nolog logger

Convenient and 'beautiful by default' logger for debugging your programs. Easy to use, complete documentation is provided on this page. Zero deps. No unsafe (by #![deny(unsafe_code)]).

See this example code

nolog features

• In most cases nolog uses a std::format_args!() (that avoids heap allocations) and compile-time level filtering by Cargo features. If you use the default setup without additional features (like logmod, logonly, logcatch, tofile), then in most cases nolog will only work on compile-time macros without using functions, methods, if or loops. In fact it will expand into the string eprintln!("{}", format_args!("{}{}{}{}"," ", "CRIT⧽", "msg", "[34] src/main.rs"));. Nothing extra.
• Filtering by module path (logmod feature).
• Display messages only from a selected section of code (logonly feature).
• Smart logging: hide all messages, show the previous Х messages if an error or crit level message was triggered (logcatch feature).
• A custom indent can be added to the message, as well as a number of blank lines before and after it. You can use variables (eg loop counter) to set the indent for the selected message.
• Easily adjust and disable all indents and newlines with features.
• Сustomization: you can create your own color scheme for the logger.
• Support for named format arguments: info!("{line_count} lines.");.
• Support for key => value syntax: info!("{server}" => "{ip}");
• Automatically disabled in the release build: cargo run --release. If you want the log to be enabled in the release build, then use release feature: nolog = {version = "*", features = ["release"]}.
• All levels are disabled by default. Turning on the debug level also turns on the levels above it: info, warn, error, crit. Level can be enabled using the console: cargo run --features debug or in Cargo.toml: nolog = {version = "*", features = ["debug"]}. To enable all levels: cargo run --features trace.
• By default, the log is writting to stderr. You can log to a file with tofile feature. You may set the buffer size. Automatic flush after each message will be used. If you want wait for the buffer to fill or to do it manually with logflush!() then use no_auto_flush feature.
• Custom output redirection. For example, to a file and to stderr at the same time.
• You can add a timestamp like [2022-07-10 06:49:33.646361181 UTC] using a third party library you like. How to add a timestamp.
• Support for chaining multiple messages into one (they must all be of the same type: usual or key-value):

info!(
        "{server}" => "{ip}";
        "Status"   => "{server_check_result}";
);

Table of contents

• Using nolog
• Using nolog with --features
• Tofile. Writing log entries to a file
• How to add a timestamp
• Styles
• Chaining
• Logmod. Filtering by module path
• Logonly. Display messages only from a selected section of code
• Logcatch. Smart logging
• Quick disable and enable messages
• Indentation and new lines
• Colors
• Level headers
• Don't disable logger in release build
• Location
• Separator
• Custom color scheme
• Custom output redirection
• Other customization options
• Logging in tests
• Changelog

Using nolog

Cargo.toml

[dependencies]
nolog = { version = "1", features = ["trace"] }

main.rs

#[macro_use]
extern crate nolog;

fn main() {
    trace!("line_count: {}", 42);
    debug!("line_count: {}", 42);
    info!("line_count: {}", 42);
    warn!("line_count: {}", 42);
    error!("line_count: {}", 42);
    crit!("line_count: {}", 42);
}

cargo run

Result:

You can enable more output filtering features in cargo.toml:

• Logonly. Display messages only from a selected section of code. Useful with trace level.
• Logcatch. Smart logging. Hide all messages, show the previous Х messages if an error or crit level message was triggered. Allows you to understand what preceded the error.
• Logmod. Filtering by module path. Receive messages only from the module you are currently working on.

Cargo.toml

[dependencies]
nolog = { version = "1", features = [
  "debug",
  "logonly",
  "logcatch",
  "logmod",
]}

In addition, you can customize the appearance settings.

Appearance settings classic:

Cargo.toml

[dependencies]
nolog = { version = "1", features = [
  "debug",
  "show_lvl_header_kv",
  "indent_ignore_all",
  "newline_ignore",
  "location_style_classic",
  "sep_colon",
]}

Appearance settings classic_plain:

Cargo.toml

[dependencies]
nolog = { version = "1", features = [
  "debug",
  "plain",
  "show_lvl_header_kv",
  "indent_ignore_all",
  "newline_ignore",
  "location_style_classic",
  "sep_colon",
]}

Indentation.

You can specify indentation in the following way:

crit!(->[X,Y,Z] "msg");

• X - Indents.
• Y - Add Y blank lines before message.
• Z - Add Z blank lines after message.

All of these arguments are optional:

crit!("msg");

//       X
crit!(->[1] "msg");

//       X Y
crit!(->[6,1] "msg");

//       X Y Z
crit!(->[1,2,3] "msg");

If you want to add blank lines and leave the default indentation:

//       X Y
crit!(->[_,1] "msg");

//       X Y Z
crit!(->[_,_,2] "msg");

The same works for each message in the chain.

debug!(
    ->[2]   "msg 1";
    ->[_,1] "msg 2";
            "msg 3";
);

Key-values ​​have the additional ability to set indentation not only for the key, but also for the value.

debug!(->[_,1] "The simulation server started successfully.");
debug!(
    "{server}" =>       "{ip}";
    "Status"   => ->[3] "{server_check_result}";
);
crit!(->[_,1] "The Universe was created with a lifetime of {} days.", universe.len());

This allows you to get nice aligned output if you want.

Read more about indentation.

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Using nolog with --features

You need to completely copy the contents of this file to your cargo.toml. Then you can write cargo run --features trace,logonly instead of cargo run --features nolog/trace,nolog/logonly See example on GitHub.

Cargo.toml

[dependencies]
nolog = { version = "1", features = [] }

[features]
nolog_setup = []

# example `classic`
#nolog_setup = ["nolog/show_lvl_header_kv", "nolog/indent_ignore_all", "nolog/newline_ignore", "nolog/location_style_classic", "nolog/sep_colon"]

# example `classic_plain`
#nolog_setup = ["nolog/plain", "nolog/show_lvl_header_kv", "nolog/indent_ignore_all", "nolog/newline_ignore", "nolog/location_style_classic", "nolog/sep_colon"]

trace = ["nolog/trace", "nolog_setup"]
debug = ["nolog/debug", "nolog_setup"]
info  = ["nolog/info",  "nolog_setup"]
warn  = ["nolog/warn",  "nolog_setup"]
error = ["nolog/error", "nolog_setup"]
crit  = ["nolog/crit",  "nolog_setup"]

logonly  = ["nolog/logonly"]
logcatch = ["nolog/logcatch"]
logmod   = ["nolog/logmod"]

The commented lines nolog_setup = ["nolog/show_lvl_h...] are the appearance settings (indents, color scheme, styles, etc). Uncomment one of them to see what happens (don't forget to remove nolog_setup = []).

Appearance settings are selected using conditional compilation, so they have a zero cost.

main.rs

#[macro_use]
extern crate nolog;

fn main() {
    trace!("line_count: {}", 42);
    debug!("line_count: {}", 42);
    info!("line_count: {}", 42);
    warn!("line_count: {}", 42);
    error!("line_count: {}", 42);
    crit!("line_count: {}", 42);
}

This example on GitHub.

nolog has the same syntax as most loggers based on the log crate. nolog extends the log crate syntax by adding new features. However, nolog is not based on log crate, it just has the same macro names. This also results in the nolog having 0 dependencies.

Therefore, switching to nolog will require minimal changes in the code.

Ok. Now we can use the following command:

cargo run --features trace

Or, for example

# The output will be empty because there are no logonly
# blocks, etc. in the code.
# This is just to demonstrate the use of several features.

cargo run --features trace,logonly,logcatch,logmod

It's the same but noisier

# The output will be empty because there are no logonly
# blocks, etc. in the code.
# This is just to demonstrate the use of several features.

cargo run --features nolog/trace,nolog/logonly,nolog/logcatch,nolog/logmod

Result:

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Tofile. Writing log entries to a file

Cargo.toml

#...
[dependencies]
nolog = { version = "1", features = ["tofile"] }
#...

main.rs

use std::fs::OpenOptions;
use std::io::{self, Read};
use std::path::PathBuf;

#[macro_use]
extern crate nolog;

fn main() -> io::Result<()> {
    let path = PathBuf::from("log.txt");
    let file = OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(true)
        //^^^^^^^ truncate the file to 0 length if it already exists.
        //.append(true)
        .open(&path)?;
        
    // Initialization
    // Don't use macros like `debug!("msg");` before initialization.
    logfile!(file);
    
    trace!("Hello from file!");
        
    let mut file = OpenOptions::new()
        .read(true)
        .open(&path)?;
    
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    println!("\n      -- In {path:?} --");
    println!("{contents}");
    
    Ok(())
}

This example on GitHub.

Optionally, you can set the buffer size.

    // Buffer `std::io::BufWriter` with capacity: 8000 bytes.
    logfile!(8000, file);

The default is to automatically flush after each message. If you want wait for the buffer to fill or to do it manually with logflush!() then use no_auto_flush feature.

Cargo.toml

#...
[dependencies]
nolog = { version = "1", features = ["tofile", "no_auto_flush"] }
#...

Then use logflush!() to flush the log manually.

main.rs

...
    // Initialization
    // Don't use macros like `debug!("msg");` before initialization.
    logfile!(8000, file);
    
    trace!("Hello from file!");
    logflush!();     
...

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How to add a timestamp

You can add a timestamp like [2022-07-10 06:49:33.646361181 UTC] using a third party library you like.

For this example, we will use chrono crate.

Cargo.toml

#...
[dependencies]
nolog = { version = "1", features = [] }
chrono = "0.4"

[features]
custom_leading    = ["nolog/custom_leading"]
custom_trailing   = ["nolog/custom_trailing"]
custom_before_msg = ["nolog/custom_before_msg"]
custom_after_msg  = ["nolog/custom_after_msg"]

nolog_setup = ["custom_leading"]
#...

We have 4 options here:

• "custom_leading" - <TIMESTAMP>CRIT⧽ msg [5] src/main.rs
• "custom_trailing" - CRIT⧽ msg [5] src/main.rs<TIMESTAMP>
• "custom_before_msg" - CRIT⧽ <TIMESTAMP>msg [5] src/main.rs
• "custom_after_msg" - CRIT⧽ msg<TIMESTAMP> [5] src/main.rs

Log entry structure:

usual: <indents><custom_leading><lvlheader><sep><custom_before_msg><msg><custom_after_msg><location><custom_trailing>

key-value: <indents><custom_leading><lvlheader><sep_kv><custom_before_msg><key><sep_key><value_indent><value><custom_after_msg><location><custom_trailing>

Here is an example:

main.rs

#[macro_use]
extern crate nolog;

#[macro_use] 
pub mod logger_setup {
    #[macro_export]
    #[cfg(feature = "custom_leading")] macro_rules!
    //               ^^^^^^^^^^^^^^
    custom_leading { 
        // usual
        ( $level:tt, $indent:expr, $($msg:expr),* ) => {
            format_args!("[{}] ", chrono::Utc::now())
            
        };
        // key-value
        ( $level:tt, $indent:expr, $($key:expr),* => $($value:expr),* ) => {
            format_args!("[{}] ", chrono::Local::now())
        };
    }
}

mod other {
    pub fn from_other_mod() -> () {
        crit!(->[0] "Other" => "Hello from other mod! This is key-value msg.");
    }
}

fn main() {
    crit!("Hello from main! This is usual msg.");
    other::from_other_mod();   
}

This example on GitHub.

Output:

[2022-09-07 09:22:09.150921578 UTC] CRIT⧽ Hello from main! This is usual msg. [34] src/main.rs
[2022-09-07 12:22:09.150973037 +03:00] Other⧽ Hello from other mod! This is key-value msg. [29] src/main.rs

With classic style:

[2022-09-07 09:29:45.859185734 UTC] CRIT: Hello from main! This is usual msg. [src/main.rs 34:5]
[2022-09-07 12:29:45.859225186 +03:00] CRIT: Other: Hello from other mod! This is key-value msg. [src/main.rs 29:9]

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Styles

Default

nolog_setup = []

classic

nolog_setup = ["nolog/show_lvl_header_kv", "nolog/indent_ignore_all", "nolog/newline_ignore", "nolog/location_style_classic", "nolog/sep_colon"]

classic_plain

nolog_setup = ["nolog/plain", "nolog/show_lvl_header_kv", "nolog/indent_ignore_all", "nolog/newline_ignore", "nolog/location_style_classic", "nolog/sep_colon"]

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Chaining

Messages in a chain should all be of the same type: usual or key-value

ususal

debug!(
    "Planet {name} thinks...";
    "Planet {name} thinks...";
);

key-value

debug!(
    "{server}" => "{ip}";
    "Status"   => "{server_check_result}";
);

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Logmod. Filtering by module path

Add it as early as possible in the code:

logmod!(
    [  ] main,
    [!=] crate::other2,
);

• [] - Include a module and all its submodules.
• [=] - the same (Include a module and all its submodules).
• [!] - Exclude a module and all its submodules.
• [==] - Include only this module without submodules.
• [!=] - Exclude only this module without submodules.

Then

cargo run --features trace,logmod

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Logonly. Display messages only from a selected section of code

This is useful for debugging to get messages from just a small piece of code.

logonly!(
    let universe = [0;3];
    crit!("The Universe was created with a lifetime of {} days.", universe.len());
    error!("Uncontrolled evolutionary processes have begun on the planet {planet_name}.");
);

Then

cargo run --features trace,logonly

You can use any brackets

logonly!(), logonly!{}, logonly![].

You can use multiple logonly!() blocks. Messages will be displayed from all.

It won't break your code when the logger turns off in release build. So you can leave these blocks in the code.

When disabled, the definition of this macro will be replaced with the following:

logonly { ( $($a:tt)* ) => { $($a)* }; }

It simply writes down the code it received.

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Logcatch. Smart logging

Hide all messages, show the previous Х messages if an error or crit level message was triggered.

By default X=10. You can change this anywhere in the code.

// This will take effect for the code below.
logcatch!(2); // now X=2

To enable this feature, use:

cargo run --features trace,logcatch

Each new line created with newline!() or ->[_,1,1](about what it will be below) counts as a separate message.

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Quick disable and enable messages

You can disable individual messages without removing them from the code. A macro like debug!([_]; "msg") will expand into an empty tuple ().

// on
info!([#]; "New {name} on planet {planet_name}.");

// off
info!([_]; "{repr}" => "{name} says: {speech}");

You can use any options you like:

On: [#], [x], [v], [+], [on], [true], [your_var]

Off: [ ], [_], [-], [off], [false], [your_var]

your_var should be bool.

To change states, you need to change only one character:

[_] --> [#].

This also works with chained messages, but disables the entire chain. You can't turn off a single message in a chain.

crit!([_];
    "The answer is {answer}."; 
    "Planet {planet_name} started watching TV.";
);

You can turn off the action of block logonly. This will not affect the code, the effect is as if macro logonly was not in this place.

logonly!{[_];
    crit!("The answer is {answer}.");
    let x = 42;
}

This way you can leave logonly!() in the code and if it is required in the future just enable it.

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Variables and expressions

If necessary, you can control messages using variables and expressions.

let my_log_enabled = true;
crit!([my_log_enabled]; "The planet {} has been destroyed.", self.name);

let status = "ok";
crit!([(status == "ok")]; "The planet {} has been destroyed.", self.name);
//     ^              ^
//     Add parentheses

fn is_message_show_fn () -> bool { false }
...
crit!([(is_message_show_fn())]; "The planet {} has been destroyed.", self.name);
//     ^                    ^
//     Add parentheses

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Indentation and new lines

new lines

newline!(2); - It will simply write the passed number of new lines to the log.

Indentation

Indents are of several types:

Base indent

Base indent will be added to every line.

• Default for all: 6 indents. One indent equals one space.

You can change base indent with cargo features:

• indent_base_zero
• indent_base_one
• indent_base_two
• indent_base_three
• indent_base_four
• indent_base_five
• indent_base_seven
• indent_base_eight
• indent_base_nine
• indent_base_ten

For example in Cargo.toml:

nolog_setup = ["nolog/indent_base_zero"]

trace = ["nolog/trace", "nolog_setup"]

The indent of the selected message

• Default for usual: 0
• Default for key-value: 6

The default indentation is used if no value has been provided by the user.

You can specify indentation in the following way:

crit!(->[X,Y,Z] "msg");

• X - Indents.
• Y - Add Y blank lines before message (same effect as newline!(Y)).
• Z - Add Z blank lines after message.

All of these arguments are optional:

crit!("msg");
crit!(->[1] "msg");
crit!(->[6,1] "msg");
crit!(->[1,2,3] "msg");
crit!([#]; ->[1,2,3] "msg");

If you want to add blank lines and leave the default indentation:

crit!(->[_,1] "msg");
crit!(->[_,_,2] "msg");

The same works for each message in the chain.

debug!(
    ->[2]   "Planet {name} thinks...";
    ->[_,1] "Planet {name} thinks...";
            "Planet {name} thinks...";
);

key => value ​​have an indentation of 6 by default, but you can reset it by setting it to zero.

error!(->[0] "{name}" => "{}!! Oh, yeaaaah!", 2*3*7);

Or you can do it via Cargo.toml for all messages.

• indent_kv_default_zero
• indent_kv_default_one
• indent_kv_default_two
• indent_kv_default_three
• indent_kv_default_four
• indent_kv_default_five
• indent_kv_default_seven
• indent_kv_default_eight
• indent_kv_default_nine
• indent_kv_default_ten

For example in Cargo.toml:

nolog_setup = ["nolog/indent_kv_default_zero"]

trace = ["nolog/trace", "nolog_setup"]

Key-values ​​have the additional ability to set indentation not only for the key, but also for the value.

debug!(
    "{server}" =>       "{ip}";
    "Status"   => ->[3] "{server_check_result}";
);

This allows you to get nice aligned output if you want.

Indent variables

You can use variables to set the indentation and add blank lines.

for i in 0..2 {
    warn!(->[i,i,i] "msg");
}

Ignore all indents

Ignore all types of indentation.

nolog_setup = ["nolog/indent_ignore_all"]

Ignore all newlines

nolog_setup = ["nolog/newline_ignore"]

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Colors

nolog colored by default, use this feature for plain output:

nolog_setup = ["nolog/plain"]

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Level headers

nolog_setup = ["nolog/show_lvl_header_kv"]

Show level name for key-value:

CRIT: Key: value [src/main.rs 90:5]`
^^^^

It's disabled by default:

Key: value [src/main.rs 90:5]

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Don't disable logger in release build

nolog = { version = "1", features = ["release"] }

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Location

Don't show location (like [src/main.rs 155:9])

nolog_setup = ["nolog/location_hide"]

Style like this: [src/main.rs 155:9]

nolog_setup = ["nolog/location_style_classic"]

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Separator

Default = "⧽ "

• ": "

nolog_setup = ["nolog/sep_colon"]

• " "

nolog_setup = ["nolog/sep_space"]

• ""

nolog_setup = ["nolog/sep_hide"]

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Custom color scheme

You can create your own color scheme for the logger.

Cargo.toml

#...
[dependencies]
nolog = { version = "1", features = [] }

[features]
custom_colors = ["nolog/custom_colors"]
nolog_setup = ["custom_colors"]
#...

Here is an example:

main.rs

#[macro_use]
extern crate nolog;

#[macro_use] 
pub mod logger_setup {
    #[macro_export]
    #[cfg(feature = "custom_colors")] macro_rules!
    //               ^^^^^^^^^^^^^
    color {
        ( [trace] ) => { "\x1B[34m"                    };
        ( [debug] ) => { "\x1B[36m"                    };
        ( [info]  ) => { "\x1B[32m"                    };
        ( [warn]  ) => { "\x1B[33m"                    };
        ( [error] ) => { "\x1B[31m"                    };
        ( [crit]  ) => { "\x1B[35m"                    };
        ( [sep]   ) => { "\x1B[1m\x1B[2m"              }; // +bold +dim
        ( [msg]   ) => { ""                            }; // default term font color
        ( [from]  ) => { "\x1B[90m\x1B[3m"             }; // `[src/main.rs 101:5]` in `location_style_classic`
        ( [sep2]  ) => { "\x1B[90m\x1B[2m"             }; // sep2 in default style
        ( [sep3]  ) => { "\x1B[90m\x1B[2m"             }; // sep3 in default style
        ( [line]  ) => { "\x1B[38;5;67m\x1B[1m\x1B[2m" }; // line number in default style
        ( [key]   ) => { "\x1B[3m\x1B[1m"              }; // +italic +bold 
        ( [value] ) => { "\x1B[3m"                     }; // +italic
        ( [rm]    ) => { "\x1B[0m"                     }; // remove previous colors
    }
}

mod other {
    pub fn from_other_mod() -> () {
        crit!(->[0] "Other" => "Hello from other mod! This is key-value msg.");
    }
}

fn main(){
    crit!("Hello from main! This is usual msg.");
    other::from_other_mod();   
}

This example on GitHub.

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Custom output redirection

It is possible to redirect output. For example, we will redirect output to stderr and to a file at the same time. The limitation is that output to stderr will be the same as to a file, it will not be colorized.

Cargo.toml

#...
[dependencies]
nolog = { version = "1", features = [] }

[features]
nolog_setup = [
    "custom_writelog_inner",
    "nolog/tofile"
]
custom_writelog_inner = ["nolog/custom_writelog_inner"]
#...

Here is an example:

main.rs

use std::fs::OpenOptions;
use std::io::{self, Read};
use std::path::PathBuf;

#[macro_use]
extern crate nolog;

// use `cargo run --features trace`

#[macro_use] 
pub mod logger_setup {
    #[macro_export]
    #[cfg(feature = "custom_writelog_inner")] macro_rules!
    writelog_inner { ( $msg:expr ) => {
            eprintln!("{}", $msg); // write to stderr
            tofile_writelog_inner_helper!($msg); // write to a file
        }
    }
}

mod other {
    pub fn from_other_mod() -> () {
        crit!(->[0] "Other" => "Hello from other mod! This is key-value msg.");
    }
}

fn main() -> io::Result<()> {
    let path = PathBuf::from("log.txt");
    let file = OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(true)
        //^^^^^^^ truncate the file to 0 length if it already exists.
        //.append(true)
        .open(&path)?;
        
    // Initialization
    // Don't use macros like `debug!("msg");` before initialization.
    logfile!(file);
    
    eprintln!("\n-- From eprintln: --");
    crit!("Hello from main! This is usual msg.");
    other::from_other_mod();
    
    let mut file = OpenOptions::new()
        .read(true)
        .open(&path)?;
    
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    println!("\n-- In {path:?} --");
    println!("{contents}");
    
    Ok(())
}

This example on GitHub.

Output:

-- From eprintln: --
CRIT: Hello from main! This is usual msg. [main.rs 54:5]
CRIT: Other: Hello from other mod! This is key-value msg. [main.rs 34:9]

-- In "log.txt" --
CRIT: Hello from main! This is usual msg. [main.rs 54:5]
CRIT: Other: Hello from other mod! This is key-value msg. [main.rs 34:9]

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Other customization options

nolog has other customization options not described here, since it is unlikely that they will be in demand by a wide range of users. Their use is similar to that described above. You can see the full up-to-date list in Cargo.toml.

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Logging in tests

Logging in tests works exactly the same, except that Rust test programs hide standard output of successful tests.

Use the following code to see the output of successful tests.

cargo test --features trace -- --nocapture

The output of failed tests will be displayed anyway.

cargo test --features trace

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Changelog

• 1.0.15 - 1.0.19 – Small changes in Readme etc.
• 1.0.12 - 1.0.14 – Small changes in Readme etc. Minor optimization fixes.
• 1.0.10 - 1.0.11 – Minor changes, an example with output redirection has been added.
• 1.0.1 - 1.0.9 – Small changes in Readme etc.
• 1.0.0 – Release. Completely rewritten.
• 0.1.1-0.2.3 – Early versions.