Introduce LocalInternedString::intern.

`LocalInternedString::intern(x)` is preferable to
`Symbol::intern(x).as_str()`, because the former involves one call to
`with_interner` while the latter involves two.

src/librustc/lint/context.rs

@@ -828,8 +828,8 @@ impl<'a, 'tcx> LateContext<'a, 'tcx> {
 
                 // This shouldn't ever be needed, but just in case:
                 Ok(vec![match trait_ref {
-                    Some(trait_ref) => Symbol::intern(&format!("{:?}", trait_ref)).as_str(),
-                    None => Symbol::intern(&format!("<{}>", self_ty)).as_str(),
+                    Some(trait_ref) => LocalInternedString::intern(&format!("{:?}", trait_ref)),
+                    None => LocalInternedString::intern(&format!("<{}>", self_ty)),
                 }])
             }
 
@@ -845,9 +845,10 @@ impl<'a, 'tcx> LateContext<'a, 'tcx> {
                 // This shouldn't ever be needed, but just in case:
                 path.push(match trait_ref {
                     Some(trait_ref) => {
-                        Symbol::intern(&format!("<impl {} for {}>", trait_ref, self_ty)).as_str()
+                        LocalInternedString::intern(&format!("<impl {} for {}>", trait_ref,
+                                                    self_ty))
                     },
-                    None => Symbol::intern(&format!("<impl {}>", self_ty)).as_str(),
+                    None => LocalInternedString::intern(&format!("<impl {}>", self_ty)),
                 });
 
                 Ok(path)

src/librustc_codegen_llvm/intrinsic.rs

@@ -20,7 +20,7 @@ use rustc::ty::layout::{self, LayoutOf, HasTyCtxt, Primitive};
 use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
 use rustc::hir;
 use syntax::ast::{self, FloatTy};
-use syntax::symbol::Symbol;
+use syntax::symbol::LocalInternedString;
 
 use rustc_codegen_ssa::traits::*;
 
@@ -213,7 +213,7 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
             }
             "type_name" => {
                 let tp_ty = substs.type_at(0);
-                let ty_name = Symbol::intern(&tp_ty.to_string()).as_str();
+                let ty_name = LocalInternedString::intern(&tp_ty.to_string());
                 self.const_str_slice(ty_name)
             }
             "type_id" => {

src/librustc_codegen_ssa/mir/block.rs

@@ -15,7 +15,7 @@ use crate::traits::*;
 
 use std::borrow::Cow;
 
-use syntax::symbol::Symbol;
+use syntax::symbol::LocalInternedString;
 use syntax_pos::Pos;
 
 use super::{FunctionCx, LocalRef};
@@ -401,7 +401,7 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
         // Get the location information.
         let loc = bx.sess().source_map().lookup_char_pos(span.lo());
-        let filename = Symbol::intern(&loc.file.name.to_string()).as_str();
+        let filename = LocalInternedString::intern(&loc.file.name.to_string());
         let line = bx.const_u32(loc.line as u32);
         let col = bx.const_u32(loc.col.to_usize() as u32 + 1);
 
@@ -423,7 +423,7 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             }
             _ => {
                 let str = msg.description();
-                let msg_str = Symbol::intern(str).as_str();
+                let msg_str = LocalInternedString::intern(str);
                 let msg_file_line_col = bx.static_panic_msg(
                     Some(msg_str),
                     filename,
@@ -535,15 +535,15 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             let layout = bx.layout_of(ty);
             if layout.abi.is_uninhabited() {
                 let loc = bx.sess().source_map().lookup_char_pos(span.lo());
-                let filename = Symbol::intern(&loc.file.name.to_string()).as_str();
+                let filename = LocalInternedString::intern(&loc.file.name.to_string());
                 let line = bx.const_u32(loc.line as u32);
                 let col = bx.const_u32(loc.col.to_usize() as u32 + 1);
 
                 let str = format!(
                     "Attempted to instantiate uninhabited type {}",
                     ty
                 );
-                let msg_str = Symbol::intern(&str).as_str();
+                let msg_str = LocalInternedString::intern(&str);
                 let msg_file_line_col = bx.static_panic_msg(
                     Some(msg_str),
                     filename,

src/libsyntax_pos/symbol.rs

@@ -1019,6 +1019,17 @@ pub struct LocalInternedString {
 }
 
 impl LocalInternedString {
+    /// Maps a string to its interned representation.
+    pub fn intern(string: &str) -> Self {
+        let string = with_interner(|interner| {
+            let symbol = interner.intern(string);
+            interner.strings[symbol.0.as_usize()]
+        });
+        LocalInternedString {
+            string: unsafe { std::mem::transmute::<&str, &str>(string) }
+        }
+    }
+
     pub fn as_interned_str(self) -> InternedString {
         InternedString {
             symbol: Symbol::intern(self.string)
@@ -1095,7 +1106,7 @@ impl fmt::Display for LocalInternedString {
 
 impl Decodable for LocalInternedString {
     fn decode<D: Decoder>(d: &mut D) -> Result<LocalInternedString, D::Error> {
-        Ok(Symbol::intern(&d.read_str()?).as_str())
+        Ok(LocalInternedString::intern(&d.read_str()?))
     }
 }