add 'must use' annotation where appropriate

Author: danieleades

src/builder.rs

@@ -87,6 +87,7 @@ use crate::{config::Config, path::Expression, source::Source, value::Value};
 /// let mut builder = ConfigBuilder::<DefaultState>::default();
 /// ```
 #[derive(Debug, Clone, Default)]
+#[must_use]
 pub struct ConfigBuilder<St: BuilderState> {
     defaults: Map<Expression, Value>,
     overrides: Map<Expression, Value>,

src/env.rs

@@ -6,6 +6,7 @@ use crate::source::Source;
 use crate::value::{Value, ValueKind};
 
 #[derive(Clone, Debug, Default)]
+#[must_use]
 pub struct Environment {
     /// Optional prefix that will limit access to the environment to only keys that
     /// begin with the defined prefix.

src/file/mod.rs

@@ -20,6 +20,7 @@ pub use self::source::string::FileSourceString;
 ///
 /// It supports optional automatic file format discovery.
 #[derive(Clone, Debug)]
+#[must_use]
 pub struct File<T, F> {
     source: T,