feat: Add Builder::try_parse method

Current implementation of the Builder::parse() method prints out
specification errors to stderr and then just ignores them. This is fine
for most console applications, but in some cases better control over
what is happening is needed:

* Sometimes there is no access to stderr, in that case there is no way
to find out what went wrong.

* For some applications it's more
desirable to fail on startup than to run with (partially) invalid
configuration.

For such cases this commit introduces a new method try_parse that does
the same thing as the parse method, but returns an Err in case an error
in the specification is found.

crates/env_filter/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 <!-- next-header -->
 ## [Unreleased] - ReleaseDate
 
+### Features
+
+- Added `env_filter::Builder::try_parse(&self, &str)` method (failable version of `env_filter::Builder::parse()`)
+
 ## [0.1.0] - 2024-01-19
 
 <!-- next-url -->

crates/env_filter/src/filter.rs

@@ -9,6 +9,7 @@ use crate::parse_spec;
 use crate::parser::ParseResult;
 use crate::Directive;
 use crate::FilterOp;
+use crate::ParseError;
 
 /// A builder for a log filter.
 ///
@@ -118,6 +119,22 @@ impl Builder {
         self
     }
 
+    /// Parses the directive string, returning an error if the given directive string is invalid.
+    ///
+    /// See the [Enabling Logging] section for more details.
+    ///
+    /// [Enabling Logging]: ../index.html#enabling-logging
+    pub fn try_parse(&mut self, filters: &str) -> Result<&mut Self, ParseError> {
+        let (directives, filter) = parse_spec(filters).ok()?;
+
+        self.filter = filter;
+
+        for directive in directives {
+            self.insert_directive(directive);
+        }
+        Ok(self)
+    }
+
     /// Build a log filter.
     pub fn build(&mut self) -> Filter {
         assert!(!self.built, "attempt to re-use consumed builder");
@@ -241,6 +258,7 @@ impl fmt::Debug for Filter {
 #[cfg(test)]
 mod tests {
     use log::{Level, LevelFilter};
+    use snapbox::{assert_data_eq, str};
 
     use super::{enabled, Builder, Directive, Filter};
 
@@ -455,6 +473,25 @@ mod tests {
         }
     }
 
+    #[test]
+    fn try_parse_valid_filter() {
+        let logger = Builder::new()
+            .try_parse("info,crate1::mod1=warn")
+            .expect("valid filter returned error")
+            .build();
+        assert!(enabled(&logger.directives, Level::Warn, "crate1::mod1"));
+        assert!(enabled(&logger.directives, Level::Info, "crate2::mod2"));
+    }
+
+    #[test]
+    fn try_parse_invalid_filter() {
+        let error = Builder::new().try_parse("info,crate1=invalid").unwrap_err();
+        assert_data_eq!(
+            error,
+            str!["error parsing logger filter: invalid logging spec 'invalid'"]
+        );
+    }
+
     #[test]
     fn match_full_path() {
         let logger = make_logger_filter(vec![

crates/env_filter/src/lib.rs

@@ -56,3 +56,4 @@ use parser::parse_spec;
 pub use filter::Builder;
 pub use filter::Filter;
 pub use filtered_log::FilteredLog;
+pub use parser::ParseError;

crates/env_filter/src/parser.rs

@@ -1,4 +1,6 @@
 use log::LevelFilter;
+use std::error::Error;
+use std::fmt::{Display, Formatter};
 
 use crate::Directive;
 use crate::FilterOp;
@@ -22,8 +24,35 @@ impl ParseResult {
     fn add_error(&mut self, message: String) {
         self.errors.push(message);
     }
+
+    pub(crate) fn ok(self) -> Result<(Vec<Directive>, Option<FilterOp>), ParseError> {
+        let Self {
+            directives,
+            filter,
+            errors,
+        } = self;
+        if let Some(error) = errors.into_iter().next() {
+            Err(ParseError { details: error })
+        } else {
+            Ok((directives, filter))
+        }
+    }
+}
+
+/// Error during logger directive parsing process.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct ParseError {
+    details: String,
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "error parsing logger filter: {}", self.details)
+    }
 }
 
+impl Error for ParseError {}
+
 /// Parse a logging specification string (e.g: `crate1,crate2::mod3,crate3::x=error/foo`)
 /// and return a vector with log directives.
 pub(crate) fn parse_spec(spec: &str) -> ParseResult {
@@ -86,11 +115,18 @@ pub(crate) fn parse_spec(spec: &str) -> ParseResult {
 
 #[cfg(test)]
 mod tests {
+    use crate::ParseError;
     use log::LevelFilter;
-    use snapbox::{assert_data_eq, str};
+    use snapbox::{assert_data_eq, str, Data, IntoData};
 
     use super::{parse_spec, ParseResult};
 
+    impl IntoData for ParseError {
+        fn into_data(self) -> Data {
+            self.to_string().into_data()
+        }
+    }
+
     #[test]
     fn parse_spec_valid() {
         let ParseResult {
@@ -460,4 +496,26 @@ mod tests {
         );
         assert_data_eq!(&errors[1], str!["invalid logging spec 'invalid'"]);
     }
+
+    #[test]
+    fn parse_error_message_single_error() {
+        let error = parse_spec("crate1::mod1=debug=info,crate2=debug")
+            .ok()
+            .unwrap_err();
+        assert_data_eq!(
+            error,
+            str!["error parsing logger filter: invalid logging spec 'crate1::mod1=debug=info'"]
+        );
+    }
+
+    #[test]
+    fn parse_error_message_multiple_errors() {
+        let error = parse_spec("crate1::mod1=debug=info,crate2=debug,crate3=invalid")
+            .ok()
+            .unwrap_err();
+        assert_data_eq!(
+            error,
+            str!["error parsing logger filter: invalid logging spec 'crate1::mod1=debug=info'"]
+        );
+    }
 }