Rustc Basics

mohitmishra786/low-level-dev-skills/skills/rust/rustc-basics

by mohitmishra786bdc58472fa9fNo license253 starsListed Oct 9, 2026Updated Oct 9, 2026Repository updated 3 months ago

Rust compiler skill for systems programming. Use when selecting RUSTFLAGS, configuring Cargo profiles, tuning release builds, reading assembly or MIR output, understanding monomorphization, or diagnosing compilation errors. Activates on queries about rustc flags, Cargo.toml profiles, opt-level, LTO, codegen-units, target-cpu, emit asm, MIR, or Rust build performance.

Instructions onlySoftware Development Learning & Education

AI-generated overview

Guides Rust compiler configuration: RUSTFLAGS, Cargo profiles, build tuning, and inspecting MIR or assembly output.

What it does: Provides instructions for invoking the Rust compiler, including choosing build modes, configuring Cargo.toml release and dev profiles, and setting RUSTFLAGS. It covers inspecting assembly, MIR, and LLVM IR output, understanding monomorphization, reducing binary size, and triaging common compilation errors. It also references a companion document on RUSTFLAGS and Cargo profile patterns.
When to use it: Use when selecting RUSTFLAGS, tuning release builds, configuring Cargo profiles, or diagnosing Rust compilation errors. Also relevant when reading assembly or MIR output, investigating monomorphization, or shrinking a Rust binary.
Requirements: No scripts; instructions only. Requires a Rust toolchain (cargo, rustc) for the described commands. Some optional steps reference third-party cargo subcommands such as cargo-show-asm, cargo-llvm-lines, and cargo-bloat, which must be installed separately.

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

bash

# Debug (default) — fast compile, no optimization, debug infocargo build
# Release — optimized, no debug info by defaultcargo build --release
# Check only (fastest, no codegen)cargo check
# Build for specific targetcargo build --release --target aarch64-unknown-linux-gnu

# Debug (default) — fast compile, no optimization, debug infocargo build
# Release — optimized, no debug info by defaultcargo build --release
# Check only (fastest, no codegen)cargo check
# Build for specific targetcargo build --release --target aarch64-unknown-linux-gnu

2. Cargo.toml profile configuration

toml

[profile.release]opt-level = 3          # 0-3, "s" (size), "z" (aggressive size)debug = false          # true = full, 1 = line tables only, 0 = nonelto = "thin"           # false | "thin" | true (fat LTO)codegen-units = 1      # 1 = max optimization, higher = faster compilepanic = "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 symbolsstrip = "none"
[profile.dev]opt-level = 1          # Speed up debug builds slightly

[profile.release]opt-level = 3          # 0-3, "s" (size), "z" (aggressive size)debug = false          # true = full, 1 = line tables only, 0 = nonelto = "thin"           # false | "thin" | true (fat LTO)codegen-units = 1      # 1 = max optimization, higher = faster compilepanic = "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 symbolsstrip = "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

bash

# Set for a single buildRUSTFLAGS="-C target-cpu=native" cargo build --release
# Enable all target CPU featuresRUSTFLAGS="-C target-cpu=native -C target-feature=+avx2,+bmi2" cargo build --release
# Control codegen at invocation levelRUSTFLAGS="-C opt-level=3 -C codegen-units=1 -C lto=on" cargo build --release

# Set for a single buildRUSTFLAGS="-C target-cpu=native" cargo build --release
# Enable all target CPU featuresRUSTFLAGS="-C target-cpu=native -C target-feature=+avx2,+bmi2" cargo build --release
# Control codegen at invocation levelRUSTFLAGS="-C opt-level=3 -C codegen-units=1 -C lto=on" cargo build --release

Persistent in .cargo/config.toml:

toml

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

[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

bash

# Using cargo-show-asm (recommended)cargo install cargo-show-asmcargo asm --release 'myapp::module::function_name'
# Using rustc directlyrustc --emit=asm -C opt-level=3 -C target-cpu=native src/lib.rscat lib.s
# View MIR (mid-level IR, before codegen)rustc --emit=mir -C opt-level=3 src/lib.rscat lib.mir
# View LLVM IRrustc --emit=llvm-ir -C opt-level=3 src/lib.rscat lib.ll
# Use Compiler Explorer (Godbolt) patterns locallyRUSTFLAGS="--emit=asm" cargo build --releasefind target/ -name "*.s"

# Using cargo-show-asm (recommended)cargo install cargo-show-asmcargo asm --release 'myapp::module::function_name'
# Using rustc directlyrustc --emit=asm -C opt-level=3 -C target-cpu=native src/lib.rscat lib.s
# View MIR (mid-level IR, before codegen)rustc --emit=mir -C opt-level=3 src/lib.rscat lib.mir
# View LLVM IRrustc --emit=llvm-ir -C opt-level=3 src/lib.rscat lib.ll
# Use Compiler Explorer (Godbolt) patterns locallyRUSTFLAGS="--emit=asm" cargo build --releasefind 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

bash

# Measure monomorphization bloatcargo install cargo-llvm-linescargo llvm-lines --release | head -30
# Shows: lines of LLVM IR per function (monomorphized copies visible)

# Measure monomorphization bloatcargo install cargo-llvm-linescargo llvm-lines --release | head -30
# Shows: lines of LLVM IR per function (monomorphized copies visible)

Mitigation strategies:

rust

// 1. Type erasure with dyn Trait (trades monomorphization for dispatch)fn process(iter: &mut dyn Iterator<Item = i32>) { ... }
// 2. Non-generic inner function patternfn my_generic<T: AsRef<str>>(s: T) {    fn inner(s: &str) { /* actual work */ }    inner(s.as_ref())  // monomorphization only in thin wrapper}

// 1. Type erasure with dyn Trait (trades monomorphization for dispatch)fn process(iter: &mut dyn Iterator<Item = i32>) { ... }
// 2. Non-generic inner function patternfn 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

bash

# Check binary size breakdowncargo install cargo-bloatcargo bloat --release --crates      # per-crate sizecargo bloat --release -n 20         # top 20 largest functions
# Compress executable (at cost of startup time)upx --best --lzma target/release/myapp

# Check binary size breakdowncargo install cargo-bloatcargo bloat --release --crates      # per-crate sizecargo 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

bash

# Show all enabled features at a given opt levelrustc -C opt-level=3 --print cfg
# List available targetsrustc --print target-list
# Show target-specific featuresrustc --print target-features --target x86_64-unknown-linux-gnu
# Explain an error coderustc --explain E0382

# Show all enabled features at a given opt levelrustc -C opt-level=3 --print cfg
# List available targetsrustc --print target-list
# Show target-specific featuresrustc --print target-features --target x86_64-unknown-linux-gnu
# Explain an error coderustc --explain E0382

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

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

Source and attribution

Source:mohitmishra786/low-level-dev-skillsinskills/rust/rustc-basicsat commitbdc5847

License: No license

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Rustc Basics

rustc Basics

Purpose

Triggers

Workflow

1. Choose a build mode

2. Cargo.toml profile configuration

3. RUSTFLAGS

4. Inspect assembly output

5. Understand monomorphization

6. Binary size reduction

7. Common error triage

8. Useful rustc flags

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