refactor(fluent-bundle): Remove unreachable MissingDefault ResolverError

Author: JasperDeSutter

fluent-bundle/src/resolver/errors.rs

@@ -53,7 +53,6 @@ where
 pub enum ResolverError {
     Reference(ReferenceKind),
     NoValue(String),
-    MissingDefault,
     Cyclic,
     TooManyPlaceables,
 }
@@ -82,7 +81,6 @@ impl std::fmt::Display for ResolverError {
                 ReferenceKind::Variable { id } => write!(f, "Unknown variable: ${}", id),
             },
             Self::NoValue(id) => write!(f, "No value: {}", id),
-            Self::MissingDefault => f.write_str("No default"),
             Self::Cyclic => f.write_str("Cyclical dependency detected"),
             Self::TooManyPlaceables => f.write_str("Too many placeables"),
         }

fluent-bundle/src/resolver/expression.rs

@@ -7,7 +7,7 @@ use std::fmt;
 use fluent_syntax::ast;
 
 use crate::memoizer::MemoizerKind;
-use crate::resolver::{ResolveValue, ResolverError};
+use crate::resolver::ResolveValue;
 use crate::resource::FluentResource;
 use crate::types::FluentValue;
 
@@ -43,13 +43,12 @@ impl<'bundle> WriteValue<'bundle> for ast::Expression<&'bundle str> {
                     _ => {}
                 }
 
-                for variant in variants {
-                    if variant.default {
-                        return variant.value.write(w, scope);
-                    }
-                }
-                scope.add_error(ResolverError::MissingDefault);
-                Ok(())
+                variants
+                    .iter()
+                    .find(|variant| variant.default)
+                    .expect("select expressions have a default variant")
+                    .value
+                    .write(w, scope)
             }
         }
     }
@@ -95,13 +94,12 @@ impl<'bundle> ResolveValue<'bundle> for ast::Expression<&'bundle str> {
                     _ => {}
                 }
 
-                for variant in variants {
-                    if variant.default {
-                        return variant.value.resolve(scope);
-                    }
-                }
-                scope.add_error(ResolverError::MissingDefault);
-                FluentValue::Error
+                variants
+                    .iter()
+                    .find(|variant| variant.default)
+                    .expect("select expressions have a default variant")
+                    .value
+                    .resolve(scope)
             }
         }
     }

fluent-bundle/tests/resolver_fixtures.rs

@@ -334,7 +334,6 @@ fn test_errors(errors: &[FluentError], reference: Option<&[TestError]>) {
                 ResolverError::TooManyPlaceables => {
                     assert_eq!(reference.error_type, "TooManyPlaceables");
                 }
-                _ => unimplemented!(),
             },
             FluentError::ParserError(_) => {
                 assert_eq!(reference.error_type, "Parser");

fluent-syntax/src/ast/mod.rs

@@ -1444,6 +1444,7 @@ pub enum Expression<S> {
     ///     [key1] Value 1
     ///    *[other] Value 2
     /// }
+    /// Invariant: exactly 1 variant must have default: true.
     /// ```
     Select {
         selector: InlineExpression<S>,