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Diff for: ‎.github/workflows/cargo-test.yml

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name: Cargo Test
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on: push
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jobs:
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cargo-test:
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runs-on: ubuntu-22.04
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steps:
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- uses: actions-rust-lang/setup-rust-toolchain@v1
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- uses: actions/checkout@v4
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- name: Run Unittests
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run: |
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cargo test

Diff for: ‎.gitignore

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/target
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/.idea
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Cargo.lock

Diff for: ‎Cargo.toml

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[workspace]
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resolver = "2"
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members = [
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"tracing-build-script",
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"tracing-build-script-tests",
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]

Diff for: ‎LICENSE-APACHE

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Apache License
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Version 2.0, January 2004
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http://www.apache.org/licenses/
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Diff for: ‎LICENSE-MIT

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MIT License
2+
3+
Copyright (c) 2025 Data Science Group at UPB
4+
5+
Permission is hereby granted, free of charge, to any person obtaining a copy
6+
of this software and associated documentation files (the "Software"), to deal
7+
in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
11+
12+
The above copyright notice and this permission notice shall be included in all
13+
copies or substantial portions of the Software.
14+
15+
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16+
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17+
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
18+
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19+
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20+
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21+
SOFTWARE.

Diff for: ‎README.md

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# tracing-build-script
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3+
A [`tracing-subscriber`](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/) compatible writer for logging events in build scripts.
4+
5+
## Getting Started
6+
### How to install
7+
Add `tracing-build-script` to your dependencies
8+
9+
```toml
10+
[package]
11+
# ...
12+
build = "build.rs"
13+
14+
[build-dependencies]
15+
# ...
16+
tracing-build-script = "0.1.0"
17+
tracing-subscriber = "0.3.19"
18+
```
19+
20+
### Quickstart
21+
> build.rs
22+
> ```rust
23+
> fn main() {
24+
> tracing_subscriber::fmt()
25+
> .with_writer(tracing_build_script::BuildScriptMakeWriter)
26+
> .init();
27+
>
28+
> ...
29+
> }
30+
> ```

Diff for: ‎tracing-build-script-tests/Cargo.toml

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[package]
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name = "tracing-build-script-tests"
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build = "build.rs"
4+
edition = "2021"
5+
6+
[build-dependencies]
7+
tracing = "0.1.41"
8+
tracing-subscriber = "0.3.19"
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tracing-build-script = { path = "../tracing-build-script" }

Diff for: ‎tracing-build-script-tests/build.rs

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1+
use tracing::metadata::LevelFilter;
2+
3+
fn write_message(msg: &str) {
4+
tracing::trace!("{msg}");
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tracing::debug!("{msg}");
6+
tracing::info!("{msg}");
7+
tracing::warn!("{msg}");
8+
tracing::error!("{msg}");
9+
}
10+
11+
fn main() {
12+
tracing_subscriber::fmt()
13+
.with_writer(tracing_build_script::BuildScriptMakeWriter)
14+
.with_ansi(false)
15+
.without_time()
16+
.with_max_level(LevelFilter::TRACE)
17+
.init();
18+
19+
write_message("simple message");
20+
write_message("with\nnew\nline");
21+
write_message("with\nnew\nlineend\n");
22+
write_message("\nwith\nnew\nlinestart");
23+
write_message("two\n\nnewlines");
24+
write_message("other\rspecial\tchar\0a\tb\"c\\");
25+
write_message("two\nnewlines\natend\n\n");
26+
}

Diff for: ‎tracing-build-script-tests/src/lib.rs

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#![cfg(test)]
2+
3+
use std::path::Path;
4+
5+
#[test]
6+
fn test_warn_and_error_output() {
7+
let out_dir = Path::new(env!("OUT_DIR"));
8+
let warn_output_file = out_dir.join("../output");
9+
let warn_output = std::fs::read_to_string(warn_output_file).unwrap();
10+
11+
assert_eq!(
12+
warn_output,
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"cargo::warning= WARN build_script_build: simple message\n\
14+
cargo::warning=ERROR build_script_build: simple message\n\
15+
cargo::warning= WARN build_script_build: with\\nnew\\nline\n\
16+
cargo::warning=ERROR build_script_build: with\\nnew\\nline\n\
17+
cargo::warning= WARN build_script_build: with\\nnew\\nlineend\\n\n\
18+
cargo::warning=ERROR build_script_build: with\\nnew\\nlineend\\n\n\
19+
cargo::warning= WARN build_script_build: \\nwith\\nnew\\nlinestart\n\
20+
cargo::warning=ERROR build_script_build: \\nwith\\nnew\\nlinestart\n\
21+
cargo::warning= WARN build_script_build: two\\n\\nnewlines\n\
22+
cargo::warning=ERROR build_script_build: two\\n\\nnewlines\n\
23+
cargo::warning= WARN build_script_build: other\\rspecial\tchar\0a\tb\"c\\\n\
24+
cargo::warning=ERROR build_script_build: other\\rspecial\tchar\0a\tb\"c\\\n\
25+
cargo::warning= WARN build_script_build: two\\nnewlines\\natend\\n\\n\n\
26+
cargo::warning=ERROR build_script_build: two\\nnewlines\\natend\\n\\n\n"
27+
);
28+
}
29+
30+
#[test]
31+
fn test_informational_output() {
32+
let out_dir = Path::new(env!("OUT_DIR"));
33+
let info_output_file = out_dir.join("../stderr");
34+
let info_output = std::fs::read_to_string(info_output_file).unwrap();
35+
36+
assert_eq!(
37+
info_output,
38+
"TRACE build_script_build: simple message\n\
39+
DEBUG build_script_build: simple message\n \
40+
INFO build_script_build: simple message\n\
41+
TRACE build_script_build: with\nnew\nline\n\
42+
DEBUG build_script_build: with\nnew\nline\n \
43+
INFO build_script_build: with\nnew\nline\n\
44+
TRACE build_script_build: with\nnew\nlineend\n\n\
45+
DEBUG build_script_build: with\nnew\nlineend\n\n \
46+
INFO build_script_build: with\nnew\nlineend\n\n\
47+
TRACE build_script_build: \nwith\nnew\nlinestart\n\
48+
DEBUG build_script_build: \nwith\nnew\nlinestart\n \
49+
INFO build_script_build: \nwith\nnew\nlinestart\n\
50+
TRACE build_script_build: two\n\nnewlines\n\
51+
DEBUG build_script_build: two\n\nnewlines\n \
52+
INFO build_script_build: two\n\nnewlines\n\
53+
TRACE build_script_build: other\rspecial\tchar\0a\tb\"c\\\n\
54+
DEBUG build_script_build: other\rspecial\tchar\0a\tb\"c\\\n \
55+
INFO build_script_build: other\rspecial\tchar\0a\tb\"c\\\n\
56+
TRACE build_script_build: two\nnewlines\natend\n\n\n\
57+
DEBUG build_script_build: two\nnewlines\natend\n\n\n \
58+
INFO build_script_build: two\nnewlines\natend\n\n\n"
59+
);
60+
}

Diff for: ‎tracing-build-script/Cargo.toml

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1+
[package]
2+
name = "tracing-build-script"
3+
version = "0.1.0"
4+
authors = ["Liss Heidrich <liss.heidrich@uni-paderborn.de>"]
5+
license = "MIT OR Apache-2.0"
6+
readme = "../README.md"
7+
homepage = "https://github.com/dice-group/tracing-build-script"
8+
description = """
9+
Provides a tracing-subscriber compatible writer for use in build scripts.
10+
"""
11+
categories = [
12+
"development-tools::debugging",
13+
"development-tools::build-utils",
14+
]
15+
keywords = ["logging", "tracing", "build-script"]
16+
edition = "2021"
17+
18+
[dependencies]
19+
tracing = "^0.1"
20+
tracing-subscriber = "^0.3"

Diff for: ‎tracing-build-script/src/lib.rs

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1+
use std::{io, io::Write};
2+
use tracing::{Level, Metadata};
3+
use tracing_subscriber::fmt::MakeWriter;
4+
5+
enum ErrorAndWarnState {
6+
/// Initial state, no output has been written yet
7+
/// cargo::warning= needs to be written next
8+
Init,
9+
/// Normal operation, cargo::warning= was already written
10+
/// and user input did not end with newline
11+
Normal,
12+
/// Normal operation, cargo::warning= was already written
13+
/// but user input ended with a newline or another char that needs escaping.
14+
/// This either means the log message is done, or there just happens to be a newline at the end of this write
15+
/// request
16+
LastCharWasSpecial(u8),
17+
}
18+
19+
fn char_is_special(ch: u8) -> bool {
20+
ch == b'\n' || ch == b'\r'
21+
}
22+
23+
fn escape_special(ch: u8) -> &'static [u8] {
24+
match ch {
25+
b'\n' => b"\\n",
26+
b'\r' => b"\\r",
27+
_ => unreachable!(),
28+
}
29+
}
30+
31+
enum BuildScriptWriterInner {
32+
Informational(io::Stderr),
33+
ErrorsAndWarnings {
34+
state: ErrorAndWarnState,
35+
writer: io::Stdout,
36+
},
37+
}
38+
39+
/// A writer intended to support the [output capturing of build scripts](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script).
40+
/// `BuildScriptWriter` can be used by [`tracing_subscriber::fmt::Subscriber`](tracing_subscriber::fmt::Subscriber) or [`tracing_subscriber::fmt::Layer`](tracing_subscriber::fmt::Layer)
41+
/// to enable capturing output in build scripts.
42+
pub struct BuildScriptWriter(BuildScriptWriterInner);
43+
44+
impl BuildScriptWriter {
45+
/// Create a writer for informational events.
46+
/// Events will be written to stderr.
47+
pub fn informational() -> Self {
48+
Self(BuildScriptWriterInner::Informational(io::stderr()))
49+
}
50+
51+
/// Create a writer for warning and error events.
52+
/// Events will be written to stdout after having `cargo::warning=` prepended.
53+
pub fn errors_and_warnings() -> Self {
54+
Self(BuildScriptWriterInner::ErrorsAndWarnings { state: ErrorAndWarnState::Init, writer: io::stdout() })
55+
}
56+
}
57+
58+
impl Drop for BuildScriptWriter {
59+
fn drop(&mut self) {
60+
if let BuildScriptWriterInner::ErrorsAndWarnings { state: ErrorAndWarnState::LastCharWasSpecial(ch), writer } =
61+
&mut self.0
62+
{
63+
let _ = writer.write(&[*ch]);
64+
}
65+
}
66+
}
67+
68+
impl Write for BuildScriptWriter {
69+
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
70+
self.write_all(buf)?;
71+
Ok(buf.len())
72+
}
73+
74+
fn flush(&mut self) -> io::Result<()> {
75+
match &mut self.0 {
76+
BuildScriptWriterInner::Informational(writer) => writer.flush(),
77+
BuildScriptWriterInner::ErrorsAndWarnings { writer, state: _ } => writer.flush(),
78+
}
79+
}
80+
81+
fn write_all(&mut self, mut buf: &[u8]) -> io::Result<()> {
82+
match &mut self.0 {
83+
BuildScriptWriterInner::Informational(writer) => writer.write_all(buf),
84+
BuildScriptWriterInner::ErrorsAndWarnings { state, writer } => {
85+
// We will need to issue multiple write calls to the writer (to avoid heap allocation)
86+
// so we need to lock it to prevent other threads from clobbering our output.
87+
let mut writer = writer.lock();
88+
89+
// depending on the current state we may need to prefix the output
90+
match *state {
91+
ErrorAndWarnState::Init => {
92+
writer.write_all(b"cargo::warning=")?;
93+
},
94+
ErrorAndWarnState::LastCharWasSpecial(ch) => {
95+
writer.write_all(escape_special(ch))?;
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},
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ErrorAndWarnState::Normal => {},
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}
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// If the last char is a newline we need to remember that but cannot immediately
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// write it out. This is because we cannot know yet if its needs to be escaped, there are two cases:
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//
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// 1. this call to write is not actually the last call to write that will happen it just happens to end with a newline
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// => we need to escape the newline
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//
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// 2. this call to write is actually the last call to write that will happen, and it ends with a newline
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// => we need to keep the newline as is, because it is the newline terminator of the log message
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// (tracing automatically appends a newline at the end of each message, like println!)
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//
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// Since we cannot decide which of these cases we are in at the moment, we need to delay writing the last character (if it is a newline) until we know that.
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// We know which case we are in
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// either when we enter this function the next time (case 1)
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// or the next time or when we enter the destructor (case 2).
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match buf.last().copied() {
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Some(ch) if char_is_special(ch) => {
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buf = &buf[..buf.len() - 1];
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*state = ErrorAndWarnState::LastCharWasSpecial(ch);
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},
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_ => {
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*state = ErrorAndWarnState::Normal;
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},
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}
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let mut last_special_char = match buf.iter().position(|ch| char_is_special(*ch)) {
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Some(pos) => {
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writer.write_all(&buf[..pos])?;
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let ret = buf[pos];
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buf = &buf[pos + 1..];
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ret
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},
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None => {
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// fast path for messages without any special chars
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writer.write_all(buf)?;
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return Ok(());
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},
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};
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loop {
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writer.write_all(escape_special(last_special_char))?;
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match buf.iter().position(|ch| char_is_special(*ch)) {
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Some(pos) => {
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writer.write_all(&buf[..pos])?;
145+
146+
last_special_char = buf[pos];
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buf = &buf[pos + 1..];
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},
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None => {
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writer.write_all(buf)?;
151+
break;
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},
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}
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}
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Ok(())
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},
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}
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}
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}
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/// [`MakeWriter`](tracing_subscriber::fmt::MakeWriter) implementation for [`BuildScriptWriter`](BuildScriptWriter)
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///
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/// # Behaviour
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/// Events for Levels Error and Warn are printed to stdout with [`cargo::warning=`](https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargo-warning) prepended.
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/// All other levels are sent to stderr, where they are only visible when running with verbose build output (`cargo build -vv`).
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///
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/// Note: this writer explicitly does **not** use the [`cargo::error=`](https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargo-error) instruction
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/// because it aborts the build with an error, which is not always desired.
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///
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/// # Example
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/// ```
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/// tracing_subscriber::fmt()
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/// .with_writer(tracing_build_script::BuildScriptMakeWriter)
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/// .init();
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/// ```
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pub struct BuildScriptMakeWriter;
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impl<'a> MakeWriter<'a> for BuildScriptMakeWriter {
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type Writer = BuildScriptWriter;
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182+
fn make_writer(&'a self) -> Self::Writer {
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BuildScriptWriter::informational()
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}
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fn make_writer_for(&'a self, meta: &Metadata) -> Self::Writer {
187+
if meta.level() == &Level::ERROR || meta.level() == &Level::WARN {
188+
BuildScriptWriter::errors_and_warnings()
189+
} else {
190+
BuildScriptWriter::informational()
191+
}
192+
}
193+
}

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