The Tool Desk
Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Rust does not send generic Rust code to LLVM and ask LLVM to specialize it. Instead, rustc determines which concrete generic instances the program needs, then lowers those instances into code-generation IR. With the LLVM backend, that IR is LLVM IR; LLVM optimizes it and emits object code.
Where monomorphization fits in Rust compilation
The compiler’s stages are often shown as a simple sequence, but rustc’s real query and correctness flow is more involved than a single linear pipeline. As a high-level model, Rust source is processed into compiler representations, including MIR (Mid-level Intermediate Representation). MIR supports analyses such as borrow checking and is also used for optimization and code generation. See the Rust Compiler Development Guide’s compiler overview.
- Build and analyze MIR. rustc derives MIR from higher-level compiler representations and performs relevant analyses and optimizations.
- Collect needed code-generation items. Before MIR lowering for code generation, rustc identifies the concrete items for which code is required and organizes them into codegen units.
- Lower concrete instances. As code generation proceeds, rustc substitutes concrete generic arguments and translates the resulting MIR into codegen IR.
- Run the backend and link. With the LLVM backend, LLVM optimizes LLVM IR and emits object code. The linker combines object files and any applicable metadata into the requested output.
This ordering matters: collection and actual instantiation are related, but they are not one isolated pass that monomorphizes every possible generic substitution before LLVM starts. The monomorphization guide distinguishes collecting the required items from translating them into concrete code.
How rustc decides which generic instances are needed
Monomorphization means producing concrete code for a generic item with particular type arguments. A generic function does not automatically require machine code for every type it could accept. rustc collects the instances required by the program’s reachable code-generation work.
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For example, the guide describes main calling banana, which in turn calls peach::<u64>. The collector lists main, banana, and the concrete instance peach::<u64> for code generation. The relevant result is the set of needed items, not an exhaustive expansion of every generic function for every imaginable type.
After collection, rustc partitions code-generation items into codegen units. These units organize code generation and can support parallel work; they are not another name for monomorphization. The guide discusses the partitioning process and its relationship to incremental builds in its section on monomorphization and codegen units.
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Generic MIR, mono items, and LLVM IR are different stages
| Stage | What it represents | What happens next |
|---|---|---|
| Generic MIR | MIR before concrete generic substitutions are translated into separate code instances. | rustc performs applicable MIR analysis and optimization, then collects the code-generation items needed. |
| Collected mono items | The concrete functions and other code-generation items that rustc determines are needed, including generic items with specific type arguments. | rustc lowers these instances for code generation and assigns work to codegen units. |
| LLVM IR (LLVM backend) | The lower-level representation produced when rustc lowers instances for its LLVM backend. | LLVM applies backend optimizations and emits object code; linking follows. |
The stages help separate responsibilities: rustc understands Rust’s generic instances and prepares their concrete code; LLVM works on LLVM IR rather than performing Rust’s generic-instance collection. The code generation guide describes codegen units, LLVM processing, object files, and linking.
Why MIR optimization happens before concrete lowering
Generic MIR is not yet monomorphized when rustc performs MIR optimizations. This allows applicable transformations to simplify the generic representation before rustc translates it into concrete instances. If an optimization reduces work in generic MIR, that can reduce work across the resulting monomorphizations. It does not mean every optimization applies identically or produces the same result for every concrete type.
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The compiler guide treats MIR optimization and monomorphization as distinct concerns: optimization acts on MIR, while monomorphization supplies concrete type arguments during code generation. More detail is available in the guide’s MIR optimizations section.
What LLVM does—and what it does not do
For the usual LLVM configuration, rustc lowers concrete instances to LLVM IR. LLVM then performs its own optimizations and generates object code. The codegen units are modules LLVM can process, potentially in parallel, before object files reach the linker.
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LLVM is a backend choice, not the definition of Rust monomorphization. rustc also documents Cranelift and GCC backends. The Rust-specific work of determining and lowering required generic instances occurs before the selected backend processes its input. In LLVM builds, some optimization may also happen at link time under particular link-time optimization (LTO) configurations, so it is not accurate to say that all optimization always finishes before linking. See the guide’s backend and code generation overview.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why monomorphization has a performance trade-off
Concrete specialization can help produce fast programs, but generating many concrete copies can increase compile time and binary size. The trade-off is qualitative here: the compiler guide identifies these costs, but does not establish a generally applicable numeric figure for their size. The impact depends on the program and its required instances.
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