Implement Display Trait instead of ToString Trait

This ToString trait is automatically implemented for any type
which implements the Display trait. As such, ToString shouldn’t
be implemented directly: Display should be implemented instead,
and you get the ToString implementation for free. With such fixing,
It will make CI with lint-clippy happy.

Fixes #165

Signed-off-by: Alex Lyn <[email protected]>

.rustfmt.toml

@@ -14,7 +14,7 @@ reorder_imports = true
 reorder_modules = true
 remove_nested_parens = true
 match_arm_leading_pipes = "Never"
-fn_args_layout = "Tall"
+fn_params_layout = "Tall"
 edition = "2018"
 merge_derives = true
 use_try_shorthand = false

src/image/config.rs

@@ -10,6 +10,7 @@ use serde::{ser::SerializeMap, Deserialize, Deserializer, Serialize, Serializer}
 use std::collections::BTreeMap;
 use std::{
     collections::HashMap,
+    fmt::Display,
     io::{Read, Write},
     path::Path,
 };
@@ -250,14 +251,19 @@ impl ImageConfiguration {
     }
 }
 
-/// Implement `ToString` directly since we cannot avoid twice memory allocation
-/// when using auto-implementaion through `Display`.
-impl ToString for ImageConfiguration {
-    fn to_string(&self) -> String {
+/// This ToString trait is automatically implemented for any type which implements the Display trait.
+/// As such, ToString shouldn’t be implemented directly: Display should be implemented instead,
+/// and you get the ToString implementation for free.
+impl Display for ImageConfiguration {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         // Serde seralization never fails since this is
         // a combination of String and enums.
-        self.to_string_pretty()
-            .expect("ImageConfiguration JSON convertion failed")
+        write!(
+            f,
+            "{}",
+            self.to_string_pretty()
+                .expect("ImageConfiguration JSON convertion failed")
+        )
     }
 }
 

src/image/index.rs

@@ -8,6 +8,7 @@ use getset::{CopyGetters, Getters, Setters};
 use serde::{Deserialize, Serialize};
 use std::{
     collections::HashMap,
+    fmt::Display,
     io::{Read, Write},
     path::Path,
 };
@@ -210,14 +211,19 @@ impl Default for ImageIndex {
     }
 }
 
-/// Implement `ToString` directly since we cannot avoid twice memory allocation
-/// when using auto-implementaion through `Display`.
-impl ToString for ImageIndex {
-    fn to_string(&self) -> String {
+/// This ToString trait is automatically implemented for any type which implements the Display trait.
+/// As such, ToString shouldn’t be implemented directly: Display should be implemented instead,
+/// and you get the ToString implementation for free.
+impl Display for ImageIndex {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         // Serde seralization never fails since this is
         // a combination of String and enums.
-        self.to_string_pretty()
-            .expect("ImageIndex to JSON convertion failed")
+        write!(
+            f,
+            "{}",
+            self.to_string_pretty()
+                .expect("ImageIndex to JSON convertion failed")
+        )
     }
 }
 

src/image/manifest.rs

@@ -8,6 +8,7 @@ use getset::{CopyGetters, Getters, MutGetters, Setters};
 use serde::{Deserialize, Serialize};
 use std::{
     collections::HashMap,
+    fmt::Display,
     io::{Read, Write},
     path::Path,
 };
@@ -221,14 +222,19 @@ impl ImageManifest {
     }
 }
 
-/// Implement `ToString` directly since we cannot avoid twice memory allocation
-/// when using auto-implementaion through `Display`.
-impl ToString for ImageManifest {
-    fn to_string(&self) -> String {
+/// This ToString trait is automatically implemented for any type which implements the Display trait.
+/// As such, ToString shouldn’t be implemented directly: Display should be implemented instead,
+/// and you get the ToString implementation for free.
+impl Display for ImageManifest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         // Serde seralization never fails since this is
         // a combination of String and enums.
-        self.to_string_pretty()
-            .expect("ImageManifest to JSON convertion failed")
+        write!(
+            f,
+            "{}",
+            self.to_string_pretty()
+                .expect("ImageManifest to JSON convertion failed")
+        )
     }
 }
 

src/runtime/linux.rs

@@ -3,7 +3,7 @@ use crate::error::{oci_error, OciSpecError};
 use derive_builder::Builder;
 use getset::{CopyGetters, Getters, MutGetters, Setters};
 use serde::{Deserialize, Serialize};
-use std::{collections::HashMap, path::PathBuf, vec};
+use std::{collections::HashMap, fmt::Display, path::PathBuf, vec};
 
 #[derive(
     Builder, Clone, Debug, Deserialize, Eq, Getters, MutGetters, Setters, PartialEq, Serialize,
@@ -282,8 +282,11 @@ pub struct LinuxDeviceCgroup {
     access: Option<String>,
 }
 
-impl ToString for LinuxDeviceCgroup {
-    fn to_string(&self) -> String {
+/// This ToString trait is automatically implemented for any type which implements the Display trait.
+/// As such, ToString shouldn’t be implemented directly: Display should be implemented instead,
+/// and you get the ToString implementation for free.
+impl Display for LinuxDeviceCgroup {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let major = self
             .major
             .map(|mj| mj.to_string())
@@ -293,7 +296,8 @@ impl ToString for LinuxDeviceCgroup {
             .map(|mi| mi.to_string())
             .unwrap_or_else(|| "*".to_string());
         let access = self.access.as_deref().unwrap_or("");
-        format!(
+        write!(
+            f,
             "{} {}:{} {}",
             &self.typ.unwrap_or_default().as_str(),
             &major,
@@ -644,9 +648,14 @@ pub struct LinuxInterfacePriority {
     priority: u32,
 }
 
-impl ToString for LinuxInterfacePriority {
-    fn to_string(&self) -> String {
-        format!("{} {}\n", self.name, self.priority)
+/// This ToString trait is automatically implemented for any type which implements the Display trait.
+/// As such, ToString shouldn’t be implemented directly: Display should be implemented instead,
+/// and you get the ToString implementation for free.
+impl Display for LinuxInterfacePriority {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // Serde seralization never fails since this is
+        // a combination of String and enums.
+        writeln!(f, "{} {}", self.name, self.priority)
     }
 }