Rollup merge of rust-lang#54932 - ljedrz:a_handful_of_string_improvements, r=alexcrichton

A handful of random string-related improvements

- remove a few allocations in `errors/diagnostic`
- make `build_helper::gnu_target` return `Cow<str>`

Author: kennytm

src/build_helper/lib.rs

@@ -91,13 +91,13 @@ pub fn try_run_suppressed(cmd: &mut Command) -> bool {
     output.status.success()
 }
 
-pub fn gnu_target(target: &str) -> String {
+pub fn gnu_target(target: &str) -> &str {
     match target {
-        "i686-pc-windows-msvc" => "i686-pc-win32".to_string(),
-        "x86_64-pc-windows-msvc" => "x86_64-pc-win32".to_string(),
-        "i686-pc-windows-gnu" => "i686-w64-mingw32".to_string(),
-        "x86_64-pc-windows-gnu" => "x86_64-w64-mingw32".to_string(),
-        s => s.to_string(),
+        "i686-pc-windows-msvc" => "i686-pc-win32",
+        "x86_64-pc-windows-msvc" => "x86_64-pc-win32",
+        "i686-pc-windows-gnu" => "i686-w64-mingw32",
+        "x86_64-pc-windows-gnu" => "x86_64-w64-mingw32",
+        s => s,
     }
 }
 

src/librustc_errors/diagnostic.rs

@@ -76,9 +76,9 @@ pub enum StringPart {
 }
 
 impl StringPart {
-    pub fn content(&self) -> String {
+    pub fn content(&self) -> &str {
         match self {
-            &StringPart::Normal(ref s) | & StringPart::Highlighted(ref s) => s.to_owned()
+            &StringPart::Normal(ref s) | & StringPart::Highlighted(ref s) => s
         }
     }
 }
@@ -398,7 +398,7 @@ impl Diagnostic {
     }
 
     pub fn message(&self) -> String {
-        self.message.iter().map(|i| i.0.to_owned()).collect::<String>()
+        self.message.iter().map(|i| i.0.as_str()).collect::<String>()
     }
 
     pub fn styled_message(&self) -> &Vec<(String, Style)> {
@@ -448,7 +448,7 @@ impl Diagnostic {
 
 impl SubDiagnostic {
     pub fn message(&self) -> String {
-        self.message.iter().map(|i| i.0.to_owned()).collect::<String>()
+        self.message.iter().map(|i| i.0.as_str()).collect::<String>()
     }
 
     pub fn styled_message(&self) -> &Vec<(String, Style)> {