Add must_use attributes and enable clippy::must-use-candidate (#232)

If a semicolon discards the result of a function or method tagged with
`#[must_use]`, the compiler will emit a lint message warning that the
return value is expected to be used. 

This lint suggests adding `#[must_use]` to public functions that:
- return something that's not already marked as `must_use`
- have no mutable *reference* args (having a mutable owned arg is fine,
  since as the method took ownership, the caller can only access the result
  via the returned self, so it therefore still must be used)
- don't mutate statics

Docs:
https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

For more on best practices of when to use `must_use`, see:
rust-lang/rust#48926 (comment)

Fixes #57.
GUS-W-10222390.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+- Add `must_use` attributes to a number of pure public methods.
+
 ## [0.4.0] 2021/12/08
 
 - Add `PartialEq` and `Eq` implementations for `Process`.

libcnb-data/src/build_plan.rs

@@ -13,6 +13,7 @@ pub struct BuildPlan {
 }
 
 impl BuildPlan {
+    #[must_use]
     pub fn new() -> Self {
         Self {
             provides: vec![],
@@ -35,6 +36,7 @@ pub struct BuildPlanBuilder {
 }
 
 impl BuildPlanBuilder {
+    #[must_use]
     pub fn new() -> Self {
         Self {
             acc: VecDeque::new(),
@@ -53,6 +55,7 @@ impl BuildPlanBuilder {
         self
     }
 
+    #[must_use]
     pub fn or(mut self) -> Self {
         self.acc
             .push_back((self.current_provides, self.current_requires));
@@ -62,6 +65,7 @@ impl BuildPlanBuilder {
         self
     }
 
+    #[must_use]
     pub fn build(self) -> BuildPlan {
         let mut xyz = self.or();
 

libcnb-data/src/launch.rs

@@ -29,6 +29,7 @@ pub struct Launch {
 /// assert!(toml::to_string(&launch_toml).is_ok());
 /// ```
 impl Launch {
+    #[must_use]
     pub fn new() -> Self {
         Self {
             bom: bom::Bom::new(),
@@ -38,6 +39,7 @@ impl Launch {
         }
     }
 
+    #[must_use]
     pub fn process(mut self, process: Process) -> Self {
         self.processes.push(process);
         self

libcnb-data/src/lib.rs

@@ -7,9 +7,6 @@
 #![warn(clippy::pedantic)]
 // This lint is too noisy and enforces a style that reduces readability in many cases.
 #![allow(clippy::module_name_repetitions)]
-// Re-disable pedantic lints that are currently failing, until they are triaged and fixed/wontfixed.
-// https://github.com/Malax/libcnb.rs/issues/57
-#![allow(clippy::must_use_candidate)]
 
 pub mod bom;
 pub mod build;

libcnb/src/build.rs

@@ -146,6 +146,7 @@ pub struct BuildResultBuilder {
 }
 
 impl BuildResultBuilder {
+    #[must_use]
     pub fn new() -> Self {
         Self {
             launch: None,
@@ -166,18 +167,21 @@ impl BuildResultBuilder {
         Ok(self.build_unwrapped())
     }
 
+    #[must_use]
     pub fn build_unwrapped(self) -> BuildResult {
         BuildResult(InnerBuildResult::Pass {
             launch: self.launch,
             store: self.store,
         })
     }
 
+    #[must_use]
     pub fn launch(mut self, launch: Launch) -> Self {
         self.launch = Some(launch);
         self
     }
 
+    #[must_use]
     pub fn store(mut self, store: Store) -> Self {
         self.store = Some(store);
         self

libcnb/src/detect.rs

@@ -46,10 +46,12 @@ pub(crate) enum InnerDetectResult {
 pub struct DetectResultBuilder;
 
 impl DetectResultBuilder {
+    #[must_use]
     pub fn pass() -> PassDetectResultBuilder {
         PassDetectResultBuilder { build_plan: None }
     }
 
+    #[must_use]
     pub fn fail() -> FailDetectResultBuilder {
         FailDetectResultBuilder {}
     }
@@ -73,12 +75,14 @@ impl PassDetectResultBuilder {
         Ok(self.build_unwrapped())
     }
 
+    #[must_use]
     pub fn build_unwrapped(self) -> DetectResult {
         DetectResult(InnerDetectResult::Pass {
             build_plan: self.build_plan,
         })
     }
 
+    #[must_use]
     pub fn build_plan(mut self, build_plan: BuildPlan) -> Self {
         self.build_plan = Some(build_plan);
         self
@@ -102,6 +106,7 @@ impl FailDetectResultBuilder {
     }
 
     #[allow(clippy::unused_self)]
+    #[must_use]
     pub fn build_unwrapped(self) -> DetectResult {
         DetectResult(InnerDetectResult::Fail)
     }

libcnb/src/env.rs

@@ -38,11 +38,13 @@ impl Env {
     /// variables afterwards will not be reflected in the returned value.
     ///
     /// See [`std::env::vars_os`]
+    #[must_use]
     pub fn from_current() -> Self {
         env::vars_os().into()
     }
 
     /// Creates an empty `Env` struct.
+    #[must_use]
     pub fn new() -> Self {
         Self {
             inner: HashMap::new(),
@@ -57,15 +59,18 @@ impl Env {
     }
 
     /// Returns a cloned value corresponding to the given key.
+    #[must_use]
     pub fn get(&self, key: impl AsRef<OsStr>) -> Option<OsString> {
         self.inner.get(key.as_ref()).cloned()
     }
 
     /// Returns true if the environment contains a value for the specified key.
+    #[must_use]
     pub fn contains_key(&self, key: impl AsRef<OsStr>) -> bool {
         self.inner.contains_key(key.as_ref())
     }
 
+    #[must_use]
     pub fn iter(&self) -> std::collections::hash_map::Iter<'_, OsString, OsString> {
         self.inner.iter()
     }

libcnb/src/generic.rs

@@ -24,6 +24,7 @@ pub struct GenericPlatform {
 }
 
 impl GenericPlatform {
+    #[must_use]
     pub fn new(env: Env) -> Self {
         Self { env }
     }

libcnb/src/layer/public_interface.rs

@@ -175,13 +175,15 @@ pub struct LayerResultBuilder<M> {
 }
 
 impl<M> LayerResultBuilder<M> {
+    #[must_use]
     pub fn new(metadata: M) -> Self {
         Self {
             metadata,
             env: None,
         }
     }
 
+    #[must_use]
     pub fn env(mut self, layer_env: LayerEnv) -> Self {
         self.env = Some(layer_env);
         self
@@ -199,6 +201,7 @@ impl<M> LayerResultBuilder<M> {
         Ok(self.build_unwrapped())
     }
 
+    #[must_use]
     pub fn build_unwrapped(self) -> LayerResult<M> {
         LayerResult {
             metadata: self.metadata,

libcnb/src/layer_env.rs

@@ -113,6 +113,7 @@ impl LayerEnv {
     /// let modified_env = layer_env.apply(TargetLifecycle::Build, &env);
     /// assert_eq!(env, modified_env);
     /// ```
+    #[must_use]
     pub fn new() -> Self {
         Self {
             all: LayerEnvDelta::new(),
@@ -143,6 +144,7 @@ impl LayerEnv {
     /// assert_eq!(modified_env.get("VAR").unwrap(), "foobar");
     /// assert_eq!(modified_env.get("VAR2").unwrap(), "previous-value");
     /// ```
+    #[must_use]
     pub fn apply(&self, target: TargetLifecycle, env: &Env) -> Env {
         let deltas = match target {
             TargetLifecycle::All => vec![&self.all],
@@ -243,6 +245,7 @@ impl LayerEnv {
     ///         ),
     /// );
     /// ```
+    #[must_use]
     pub fn chainable_insert(
         mut self,
         target: TargetLifecycle,

libcnb/src/lib.rs

@@ -11,9 +11,6 @@
 #![allow(clippy::module_name_repetitions)]
 // This lint triggers when both layer_dir and layers_dir are present which are quite common.
 #![allow(clippy::similar_names)]
-// Re-disable pedantic lints that are currently failing, until they are triaged and fixed/wontfixed.
-// https://github.com/Malax/libcnb.rs/issues/57
-#![allow(clippy::must_use_candidate)]
 
 pub mod build;
 pub mod detect;