rustc Basics

Purpose

Guide agents through Rust compiler invocation: RUSTFLAGS, Cargo profile configuration, build modes, MIR and assembly inspection, monomorphization, and common compilation error patterns.

Triggers

• "How do I configure a release build in Rust for maximum performance?"
• "How do I see the assembly output for a Rust function?"
• "What is monomorphization and why is it making my compile slow?"
• "How do I enable LTO in Rust?"
• "My Rust binary is too large — how do I shrink it?"
• "How do I read Rust MIR output?"

Workflow

1. Choose a build mode

# Debug (default) — fast compile, no optimization, debug info
cargo build

# Release — optimized, no debug info by default
cargo build --release

# Check only (fastest, no codegen)
cargo check

# Build for specific target
cargo build --release --target aarch64-unknown-linux-gnu

2. Cargo.toml profile configuration

[profile.release]
opt-level = 3          # 0-3, "s" (size), "z" (aggressive size)
debug = false          # true = full, 1 = line tables only, 0 = none
lto = "thin"           # false | "thin" | true (fat LTO)
codegen-units = 1      # 1 = max optimization, higher = faster compile
panic = "abort"        # "unwind" (default) | "abort" (smaller binary)
strip = "symbols"      # "none" | "debuginfo" | "symbols"
overflow-checks = false # default true in debug, false in release

[profile.release-with-debug]
inherits = "release"
debug = true           # release build with debug symbols
strip = "none"

[profile.dev]
opt-level = 1          # Speed up debug builds slightly

Setting	Impact
lto = true (fat)	Best optimization, slowest link
lto = "thin"	Good optimization, parallel link
codegen-units = 1	Best inlining, slower compile
panic = "abort"	Removes unwind tables, smaller binary
opt-level = "z"	Aggressive size reduction

3. RUSTFLAGS

# Set for a single build
RUSTFLAGS="-C target-cpu=native" cargo build --release

# Enable all target CPU features
RUSTFLAGS="-C target-cpu=native -C target-feature=+avx2,+bmi2" cargo build --release

# Control codegen at invocation level
RUSTFLAGS="-C opt-level=3 -C codegen-units=1 -C lto=on" cargo build --release

Persistent in .cargo/config.toml:

[build]
rustflags = ["-C", "target-cpu=native"]

[target.x86_64-unknown-linux-gnu]
rustflags = ["-C", "target-cpu=native", "-C", "link-arg=-fuse-ld=lld"]

4. Inspect assembly output

# Using cargo-show-asm (recommended)
cargo install cargo-show-asm
cargo asm --release 'myapp::module::function_name'

# Using rustc directly
rustc --emit=asm -C opt-level=3 -C target-cpu=native src/lib.rs
cat lib.s

# View MIR (mid-level IR, before codegen)
rustc --emit=mir -C opt-level=3 src/lib.rs
cat lib.mir

# View LLVM IR
rustc --emit=llvm-ir -C opt-level=3 src/lib.rs
cat lib.ll

# Use Compiler Explorer (Godbolt) patterns locally
RUSTFLAGS="--emit=asm" cargo build --release
find target/ -name "*.s"

5. Understand monomorphization

Rust generics are monomorphized — each concrete type instantiation produces separate code. This causes:

• Binary size bloat
• Longer compile times
• Potential i-cache pressure

# Measure monomorphization bloat
cargo install cargo-llvm-lines
cargo llvm-lines --release | head -30

# Shows: lines of LLVM IR per function (monomorphized copies visible)

Mitigation strategies:

// 1. Type erasure with dyn Trait (trades monomorphization for dispatch)
fn process(iter: &mut dyn Iterator<Item = i32>) { ... }

// 2. Non-generic inner function pattern
fn my_generic<T: AsRef<str>>(s: T) {
    fn inner(s: &str) { /* actual work */ }
    inner(s.as_ref())  // monomorphization only in thin wrapper
}

6. Binary size reduction

[profile.release]
opt-level = "z"
lto = true
codegen-units = 1
panic = "abort"
strip = "symbols"

# Check binary size breakdown
cargo install cargo-bloat
cargo bloat --release --crates      # per-crate size
cargo bloat --release -n 20         # top 20 largest functions

# Compress executable (at cost of startup time)
upx --best --lzma target/release/myapp

7. Common error triage

Error	Cause	Fix
cannot find function in this scope	Missing use or wrong module path	Add use crate::module::fn_name
the trait X is not implemented for Y	Missing impl or wrong generic bound	Implement trait or adjust bounds
lifetime may not live long enough	Borrow checker lifetime issue	Add explicit lifetime annotations
cannot borrow as mutable because also borrowed as immutable	Aliasing violation	Restructure borrows to not overlap
use of moved value	Value used after move into closure or function	Use .clone() or borrow instead
mismatched types: expected &str found String	String vs &str confusion	Use .as_str() or &my_string

8. Useful rustc flags

# Show all enabled features at a given opt level
rustc -C opt-level=3 --print cfg

# List available targets
rustc --print target-list

# Show target-specific features
rustc --print target-features --target x86_64-unknown-linux-gnu

# Explain an error code
rustc --explain E0382

For RUSTFLAGS reference and Cargo profile patterns, see references/rustflags-profiles.md.

Related skills

• Use skills/rust/cargo-workflows for workspace management and Cargo tooling
• Use skills/rust/rust-debugging for debugging Rust binaries with GDB/LLDB
• Use skills/rust/rust-profiling for profiling and flamegraphs
• Use skills/rust/rust-sanitizers-miri for memory safety validation