feat(es/minifier): Add merge_imports optimization pass to reduce bundle size

.changeset/lazy-weeks-allow.md

@@ -0,0 +1,6 @@
+---
+swc_ecma_minifier: patch
+swc_core: patch
+---
+
+feat(es/minifier): Add merge_imports optimization pass to reduce bundle size

crates/swc/tests/tsc-references/es6modulekindWithES5Target9.2.minified.js

@@ -1,8 +1,7 @@
 //// [es6modulekindWithES5Target9.ts]
-import d from "mod";
-import { a } from "mod";
-import * as M from "mod";
 export * from "mod";
 export { b } from "mod";
 export default d;
+import d, { a } from "mod";
+import * as M from "mod";
 export { a, M, d };

crates/swc/tests/tsc-references/esnextmodulekindWithES5Target9.2.minified.js

@@ -1,8 +1,7 @@
 //// [esnextmodulekindWithES5Target9.ts]
-import d from "mod";
-import { a } from "mod";
-import * as M from "mod";
 export * from "mod";
 export { b } from "mod";
 export default d;
+import d, { a } from "mod";
+import * as M from "mod";
 export { a, M, d };

crates/swc/tests/tsc-references/exportNamespace12.2.minified.js

@@ -1,7 +1,7 @@
 //// [main.ts]
-import { c } from './types';
-import * as types from './types';
 console.log(c), console.log(types.c);
+import * as types from "./types";
+import { c } from "./types";
 //// [types.ts]
 export { };
 //// [values.ts]

crates/swc_ecma_minifier/src/option/mod.rs

@@ -282,6 +282,10 @@ pub struct CompressOptions {
     #[cfg_attr(feature = "extra-serde", serde(alias = "loops"))]
     pub loops: bool,
 
+    #[cfg_attr(feature = "extra-serde", serde(default = "true_by_default"))]
+    #[cfg_attr(feature = "extra-serde", serde(alias = "merge_imports"))]
+    pub merge_imports: bool,
+
     #[cfg_attr(feature = "extra-serde", serde(default))]
     pub module: bool,
 
@@ -451,6 +455,7 @@ impl Default for CompressOptions {
             keep_fnames: false,
             keep_infinity: false,
             loops: true,
+            merge_imports: true,
             module: false,
             negate_iife: true,
             passes: default_passes(),

crates/swc_ecma_minifier/src/option/terser.rs

@@ -343,6 +343,7 @@ impl TerserCompressorOptions {
             keep_fnames: self.keep_fnames,
             keep_infinity: self.keep_infinity,
             loops: self.loops.unwrap_or(self.defaults),
+            merge_imports: self.defaults,
             module: self.module,
             negate_iife: self.negate_iife.unwrap_or(self.defaults),
             passes: self.passes,

crates/swc_ecma_minifier/src/pass/postcompress.rs

@@ -1,4 +1,5 @@
-use swc_common::util::take::Take;
+use rustc_hash::FxHashMap;
+use swc_common::{util::take::Take, DUMMY_SP};
 use swc_ecma_ast::*;
 
 use crate::option::CompressOptions;
@@ -53,4 +54,294 @@ pub fn postcompress_optimizer(program: &mut Program, options: &CompressOptions)
             }
         }
     }
+
+    // Merge duplicate imports if enabled
+    if options.merge_imports {
+        merge_imports_in_module(module);
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+struct ImportKey {
+    src: String,
+    type_only: bool,
+    phase: ImportPhase,
+    /// Hash of the `with` clause to group imports with the same assertions
+    with_hash: Option<u64>,
+}
+
+impl ImportKey {
+    fn from_import_decl(decl: &ImportDecl) -> Self {
+        use std::{
+            collections::hash_map::DefaultHasher,
+            hash::{Hash, Hasher},
+        };
+
+        let with_hash = decl.with.as_ref().map(|w| {
+            let mut hasher = DefaultHasher::new();
+            // Hash the with clause structure
+            format!("{w:?}").hash(&mut hasher);
+            hasher.finish()
+        });
+
+        Self {
+            src: decl.src.value.to_string(),
+            type_only: decl.type_only,
+            phase: decl.phase,
+            with_hash,
+        }
+    }
+}
+
+/// Key to identify unique import specifiers.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+enum SpecifierKey {
+    /// Named import: (imported name, local name, is_type_only)
+    Named(String, String, bool),
+    /// Default import: (local name)
+    Default(String),
+    /// Namespace import: (local name)
+    Namespace(String),
+}
+
+impl SpecifierKey {
+    fn from_specifier(spec: &ImportSpecifier) -> Self {
+        match spec {
+            ImportSpecifier::Named(named) => {
+                let imported = named
+                    .imported
+                    .as_ref()
+                    .map(|n| match n {
+                        ModuleExportName::Ident(id) => id.sym.to_string(),
+                        ModuleExportName::Str(s) => s.value.to_string(),
+                    })
+                    .unwrap_or_else(|| named.local.sym.to_string());
+
+                SpecifierKey::Named(imported, named.local.sym.to_string(), named.is_type_only)
+            }
+            ImportSpecifier::Default(default) => {
+                SpecifierKey::Default(default.local.sym.to_string())
+            }
+            ImportSpecifier::Namespace(ns) => SpecifierKey::Namespace(ns.local.sym.to_string()),
+        }
+    }
+}
+
+/// Merge duplicate import statements from the same module source.
+///
+/// This optimization reduces bundle size by combining multiple imports from
+/// the same source into a single import declaration.
+fn merge_imports_in_module(module: &mut Module) {
+    // Group imports by source and metadata
+    let mut import_groups: FxHashMap<ImportKey, Vec<ImportDecl>> = FxHashMap::default();
+
+    for item in module.body.iter() {
+        if let ModuleItem::ModuleDecl(ModuleDecl::Import(import_decl)) = item {
+            // Skip side-effect only imports (no specifiers)
+            if import_decl.specifiers.is_empty() {
+                continue;
+            }
+
+            let key = ImportKey::from_import_decl(import_decl);
+            import_groups
+                .entry(key)
+                .or_default()
+                .push(import_decl.clone());
+        }
+    }
+
+    // Remove all imports that will be merged (except side-effect imports)
+    module.body.retain(|item| {
+        if let ModuleItem::ModuleDecl(ModuleDecl::Import(import_decl)) = item {
+            // Keep side-effect imports
+            if import_decl.specifiers.is_empty() {
+                return true;
+            }
+
+            let key = ImportKey::from_import_decl(import_decl);
+            // Only keep if there's just one import for this key (no merging needed)
+            import_groups.get(&key).map_or(true, |v| v.len() <= 1)
+        } else {
+            true
+        }
+    });
+
+    // Create merged imports and add them back
+    for (key, import_decls) in import_groups.iter() {
+        if import_decls.len() <= 1 {
+            // No merging needed, already retained above
+            continue;
+        }
+
+        let merged_imports = merge_import_decls(import_decls, key);
+        for merged in merged_imports {
+            module
+                .body
+                .push(ModuleItem::ModuleDecl(ModuleDecl::Import(merged)));
+        }
+    }
+}
+
+/// Merge multiple ImportDecl nodes.
+/// Returns a Vec because in some cases (namespace + named), we need to create
+/// multiple import statements since they cannot be combined in valid ES syntax.
+fn merge_import_decls(decls: &[ImportDecl], key: &ImportKey) -> Vec<ImportDecl> {
+    let mut default_spec: Option<ImportSpecifier> = None;
+    let mut namespace_spec: Option<ImportSpecifier> = None;
+    let mut named_specs: Vec<ImportSpecifier> = Vec::new();
+    let mut seen_named: FxHashMap<SpecifierKey, ()> = FxHashMap::default();
+
+    let first_decl = &decls[0];
+    let span = first_decl.span;
+
+    // Separate specifiers by type
+    for decl in decls {
+        for spec in &decl.specifiers {
+            match spec {
+                ImportSpecifier::Default(_) => {
+                    if default_spec.is_none() {
+                        default_spec = Some(spec.clone());
+                    }
+                }
+                ImportSpecifier::Namespace(_) => {
+                    if namespace_spec.is_none() {
+                        namespace_spec = Some(spec.clone());
+                    }
+                }
+                ImportSpecifier::Named(_) => {
+                    let spec_key = SpecifierKey::from_specifier(spec);
+                    if let std::collections::hash_map::Entry::Vacant(e) = seen_named.entry(spec_key)
+                    {
+                        e.insert(());
+                        named_specs.push(spec.clone());
+                    }
+                }
+            }
+        }
+    }
+
+    let mut result = Vec::new();
+
+    // Valid combinations in ES modules:
+    // - default only
+    // - namespace only
+    // - named only
+    // - default + named
+    // - default + namespace (ONLY these two, no named allowed)
+    // Note: namespace + named (without default) is NOT valid - must split
+    // Note: default + namespace + named is NOT valid - must split
+
+    if let Some(namespace) = namespace_spec {
+        if default_spec.is_some() {
+            if named_specs.is_empty() {
+                // default + namespace only (valid combination)
+                result.push(ImportDecl {
+                    span,
+                    specifiers: vec![default_spec.unwrap(), namespace],
+                    src: Box::new(Str {
+                        span: DUMMY_SP,
+                        value: key.src.clone().into(),
+                        raw: None,
+                    }),
+                    type_only: key.type_only,
+                    with: first_decl.with.clone(),
+                    phase: key.phase,
+                });
+            } else {
+                // default + namespace + named - MUST SPLIT
+                // Create one import for default + named
+                let mut specs = vec![default_spec.unwrap()];
+                specs.extend(named_specs);
+                result.push(ImportDecl {
+                    span,
+                    specifiers: specs,
+                    src: Box::new(Str {
+                        span: DUMMY_SP,
+                        value: key.src.clone().into(),
+                        raw: None,
+                    }),
+                    type_only: key.type_only,
+                    with: first_decl.with.clone(),
+                    phase: key.phase,
+                });
+                // Create one import for namespace
+                result.push(ImportDecl {
+                    span,
+                    specifiers: vec![namespace],
+                    src: Box::new(Str {
+                        span: DUMMY_SP,
+                        value: key.src.clone().into(),
+                        raw: None,
+                    }),
+                    type_only: key.type_only,
+                    with: first_decl.with.clone(),
+                    phase: key.phase,
+                });
+            }
+        } else if named_specs.is_empty() {
+            // Just namespace
+            result.push(ImportDecl {
+                span,
+                specifiers: vec![namespace],
+                src: Box::new(Str {
+                    span: DUMMY_SP,
+                    value: key.src.clone().into(),
+                    raw: None,
+                }),
+                type_only: key.type_only,
+                with: first_decl.with.clone(),
+                phase: key.phase,
+            });
+        } else {
+            // namespace + named without default - MUST SPLIT
+            // Create one import for namespace
+            result.push(ImportDecl {
+                span,
+                specifiers: vec![namespace],
+                src: Box::new(Str {
+                    span: DUMMY_SP,
+                    value: key.src.clone().into(),
+                    raw: None,
+                }),
+                type_only: key.type_only,
+                with: first_decl.with.clone(),
+                phase: key.phase,
+            });
+            // Create one import for named
+            result.push(ImportDecl {
+                span,
+                specifiers: named_specs,
+                src: Box::new(Str {
+                    span: DUMMY_SP,
+                    value: key.src.clone().into(),
+                    raw: None,
+                }),
+                type_only: key.type_only,
+                with: first_decl.with.clone(),
+                phase: key.phase,
+            });
+        }
+    } else {
+        // No namespace - merge default and/or named
+        let mut specs = Vec::new();
+        if let Some(default) = default_spec {
+            specs.push(default);
+        }
+        specs.extend(named_specs);
+
+        result.push(ImportDecl {
+            span,
+            specifiers: specs,
+            src: Box::new(Str {
+                span: DUMMY_SP,
+                value: key.src.clone().into(),
+                raw: None,
+            }),
+            type_only: key.type_only,
+            with: first_decl.with.clone(),
+            phase: key.phase,
+        });
+    }
+
+    result
 }