Podcast Summary: Software Engineering Daily – Carbon and Modernizing C++ with Chandler Carruth

Episode Information

• Title: Software Engineering Daily
• Host: Kevin Ball
• Guest: Chandler Carruth (Creator of Carbon, Leader of C, C++ Lang, and LLVM Teams at Google)
• Release Date: August 14, 2025
• Description: Technical discussions on modern software engineering topics, focusing on Carbon programming language and its implications for modernizing C++.

Introduction

In this episode of Software Engineering Daily, host Kevin Ball engages in an in-depth conversation with Chandler Carruth, the creator of the Carbon programming language. The discussion centers around Carbon's objectives, its interoperability with C, unique compiler architecture, language features, migration paths from C, governance structure, and the future roadmap of the language.

Chandler Carruth’s Background and the Genesis of Carbon

Chandler Carruth, a seasoned software engineer at Google, shares his journey leading to the creation of Carbon. His initial frustrations with C’s limitations, particularly in handling large codebases and slow compilation times, prompted him to explore deeper into compiler technologies and ultimately spearhead the development of Carbon.

[01:34] Chandler Carruth:
"Working on the C code base was painful. The tools were bad, the experience was bad, and it became an overwhelming problem that we needed to solve... And that's where we started working on Carbon."

The Necessity for Carbon: Addressing C’s Shortcomings

Carbon was conceived as a successor to C, aiming to integrate modern safety features without sacrificing performance. Chandler emphasizes the need to modernize C to maintain and enhance Google's extensive C codebase and the broader industry reliance on C and its derivatives.

[01:34] Chandler Carruth:
"We need something that we could bring all of that code along with us, but get the kind of radical transformative change we're hoping for here."

Interoperability with C: A Unique Approach

Kevin Ball probes into why Carbon opts for deeper integration with C rather than leveraging existing modern languages like Go, Swift, Kotlin, or Rust through message passing or APIs. Chandler explains Carbon’s unique approach to interoperability, which avoids the limitations of traditional abstraction boundaries.

[04:33] Chandler Carruth:
"These abstraction boundaries tend to need to be pretty robust... What about all the code we need to write that doesn't have that abstraction boundary... that's the code that's getting left behind."

He draws parallels with TypeScript’s superset model but highlights the constraints C imposes, which Carbon aims to overcome by maintaining low-level control and ensuring seamless interoperability without performance penalties.

Carbon's Compiler Architecture: Semantic IR and Beyond

A significant portion of the discussion delves into Carbon's novel compiler architecture. Unlike traditional compilers that rely on Abstract Syntax Trees (ASTs), Carbon employs a Semantic Intermediate Representation (IR) that retains detailed semantic information, facilitating advanced features like compile-time evaluation and metaprogramming.

[08:05] Chandler Carruth:
"From that very syntactical and structural parse tree, we generate what we call a semantic IR... It's something you could evaluate or lower into LLVM's IR."

This architecture not only enhances compile-time performance but also simplifies the lowering process to LLVM IR, thereby streamlining integration with C through Clang.

Performance Implications: Efficient Interoperability

The conversation shifts to performance considerations when crossing interop boundaries between C and Carbon. Chandler elaborates on Carbon’s design to maintain a zero-copy memory model, ensuring that data passed between languages remains efficient without incurring runtime overheads.

[14:55] Chandler Carruth:
"We have basically the same memory model for all of the interop. We have to be able to realize kind of the identical ABI and memory layout on each side."

This meticulous handling ensures that interoperability does not compromise performance, a critical factor for high-performance applications reliant on C.

Language Features: Fast Compile Times, Generics, and Memory Safety

Chandler outlines the primary features Carbon introduces to modernize C:

1. Fast Compile Times: Carbon aims to drastically reduce compile times from minutes to milliseconds, enhancing developer productivity.

   [21:00] Chandler Carruth:
   "We shouldn't be measuring compile times in minutes or in seconds. We need to get down to that."

2. Generics: Unlike C’s cumbersome template system, Carbon introduces a definition-checked generics system that improves error messaging, reduces code bloat, and enhances type safety.

   [22:15] Chandler Carruth:
   "The very first thing, and probably the biggest single thing that we're adding in Carbon is this idea of definition-checked generics."

3. Memory Safety: Building upon Rust’s advancements, Carbon plans to incorporate memory safety without sacrificing performance, leveraging parametric type systems to enforce lifetimes and ownership.

   [22:45] Chandler Carruth:
   "Rust has really taught us what we needed... the information that kind of enforces correctness, enforces safety of lifetimes and ownership."

Migration Path: Transitioning from C to Carbon

The discussion covers the practical aspects of migrating existing C codebases to Carbon. Chandler emphasizes an incremental and automated approach, allowing developers to translate C code to Carbon with minimal overhead and gradually adopt generics and memory safety features.

[27:31] Chandler Carruth:
"We want that to be kind of click button in the IDE, like as automated as conceivably possible."

This approach ensures that large, complex C projects can transition to Carbon without massive rewrites, facilitating broader adoption.

Governance Structure: Open and Sustainable

Chandler details Carbon’s governance model, designed to balance rapid development with community involvement. The structure comprises three leads to avoid bottlenecks and a unique "painter" role to resolve aesthetic decisions without hindering progress.

[41:56] Chandler Carruth:
"All the decisions we make are public... and then we can kind of consider that statement as leads and maybe make a decision that reflects it."

This transparent and flexible governance aims to foster community trust and ensure sustainable project growth.

Current Status and Roadmap

Chandler candidly discusses the current state of Carbon, acknowledging that while significant progress has been made in toolchain development, full interoperability with C remains a work in progress. The upcoming year focuses on advancing interop capabilities and concretizing memory safety features.

[52:41] Chandler Carruth:
"This year our goals are twofold. One, we're going to try and get C Interop from kind of the nascent state... And then the second thing is, we've gotten feedback already that we really need to accelerate, kind of telling a concrete story about how memory safety fits into this picture."

Addressing Community Feedback and Criticisms

The podcast addresses several criticisms from the community, particularly regarding syntax changes and the initial exclusion of memory safety features. Chandler explains the rationale behind syntax alterations, emphasizing compile-time performance benefits and alignment with modern language practices.

[55:15] Chandler Carruth:
"The first part is, I think, the cost... Syntax is relatively cheap now... we keep finding places where having the simplified syntax... actually helps make our compile times faster."

He acknowledges the challenges in balancing backward compatibility with innovation, advocating for iterative improvements based on community feedback.

Conclusion

The episode concludes with Chandler Carruth reiterating Carbon’s mission to modernize C by introducing essential safety and performance features while ensuring seamless interoperability. He underscores the importance of community involvement and transparent governance in achieving Carbon's long-term goals.

[61:58] Chandler Carruth:
"We have to make sure that that works. We still have to make sure we can add memory safety to this... it's a necessary goal for us to get to."

Key Takeaways:

• Carbon is designed to address C’s limitations, offering modern language features without compromising performance.
• Interoperability with C is central, enabling large-scale adoption without extensive rewrites.
• Unique compiler architecture leverages a Semantic IR for enhanced compile-time performance and advanced features.
• Rapid and transparent governance ensures sustainable project growth and community trust.
• Future Focus: Advancing C interop and concretizing memory safety features are top priorities.

For developers and organizations reliant on C who seek modern safety and performance enhancements without abandoning their existing codebases, Carbon presents a promising evolution in programming language design.