I want rust code compiled when the compilation attribute debug_assertions is false (or not enabled), i.e. a "debug build".
Is this possible? What is the syntax?
For example, I can compile function build_type when compiling for debug build (i.e. option --release is not passed to command cargo build).
#[cfg(debug_assertions)]
pub fn build_type() -> String {
String::from("debug")
}
In this case, I want a "release version" of the function,
#[cfg(debug_assertions)]
pub fn build_type() -> String {
String::from("debug")
}
#[!cfg(debug_assertions)]
pub fn build_type() -> String {
String::from("release")
}
However, the syntax #[!cfg(debug_assertions)] results in cargo build error expected identifier, found '!'.
Other failed syntax variations were:
#[cfg(!debug_assertions)]
#[cfg(debug_assertions = false)]
#[cfg(debug_assertions = "false")]
In book Rust By Example, the section on cfg has this snipped code sample:
// And this function only gets compiled if the target OS is *not* linux
#[cfg(not(target_os = "linux"))]
fn are_you_on_linux() {
println!("You are *not* running linux!");
}
Use syntax #[cfg(not(debug_assertions))] to conditionally compile for cargo build --release.
The book The Rust Reference has the full list of conditional compilation predicates.
You can do this, based on rust book
if cfg!(not(target_os = "linux")) {
// Your code
} else {
// Your else code
}
And to check if it is a debug binary, you can do the same, but with the debug_assertions cfg, like this
if cfg!(debug_assertions) {
// Your code if is debug
}
Related
I want to print the compile-time attributes to the console, ("conditional predicates").
Is there built-in function or macro that does this?
I'm imagining code like:
#[allow(non_snake_case)]
fn main() {
eprintln!(cfg_as_str!());
}
that might print
allow.non_snake_case=true
allow.dead_code=false
...
cfg.debug_attributes=true
cfg.test=false
cfg.bench=false
...
target_arch="x86_64"
...
I want to better understand the state of the rust compiler at different lines of code. However, it's tedious to do so by trial-and-error.
Of course, I could write this on my own. But I'd guess someone else already has.
I don't think you'll get the lint attributes but for the cfg options you could add a build.rs with:
use std::env;
fn main() {
for (key, value) in env::vars() {
if key.starts_with("CARGO") {
eprintln!("{}: {}", key, value);
}
}
}
Then building with cargo build -vv will output cfgs.
See: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts
There are several ways of doing something in my crate, some result in fast execution, some in low binary size, some have other advantages, so I provide the user interfaces to all of them. Unused functions will be optimized away by the compiler. Internal functions in my crate have to use these interfaces as well, and I would like them to respect the user choice at compile time.
There are conditional compilation attributes like target_os, which store a value like linux or windows. How can I create such an attribute, for example prefer_method, so I and the user can use it somewhat like in the following code snippets?
My crate:
#[cfg(not(any(
not(prefer_method),
prefer_method = "fast",
prefer_method = "small"
)))]
compile_error("invalid `prefer_method` value");
pub fn bla() {
#[cfg(prefer_method = "fast")]
foo_fast();
#[cfg(prefer_method = "small")]
foo_small();
#[cfg(not(prefer_method))]
foo_default();
}
pub fn foo_fast() {
// Fast execution.
}
pub fn foo_small() {
// Small binary file.
}
pub fn foo_default() {
// Medium size, medium fast.
}
The user crate:
#[prefer_method = "small"]
extern crate my_crate;
fn f() {
// Uses the `foo_small` function, the other `foo_*` functions will not end up in the binary.
my_crate::bla();
// But the user can also call any function, which of course will also end up in the binary.
my_crate::foo_default();
}
I know there are --cfg attributes, but AFAIK these only represent boolean flags, not enumeration values, which allow setting multiple flags when only one enumeration value is valid.
Firstly, the --cfg flag supports key-value pairs using the syntax --cfg 'prefer_method="fast"'. This will allow you to write code like:
#[cfg(prefer_method = "fast")]
fn foo_fast() { }
You can also set these cfg options from a build script. For example:
// build.rs
fn main() {
println!("cargo:rustc-cfg=prefer_method=\"method_a\"");
}
// src/main.rs
#[cfg(prefer_method = "method_a")]
fn main() {
println!("It's A");
}
#[cfg(prefer_method = "method_b")]
fn main() {
println!("It's B");
}
#[cfg(not(any(prefer_method = "method_a", prefer_method = "method_b")))]
fn main() {
println!("No preferred method");
}
The above code will result in an executable that prints "It's A".
There's no syntax like the one you suggest to specify cfg settings. The best thing to expose these options to your crates' users is through Cargo features.
For example:
# Library Cargo.toml
# ...
[features]
method_a = []
method_b = []
// build.rs
fn main() {
// prefer method A if both method A and B are selected
if cfg!(feature = "method_a") {
println!("cargo:rustc-cfg=prefer_method=\"method_a\"");
} else if cfg!(feature = "method_b") {
println!("cargo:rustc-cfg=prefer_method=\"method_b\"");
}
}
# User Cargo.toml
# ...
[dependencies.my_crate]
version = "..."
features = ["method_a"]
However, in this case, I'd recommend just using the Cargo features directly in your code (i.e. #[cfg(feature = "fast")]) rather than adding the build script since there's a one-to-one correspondence between the cargo feature and the rustc-cfg being added.
I am new to Rust and working on the exercise in Chapter 12.3 in the book.
I am pretty confident that my code is the same as that in the book (hard to tell for sure because of the 'snips'). However I get an unresolved import error when I try cargo build or cargo run from the project directory, minigrep/
src/main.rs
use std::env;
use std::process;
use minigrep;
use minigrep::Config;
fn main() {
let args: Vec<String> = env::args().collect();
let config = Config::new(&args).unwrap_or_else(|err| {
println!("Problem parsing args: {}", err);
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.filename);
if let Err(e) = minigrep::run(config) {
println!("Application error: {}", e);
process::exit(1);
}
}
src/lib.rs
use std::fs;
use std::error::Error;
pub struct Config {
pub query: String,
pub filename: String,
}
impl Config {
pub fn new(args: &[String]) -> Result <Config, &'static str> {
if args.len() < 3 {
return Err("not enough args");
}
let query = args[1].clone();
let filename = args[2].clone();
Ok(Config { query, filename })
}
}
pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
let contents = fs::read_to_string(config.filename)?;
println!("With text:\n {}", contents);
Ok(())
}
There is a contradiction between the code samples for the exercise in chapter 12.3 and the earlier section in chapter 7: '7.2 -- Modules and use to control scope -- Separating modules into different files'.
Using 7.2's syntax complies:
mod lib; //.. instead of 'use minigrep'
use lib::Config; //.. instead of 'use minigrep::Config'
Neither of the above answers is correct. Using cargo clean is the correct solution. The incremental build process that produces that red underline in the IDE is a result of cached information about the program and library.
$ cargo clean
This seems to be a VSCode issue. Simply restarting VSCode makes the error go away, as discussed on this GitHub issue: https://github.com/rust-lang/vscode-rust/issues/686
I had this issue because I accidentally chose the wrong file name for the lib.rs. Make sure the file is named lib.rs without a typo. Other file names do not work here because of the use statement.
When you write use minigrep::Config, the compiler will look for a lib.rs file in the current project minigrep to resolve the imports.
I just ran across the same problem when reviewing The Rust Programming Language. The way I got it to compile was to remove the line use minigrep; from src/main.rs and add extern crate minigrep to the top of the file. The other file can stay as it is. Not sure if this a typo in the book (if it is then it is in multiple versions) or if there is a change to the behavior of the Rust compiler to accommodate the new syntax and we are just using a version that is too old. For comparison my version is 1.31.0.
I want to make it so that when my program starts it prints to stderr:
This is Program X v. 0.1.0 compiled on 20180110. Now listening on stdin,
quit with SIGINT (^C). EOF is ignored. For licensing information, read
LICENSE. To suppress this message, supply --quiet or --suppress-greeting
In C/C++, I would achieve this with a Makefile, e.g.:
VERSION = 0.1.0
FLAGS = -Wall -pipe -O3 -funroll-loops -Wall -DVERSION="\"$(VERSION)\"" -DCOMPILED_AT="\"`date +%Y%m%d`\""
Then, in the source code, I would use those constants as I pleased, perhaps in a call to fprintf. After checking if they actually existed with #ifdef, of course.
How can this be achieved in Rust? Do I need to use a procedural macro? Can I use cargo somehow?
I know that env!("CARGO_PKG_VERSION") can be used as a replacement for VERSION, but what about COMPILED_AT?
There are two ways to do this.
Use a build.rs script
The benefit of using a build.rs script is that other users compiling your program will not have to invoke cargo in a special way or set up their environment. Here is a minimal example of how to do that.
build.rs
use std::process::{Command, exit};
use std::str;
static CARGOENV: &str = "cargo:rustc-env=";
fn main() {
let time_c = Command::new("date").args(&["+%Y%m%d"]).output();
match time_c {
Ok(t) => {
let time;
unsafe {
time = str::from_utf8_unchecked( &t.stdout );
}
println!("{}COMPILED_AT={}", CARGOENV, time);
}
Err(_) => exit(1)
}
}
src/main.rs
fn main() {
println!("This is Example Program {} compiled at {}", env!("CARGO_PKG_VERSION"), env!("COMPILED_AT"));
}
Cargo.toml
[package]
name = "compiled_at"
version = "0.1.0"
authors = ["Fredrick Brennan <copypaste#kittens.ph>"]
build = "build.rs"
[dependencies]
Obviously this can be tweaked to get it to work on other platforms which don't have a /bin/date or to compile in other things such as the Git version number. This script was based on the example provided by Jmb, which shows how to add Mercurial information into your program at compile time.
This guarantees that COMPILED_AT will either be set or the build will fail. This way other Rust programmers can build just by doing cargo build.
[osboxes#osboxes compiled_at]$ cargo run
Compiling compiled_at v0.1.0 (file:///home/osboxes/Workspace/rust/compiled_at)
Finished dev [unoptimized + debuginfo] target(s) in 1.27 secs
Running `target/debug/compiled_at`
This is Example Program 0.1.0 compiled at 20180202
Use env!(), then require users to set their environment prior to building
It occurred to me after I asked this question to try the following, and it does work (vim is in use, thus the escaped %s), cargo does pass my environment down to rustc:
COMPILED_AT=`date +\%Y\%m\%d` cargo run
Then, in Rust:
fn main() {
eprintln!("{}", env!("COMPILED_AT"));
}
The Rust compiler refuses to compile the code if I don't supply the environment variable, which is a nice touch.
error: environment variable `COMPILED_AT` not defined
--> src/main.rs:147:21
|
147 | eprintln!("{}", env!("COMPILED_AT"));
| ^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
This way is extremely hacky and is guaranteed to annoy other Rust programmers who just expect to build with cargo build, but it does work. If you can, it is recommended to use the build.rs instead.
You can use the build.rs script to add an environment variable to the cargo environment or to create an extra source file that contains the information you need. That way you can add a lot of information about the build environment. Here is a full example that creates a build_info.rs source file containing:
Version information built from the Mercurial tag, revision hash and status (including the date if the source folder is different from the Mercurial folder).
Toolchain information, including the compiler version and the build profile (eg. debug or release) thanks to the rustc_version crate.
Plus it extracts some information from cargo (like the package name) so it doesn't have to be duplicated between the Cargo.toml and the source code).
#[macro_use] extern crate map_for;
extern crate rustc_version;
extern crate time;
use rustc_version::{ Channel, version_meta };
use std::collections::HashMap;
use std::env;
use std::ffi::OsStr;
use std::fs::File;
use std::io::{BufRead, BufReader, Write};
use std::process::Command;
/// Run mercurial with the given arguments and return the output.
fn run_hg<S: AsRef<OsStr>> (args: &[S]) -> Option<String> {
Command::new ("hg")
.env ("HGRCPATH", "")
.env ("LANG", "C")
.args (args)
.output()
.ok()
.and_then (|output|
String::from_utf8 (output.stdout)
.ok())
}
/// Get the version from a mercurial repository.
///
/// Version numbers follow the Python PEP440 conventions. If the
/// current folder corresponds to a version tag, then return that tag.
/// Otherwise, identify the closest tag and return a string of the
/// form _tag_.dev_N_+_hash_. In both cases, if the current folder has
/// been modified, then add the current date as `YYYYMMDD` to the
/// local version label.
fn get_mercurial_version_tag() -> Option<String> {
let output = run_hg (&[ "id", "-i", "-t" ]);
let mut iter = output.iter().flat_map (|s| s.split_whitespace()).fuse();
let hash = match iter.next() {
Some (hash) => hash,
_ => { return None },
};
let clean = !hash.ends_with ("+");
fn mkdate() -> String { time::strftime ("%Y%m%d", &time::now()).unwrap() }
map_for!(
version <- iter.find (|s| s.chars().next()
.map (|c| ('0' <= c) && (c <= '9'))
.unwrap_or (false));
// The current folder corresponds to a version tag (i.e. a
// tag that starts with a digit).
=> (if clean { version.into() }
else { format!("{}+{}", version, mkdate()) }))
.or_else (|| {
// The current folder does not correspond to a version tag.
// Find the closest tag and build the version from that. Note
// that this may return a wrong version number if the closest
// tag is not a version tag.
let version = run_hg (
&[ "parents",
"--template",
"{latesttag}.dev{latesttagdistance}+{node|short}" ]);
if clean { version }
else { version.map (|s| format!("{}.{}", s, mkdate())) }
})
}
/// Get the version from Mercurial archive information.
///
/// The Mercurial `archive` command creates a file named
/// `.hg_archival.txt` that contains information about the archived
/// version. This function tries to use this information to create a
/// version string similar to what `get_mercurial_version_tag` would
/// have created for this version.
fn get_mercurial_archived_version_tag() -> Option<String> {
use map_for::FlatMap;
// Parse the contents of `.hg_archival.txt` into a hash map.
let info = &File::open (".hg_archival.txt")
.iter()
.flat_map (|f| BufReader::new (f).lines())
.filter_map (|l| l.ok())
.map (|l| l.splitn (2, ':')
.map (String::from)
.collect::<Vec<_>>())
.filter_map (
|v| if v.len() == 2
{ Some ((String::from (v[0].trim()),
String::from (v[1].trim()))) }
else { None })
.collect::<HashMap<_,_>>();
// Extract version information from the hash map.
map_for!(
tag <- info.get ("tag");
=> format!("{}+archive.{}", tag, time::strftime ("%Y%m%d", &time::now()).unwrap()))
.or_else (|| map_for!{
tag <- info.get ("latesttag");
distance <- info.get ("latesttagdistance");
node <- info.get ("node");
=> format!("{}.dev{}+archive.{:.12}.{}",
tag, distance, node,
time::strftime ("%Y%m%d", &time::now()).unwrap()) })
.map (String::from)
}
/// Get the version information.
///
/// This function will first try to get the version from a Mercurial
/// repository. If that fails, it will try to get the version from a
/// `.hg_archival.txt` file. If both fail, it will return a version of
/// the form: "unknown-date".
fn get_version() -> String {
get_mercurial_version_tag()
.or_else (get_mercurial_archived_version_tag)
.unwrap_or_else (
|| format!("{}+cargo.{}",
env::var ("CARGO_PKG_VERSION").unwrap(),
time::strftime ("%Y%m%d", &time::now()).unwrap())
.into())
}
fn main()
{
let mut f = File::create ("src/build_info.rs").unwrap();
let version = version_meta().unwrap();
writeln!(f, "pub const RUST_VERSION: &'static str = \"{} {} v{}\";",
env::var ("RUSTC").unwrap_or ("rustc".into()),
match version.channel {
Channel::Dev => "dev",
Channel::Nightly => "nightly",
Channel::Beta => "beta",
Channel::Stable => "stable",
},
version.semver).unwrap();
writeln!(f, "pub const PROFILE: &'static str = \"{}\";",
env::var ("PROFILE").unwrap_or ("unknown".into()))
.unwrap();
writeln!(f, "pub const TARGET: &'static str = \"{}\";",
env::var ("TARGET").unwrap_or ("unknown".into()))
.unwrap();
writeln!(f, "pub const PKG_NAME: &'static str = \"{} {} {}\";",
env::var ("CARGO_PKG_NAME").unwrap(),
get_version(),
env::var ("PROFILE").unwrap_or ("".into()))
.unwrap();
writeln!(f, "pub const PKG_VERSION: &'static str = \"{}\";",
get_version())
.unwrap();
}
In Rust, it's possible to perform conditional compilation as follows.
#[cfg(rust_version = "1.10")]
fn my_func() {}
Is it possible to define variables for cfg to check within the same source file?
For example:
// leave off, just a quick test to enable when troubleshooting.
#define use_counter 1 // C style (not valid Rust)
#[cfg(use_counter == "1")]
static mut fn_counter: usize = 0;
fn my_func() {
#[cfg(use_counter = "1")]
unsafe { fn_counter += 1; }
}
main () {
// code calling 'my_func'
// print how many times the function is called.
#[cfg(use_counter = "1")]
unsafe { println!("Function count {}", fn_counter); }
}
I'm not asking how to write a function counter, it's just an example of optionally inserting logic into a source file.
Yes, this is written as #[cfg(use_counter)]. Such flags can be enabled or disabled on the command line at compile time and are not exposed in Cargo.toml.
fn main() {
#[cfg(use_counter)]
println!("counter is enabled");
}
Using Cargo, run with the feature disabled:
$ cargo run
Using Cargo, run with the feature enabled:
$ RUSTFLAGS="--cfg use_counter" cargo run
Compile directly with the feature disabled:
$ rustc src/main.rs
Compile with the feature enabled:
$ rustc src/main.rs --cfg use_counter