Remove useless_const optimization

There could be jump depending on the Pop being there, and then it
breaks.

src/compile.rs

@@ -848,7 +848,9 @@ impl<O: OutputStream> Compiler<O> {
 
         // Emit None at end:
         match body.last().map(|s| &s.node) {
-            Some(ast::StatementType::Return { .. }) => {}
+            Some(ast::StatementType::Return { .. }) => {
+                // the last instruction is a ReturnValue already, we don't need to emit it
+            }
             _ => {
                 self.emit(Instruction::LoadConst {
                     value: bytecode::Constant::None,

src/peephole.rs

@@ -92,7 +92,7 @@ impl<O: OutputStream> PeepholeOptimizer<O> {
     }
 
     fn optimize(&mut self) {
-        apply_optimizations!(self, operator, unpack, useless_const);
+        apply_optimizations!(self, operator, unpack);
     }
 }
 
@@ -123,8 +123,6 @@ impl<O: OutputStream> OptimizationBuffer for PeepholeOptimizer<O> {
     }
 }
 
-// OPTIMIZATION
-
 pub trait OptimizationBuffer {
     fn emit(&mut self, instruction: Instruction, meta: InstructionMetadata);
     fn pop(&mut self) -> (Instruction, InstructionMetadata);

src/peephole/optimizations.rs

@@ -2,6 +2,11 @@ use rustpython_bytecode::bytecode::{self, Instruction};
 
 use super::{InstructionMetadata, OptimizationBuffer};
 
+macro_rules! metas {
+    [$($metas:expr),*$(,)?] => {
+        InstructionMetadata::from(vec![$($metas),*])
+    };
+}
 macro_rules! lc {
     ($name:ident {$($field:tt)*}) => {
         Instruction::LoadConst {
@@ -13,10 +18,10 @@ macro_rules! lc {
     };
 }
 macro_rules! emitconst {
-    ($buf:expr, [$($metas:expr),*], $($arg:tt)*) => {
+    ($buf:expr, [$($metas:expr),*$(,)?], $($arg:tt)*) => {
         $buf.emit(
             lc!($($arg)*),
-            InstructionMetadata::from(vec![$($metas),*]),
+            metas![$($metas),*],
         )
     };
 }
@@ -91,20 +96,3 @@ pub fn unpack(buf: &mut impl OptimizationBuffer) {
         buf.emit(instruction, meta)
     }
 }
-
-pub fn useless_const(buf: &mut impl OptimizationBuffer) {
-    let (instruction, meta) = buf.pop();
-    if instruction == Instruction::Pop {
-        let (arg, arg_meta) = buf.pop();
-        if let Instruction::LoadConst { .. } = arg {
-            // just ignore it all
-            drop(arg);
-            drop(instruction);
-        } else {
-            buf.emit(arg, arg_meta);
-            buf.emit(instruction, meta);
-        }
-    } else {
-        buf.emit(instruction, meta);
-    }
-}