Remove the AsRef impl for SymbolStr.

Because it's highly magical, which goes against the goal of keeping
`SymbolStr` simple. Plus it's only used in a handful of places that
only require minor changes.

Author: nnethercote

Diff for: src/librustc_codegen_ssa/back/command.rs

@@ -53,7 +53,7 @@ impl Command {
     }
 
     pub fn sym_arg(&mut self, arg: Symbol) -> &mut Command {
-        self.arg(&arg.as_str());
+        self.arg(&*arg.as_str());
         self
     }
 

Diff for: src/libsyntax/parse/parser/module.rs

@@ -210,7 +210,7 @@ impl<'a> Parser<'a> {
             // `/` to `\`.
             #[cfg(windows)]
             let s = s.replace("/", "\\");
-            Some(dir_path.join(s))
+            Some(dir_path.join(&*s))
         } else {
             None
         }
@@ -314,7 +314,7 @@ impl<'a> Parser<'a> {
 
     fn push_directory(&mut self, id: Ident, attrs: &[Attribute]) {
         if let Some(path) = attr::first_attr_value_str_by_name(attrs, sym::path) {
-            self.directory.path.to_mut().push(&path.as_str());
+            self.directory.path.to_mut().push(&*path.as_str());
             self.directory.ownership = DirectoryOwnership::Owned { relative: None };
         } else {
             // We have to push on the current module name in the case of relative
@@ -325,10 +325,10 @@ impl<'a> Parser<'a> {
             // directory path to `/x/y/z`, not `/x/z` with a relative offset of `y`.
             if let DirectoryOwnership::Owned { relative } = &mut self.directory.ownership {
                 if let Some(ident) = relative.take() { // remove the relative offset
-                    self.directory.path.to_mut().push(ident.as_str());
+                    self.directory.path.to_mut().push(&*ident.as_str());
                 }
             }
-            self.directory.path.to_mut().push(&id.as_str());
+            self.directory.path.to_mut().push(&*id.as_str());
         }
     }
 }

Diff for: src/libsyntax_pos/symbol.rs

@@ -1099,16 +1099,6 @@ pub struct SymbolStr {
     string: &'static str,
 }
 
-impl<U: ?Sized> std::convert::AsRef<U> for SymbolStr
-where
-    str: std::convert::AsRef<U>
-{
-    #[inline]
-    fn as_ref(&self) -> &U {
-        self.string.as_ref()
-    }
-}
-
 // This impl allows a `SymbolStr` to be directly equated with a `String` or
 // `&str`.
 impl<T: std::ops::Deref<Target = str>> std::cmp::PartialEq<T> for SymbolStr {