Optimize simple literals for Display::fmt

Compiler is unable to generate as efficient code for `write!(f, "text")` as it does for `f.write_str("text")`.  This PR checks if the `#[error("text")]` uses a simple string literal without the `{` and `}` characters, and without arguments, and uses `write_str` if so.

impl/src/attr.rs

@@ -196,8 +196,24 @@ impl ToTokens for Display<'_> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let fmt = &self.fmt;
         let args = &self.args;
-        tokens.extend(quote! {
-            ::core::write!(__formatter, #fmt #args)
+
+        // Currently compiler is unable to generate as efficient code for
+        //    write!(f, "text")   as it does for   f.write_str("text"),
+        // so handle it here when the literal string has no braces/no args.
+        let mut use_write_str = self.args.is_empty();
+        if use_write_str {
+            let fmt = self.fmt.value();
+            use_write_str = !fmt.contains('{') && !fmt.contains('}');
+        }
+
+        tokens.extend(if use_write_str {
+            quote! {
+                __formatter.write_str(#fmt)
+            }
+        } else {
+            quote! {
+                ::core::write!(__formatter, #fmt #args)
+            }
         });
     }
 }

tests/test_display.rs

@@ -301,3 +301,59 @@ fn test_keyword() {
 
     assert("error: 1", Error);
 }
+
+#[test]
+fn test_str_special_chars() {
+    #[derive(Error, Debug)]
+    pub enum Error {
+        #[error("text")]
+        Text,
+        #[error("braces {{}}")]
+        Braces,
+        #[error("braces2 \x7B\x7B\x7D\x7D")]
+        Braces2,
+        #[error("braces3 \u{7B}\u{7B}\u{7D}\u{7D}")]
+        Braces3,
+        #[error(
+            "new_\
+line"
+        )]
+        NewLine,
+        #[error("escape24 \u{78}")]
+        Escape24,
+    }
+
+    assert("text", Error::Text);
+    assert("braces {}", Error::Braces);
+    assert("braces2 {}", Error::Braces2);
+    assert("braces3 {}", Error::Braces3);
+    assert("new_line", Error::NewLine);
+    assert("escape24 x", Error::Escape24);
+}
+
+#[test]
+fn test_raw_str() {
+    #[derive(Error, Debug)]
+    pub enum Error {
+        #[error(r#"raw_text"#)]
+        Text,
+        #[error(r#"raw_braces {{}}"#)]
+        Braces,
+        #[error(r#"raw_braces2 \x7B\x7D"#)]
+        Braces2,
+        #[error(
+            r#"raw_new_\
+line"#
+        )]
+        NewLine,
+    }
+
+    assert(r#"raw_text"#, Error::Text);
+    assert(r#"raw_braces {}"#, Error::Braces);
+    assert(r#"raw_braces2 \x7B\x7D"#, Error::Braces2);
+    assert(
+        r#"raw_new_\
+line"#,
+        Error::NewLine,
+    );
+}