Auto merge of #60630 - nnethercote:use-Symbol-more, r=petrochenkov

Use `Symbol` more

A `Symbol` can be equated with a string (e.g. `&str`). This involves a
TLS lookup to get the chars (and a Mutex lock in a parallel compiler)
and then a char-by-char comparison. This functionality is convenient but
avoids one of the main benefits of `Symbol`s, which is fast equality
comparisons.

This PR removes the `Symbol`/string equality operations, forcing a lot
of existing string occurrences to become `Symbol`s. Fortunately, these
are almost all static strings (many are attribute names) and we can add
static `Symbol`s as necessary, and very little extra interning occurs.
The benefits are (a) a slight speedup (possibly greater in a parallel
compiler), and (b) the code is a lot more principled about `Symbol` use.
The main downside is verbosity, particularly with more `use
syntax::symbol::symbols` items.

r? @Zoxc

3 files changed, 8 insertions, 6 deletions

diff --git a/src/librustc_codegen_utils/lib.rs b/src/librustc_codegen_utils/lib.rs
index 437515f1e9a..b4ba90c61f6 100644
--- a/src/librustc_codegen_utils/lib.rs
+++ b/src/librustc_codegen_utils/lib.rs
@@ -23,6 +23,7 @@ extern crate rustc;
 
 use rustc::ty::TyCtxt;
 use rustc::hir::def_id::LOCAL_CRATE;
+use syntax::symbol::sym;
 
 pub mod link;
 pub mod codegen_backend;
@@ -35,7 +36,7 @@ pub mod symbol_names_test;
 /// reporting an error.
 pub fn check_for_rustc_errors_attr(tcx: TyCtxt<'_, '_, '_>) {
     if let Some((def_id, _)) = tcx.entry_fn(LOCAL_CRATE) {
-        if tcx.has_attr(def_id, "rustc_error") {
+        if tcx.has_attr(def_id, sym::rustc_error) {
             tcx.sess.span_fatal(tcx.def_span(def_id), "compilation successful");
         }
     }
diff --git a/src/librustc_codegen_utils/link.rs b/src/librustc_codegen_utils/link.rs
index f3a1b219f8a..a2ac64fa7e0 100644
--- a/src/librustc_codegen_utils/link.rs
+++ b/src/librustc_codegen_utils/link.rs
@@ -2,6 +2,7 @@ use rustc::session::config::{self, OutputFilenames, Input, OutputType};
 use rustc::session::Session;
 use std::path::{Path, PathBuf};
 use syntax::{ast, attr};
+use syntax::symbol::sym;
 use syntax_pos::Span;
 
 pub fn out_filename(sess: &Session,
@@ -49,13 +50,13 @@ pub fn find_crate_name(sess: Option<&Session>,
     // as used. After doing this, however, we still prioritize a crate name from
     // the command line over one found in the #[crate_name] attribute. If we
     // find both we ensure that they're the same later on as well.
-    let attr_crate_name = attr::find_by_name(attrs, "crate_name")
+    let attr_crate_name = attr::find_by_name(attrs, sym::crate_name)
         .and_then(|at| at.value_str().map(|s| (at, s)));
 
     if let Some(sess) = sess {
         if let Some(ref s) = sess.opts.crate_name {
             if let Some((attr, name)) = attr_crate_name {
-                if name != &**s {
+                if name.as_str() != *s {
                     let msg = format!("--crate-name and #[crate_name] are \
                                        required to match, but `{}` != `{}`",
                                       s, name);
diff --git a/src/librustc_codegen_utils/symbol_names_test.rs b/src/librustc_codegen_utils/symbol_names_test.rs
index 6a2b6f1321b..27ae0b97e59 100644
--- a/src/librustc_codegen_utils/symbol_names_test.rs
+++ b/src/librustc_codegen_utils/symbol_names_test.rs
@@ -6,11 +6,11 @@
 
 use rustc::hir;
 use rustc::ty::TyCtxt;
-
 use rustc_mir::monomorphize::Instance;
+use syntax::symbol::{Symbol, sym};
 
-const SYMBOL_NAME: &'static str = "rustc_symbol_name";
-const DEF_PATH: &'static str = "rustc_def_path";
+const SYMBOL_NAME: Symbol = sym::rustc_symbol_name;
+const DEF_PATH: Symbol = sym::rustc_def_path;
 
 pub fn report_symbol_names<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     // if the `rustc_attrs` feature is not enabled, then the