Simplify macro generating ToString implementations for &…&str

Use deref coercion to let the compiler remove any amount of references.

Author: tbu-

library/alloc/src/string.rs

@@ -2949,52 +2949,39 @@ impl SpecToString for i8 {
     }
 }
 
-// Generic/generated code can sometimes have multiple, nested references
-// for strings, including `&&&str`s that would never be written
-// by hand. This macro generates twelve layers of nested `&`-impl
-// for primitive strings.
-#[cfg(not(no_global_oom_handling))]
-macro_rules! to_string_str_wrap_in_ref {
-    {x $($x:ident)*} => {
-        &to_string_str_wrap_in_ref! { $($x)* }
-    };
-    {} => { str };
-}
-#[cfg(not(no_global_oom_handling))]
-macro_rules! to_string_expr_wrap_in_deref {
-    {$self:expr ; x $($x:ident)*} => {
-        *(to_string_expr_wrap_in_deref! { $self ; $($x)* })
-    };
-    {$self:expr ;} => { $self };
-}
 #[cfg(not(no_global_oom_handling))]
 macro_rules! to_string_str {
-    {$($($x:ident)*),+} => {
+    {$($type:ty,)*} => {
         $(
-            impl SpecToString for to_string_str_wrap_in_ref!($($x)*) {
+            impl SpecToString for $type {
                 #[inline]
                 fn spec_to_string(&self) -> String {
-                    String::from(to_string_expr_wrap_in_deref!(self ; $($x)*))
+                    let s: &str = self;
+                    String::from(s)
                 }
             }
-        )+
+        )*
     };
 }
 
 #[cfg(not(no_global_oom_handling))]
 to_string_str! {
-    x x x x x x x x x x x x,
-    x x x x x x x x x x x,
-    x x x x x x x x x x,
-    x x x x x x x x x,
-    x x x x x x x x,
-    x x x x x x x,
-    x x x x x x,
-    x x x x x,
-    x x x x,
-    x x x,
-    x x,
-    x,
+    // Generic/generated code can sometimes have multiple, nested references
+    // for strings, including `&&&str`s that would never be written
+    // by hand.
+    &&&&&&&&&&&&str,
+    &&&&&&&&&&&str,
+    &&&&&&&&&&str,
+    &&&&&&&&&str,
+    &&&&&&&&str,
+    &&&&&&&str,
+    &&&&&&str,
+    &&&&&str,
+    &&&&str,
+    &&&str,
+    &&str,
+    &str,
+    str,
 }
 
 #[cfg(not(no_global_oom_handling))]