From 1fd242ed2cf5ecd6acb85eac0d23dbd42f2ce141 Mon Sep 17 00:00:00 2001
From: Stephen Chung <schungx@live.com>
Date: Thu, 30 Dec 2021 12:14:54 +0800
Subject: [PATCH] Flatten nested block scopes.

---
 src/ast/stmt.rs  | 24 +++++++++++++++++++++++-
 src/optimizer.rs | 32 +++++++++++++++++++++++++++++---
 2 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/src/ast/stmt.rs b/src/ast/stmt.rs
index 1754ee56..c90d405e 100644
--- a/src/ast/stmt.rs
+++ b/src/ast/stmt.rs
@@ -1,6 +1,7 @@
 //! Module defining script statements.
 
 use super::{ASTNode, Expr, FnCallExpr, Ident, OptionFlags, AST_OPTION_FLAGS};
+use crate::engine::KEYWORD_EVAL;
 use crate::tokenizer::Token;
 use crate::{calc_fn_hash, Position, StaticVec, INT};
 #[cfg(feature = "no_std")]
@@ -466,12 +467,33 @@ impl Stmt {
             Self::Share(_) => false,
         }
     }
+    /// Does this statement's behavior depend on its containing block?
+    ///
+    /// A statement that depends on its containing block behaves differently when promoted
+    /// to an upper block.
+    ///
+    /// Currently only variable definitions (i.e. `let` and `const`), `import`/`export` statements,
+    /// and `eval` calls (which may in turn call define variables) fall under this category.
+    #[inline]
+    #[must_use]
+    pub fn is_block_dependent(&self) -> bool {
+        match self {
+            Self::Var(_, _, _, _) => true,
+
+            Self::FnCall(x, _) if x.name == KEYWORD_EVAL => true,
+
+            #[cfg(not(feature = "no_module"))]
+            Self::Import(_, _, _) | Self::Export(_, _) => true,
+
+            _ => false,
+        }
+    }
     /// Is this statement _pure_ within the containing block?
     ///
     /// An internally pure statement only has side effects that disappear outside the block.
     ///
     /// Currently only variable definitions (i.e. `let` and `const`) and `import`/`export`
-    /// statements are internally pure.
+    /// statements are internally pure, other than pure expressions.
     #[inline]
     #[must_use]
     pub fn is_internally_pure(&self) -> bool {
diff --git a/src/optimizer.rs b/src/optimizer.rs
index aca3df1c..fb3c7cd4 100644
--- a/src/optimizer.rs
+++ b/src/optimizer.rs
@@ -189,6 +189,32 @@ fn optimize_stmt_block(
         Stmt::is_pure
     };
 
+    // Flatten blocks
+    loop {
+        if let Some(n) = statements.iter().enumerate().find_map(|(i, s)| match s {
+            Stmt::Block(block, _) if !block.iter().any(Stmt::is_block_dependent) => Some(i),
+            _ => None,
+        }) {
+            let (first, second) = statements.split_at_mut(n);
+            let stmt = mem::take(&mut second[0]);
+            let mut stmts = match stmt {
+                Stmt::Block(block, _) => block,
+                _ => unreachable!("Stmt::Block expected but gets {:?}", stmt),
+            };
+            statements = first
+                .iter_mut()
+                .map(mem::take)
+                .chain(stmts.iter_mut().map(mem::take))
+                .chain(second.iter_mut().skip(1).map(mem::take))
+                .collect();
+        } else {
+            break;
+        }
+
+        is_dirty = true;
+    }
+
+    // Optimize
     loop {
         state.clear_dirty();
 
@@ -411,7 +437,7 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut OptimizerState, preserve_result: b
                         x.2 = value;
                     }
                 }
-                _ => unreachable!(),
+                ref expr => unreachable!("Expr::FnCall expected but gets {:?}", expr),
             }
         }
 
@@ -695,8 +721,8 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut OptimizerState, preserve_result: b
                     state.set_dirty();
                     *stmt = Stmt::Noop(*pos);
                 }
-                // Only one statement - promote
-                [s] => {
+                // Only one statement which is not block-dependent - promote
+                [s] if !s.is_block_dependent() => {
                     state.set_dirty();
                     *stmt = mem::take(s);
                 }