2, "pub const fn encoded_length(&self) -> usize {\n");
         indent(sb, 3, "self.limit - self.offset\n");
         indent(sb, 2, "}\n\n");
 

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/SubGroup.java

@@ -103,7 +103,7 @@ void generateEncoder(
 
         // block_length function
         indent(sb, level, "#[inline]\n");
-        indent(sb, level, "pub fn block_length() -> %s {\n", rustTypeName(blockLengthPrimitiveType));
+        indent(sb, level, "pub const fn block_length() -> %s {\n", rustTypeName(blockLengthPrimitiveType));
         indent(sb, level + 1, "%d\n", this.groupToken.encodedLength());
         indent(sb, level, "}\n\n");