Use variable names directly in the format strings (#7906)

# Description

Lint: `clippy::uninlined_format_args`

More readable in most situations.
(May be slightly confusing for modifier format strings
https://doc.rust-lang.org/std/fmt/index.html#formatting-parameters)

Alternative to #7865

# User-Facing Changes

None intended

# Tests + Formatting

(Ran `cargo +stable clippy --fix --workspace -- -A clippy::all -D
clippy::uninlined_format_args` to achieve this. Depends on Rust `1.67`)

crates/nu-plugin/src/plugin/declaration.rs

@@ -63,7 +63,7 @@ impl Command for PluginDeclaration {
             let decl = engine_state.get_decl(call.decl_id);
             ShellError::GenericError(
                 format!("Unable to spawn plugin for {}", decl.name()),
-                format!("{}", err),
+                format!("{err}"),
                 Some(call.head),
                 None,
                 Vec::new(),
@@ -87,7 +87,7 @@ impl Command for PluginDeclaration {
                                 "Plugin {} can not handle the custom value {}",
                                 self.name, custom_value_name
                             ),
-                            format!("custom value {}", custom_value_name),
+                            format!("custom value {custom_value_name}"),
                             Some(span),
                             None,
                             Vec::new(),

crates/nu-plugin/src/plugin/mod.rs

@@ -130,11 +130,11 @@ pub fn get_signature(
                     format!("Can't find {prog_name}, please make sure that {prog_name} is in PATH.")
                 }
                 _ => {
-                    format!("Error spawning child process: {}", err)
+                    format!("Error spawning child process: {err}")
                 }
             },
             _ => {
-                format!("Error spawning child process: {}", err)
+                format!("Error spawning child process: {err}")
             }
         };
 
@@ -173,7 +173,7 @@ pub fn get_signature(
 
     match child.wait() {
         Ok(_) => Ok(signatures),
-        Err(err) => Err(ShellError::PluginFailedToLoad(format!("{}", err))),
+        Err(err) => Err(ShellError::PluginFailedToLoad(format!("{err}"))),
     }
 }
 
@@ -319,7 +319,7 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
             })
             .and_then(|_| {
                 let flags = get_flags_section(signature);
-                write!(help, "{}", flags)
+                write!(help, "{flags}")
             })
             .and_then(|_| writeln!(help, "\nParameters:"))
             .and_then(|_| {
@@ -360,11 +360,11 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
             .and_then(|_| writeln!(help, "======================"));
 
         if res.is_err() {
-            println!("{:?}", res)
+            println!("{res:?}")
         }
     });
 
-    println!("{}", help)
+    println!("{help}")
 }
 
 pub fn get_plugin_encoding(child_stdout: &mut ChildStdout) -> Result<EncodingType, ShellError> {

crates/nu-plugin/src/protocol/mod.rs

@@ -60,13 +60,13 @@ impl From<ShellError> for LabeledError {
                 LabeledError { label, msg, span }
             }
             ShellError::CantConvert(expected, input, span, _help) => LabeledError {
-                label: format!("Can't convert to {}", expected),
-                msg: format!("can't convert {} to {}", expected, input),
+                label: format!("Can't convert to {expected}"),
+                msg: format!("can't convert {expected} to {input}"),
                 span: Some(span),
             },
             ShellError::DidYouMean(suggestion, span) => LabeledError {
                 label: "Name not found".into(),
-                msg: format!("did you mean '{}'", suggestion),
+                msg: format!("did you mean '{suggestion}'"),
                 span: Some(span),
             },
             ShellError::PluginFailedToLoad(msg) => LabeledError {

crates/nu-plugin/src/protocol/plugin_custom_value.rs

@@ -56,7 +56,7 @@ impl CustomValue for PluginCustomValue {
                     "Unable to spawn plugin for {} to get base value",
                     self.source
                 ),
-                format!("{}", err),
+                format!("{err}"),
                 Some(span),
                 None,
                 Vec::new(),
@@ -85,7 +85,7 @@ impl CustomValue for PluginCustomValue {
                     "Unable to decode call for {} to get base value",
                     self.source
                 ),
-                format!("{}", err),
+                format!("{err}"),
                 Some(span),
                 None,
                 Vec::new(),