Fix symbol ordering for confusable idents detection.

Confusable idents detection uses a type `BTreeMap<Symbol, Span>`. This is
highly dubious given that `Symbol` doesn't guarantee a meaningful order. (In
practice, it currently gives an order that mostly matches source code order.)

As a result, changes in `Symbol` representation make the
`lint-confusable-idents.rs` test fail, because this error message:

> identifier pair considered confusable between `ｓ` and `s`

is changed to this:

> identifier pair considered confusable between `s` and `ｓ`

and the corresponding span pointers get swapped erroneously, leading to
an incorrect "previous identifier" label.

This commit sorts the relevant symbols by span before doing the checking,
which ensures that the ident that appears first in the code will be mentioned
first in the message. The commit also extends the test slightly to be more
thorough.

src/librustc_lint/non_ascii_idents.rs

@@ -58,6 +58,12 @@ impl EarlyLintPass for NonAsciiIdents {
 
         let mut has_non_ascii_idents = false;
         let symbols = cx.sess.parse_sess.symbol_gallery.symbols.lock();
+
+        // Sort by `Span` so that error messages make sense with respect to the
+        // order of identifier locations in the code.
+        let mut symbols: Vec<_> = symbols.iter().collect();
+        symbols.sort_by_key(|k| k.1);
+
         for (symbol, &sp) in symbols.iter() {
             let symbol_str = symbol.as_str();
             if symbol_str.is_ascii() {

src/librustc_session/parse.rs

@@ -13,7 +13,6 @@ use rustc_span::hygiene::ExpnId;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
 use rustc_span::{MultiSpan, Span, Symbol};
 
-use std::collections::BTreeMap;
 use std::path::PathBuf;
 use std::str;
 
@@ -64,7 +63,7 @@ impl GatedSpans {
 #[derive(Default)]
 pub struct SymbolGallery {
     /// All symbols occurred and their first occurrence span.
-    pub symbols: Lock<BTreeMap<Symbol, Span>>,
+    pub symbols: Lock<FxHashMap<Symbol, Span>>,
 }
 
 impl SymbolGallery {

src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.rs

@@ -3,9 +3,11 @@
 #![allow(uncommon_codepoints, non_upper_case_globals)]
 
 const ｓ: usize = 42;
+const s_s: usize = 42;
 
 fn main() {
     let s = "rust"; //~ ERROR identifier pair considered confusable
+    let ｓ_ｓ = "rust2"; //~ ERROR identifier pair considered confusable
     not_affected();
 }
 

src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.stderr

@@ -1,5 +1,5 @@
 error: identifier pair considered confusable between `ｓ` and `s`
-  --> $DIR/lint-confusable-idents.rs:8:9
+  --> $DIR/lint-confusable-idents.rs:9:9
    |
 LL | const ｓ: usize = 42;
    |       -- this is where the previous identifier occurred
@@ -13,5 +13,14 @@ note: the lint level is defined here
 LL | #![deny(confusable_idents)]
    |         ^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: identifier pair considered confusable between `s_s` and `ｓ_ｓ`
+  --> $DIR/lint-confusable-idents.rs:10:9
+   |
+LL | const s_s: usize = 42;
+   |       --- this is where the previous identifier occurred
+...
+LL |     let ｓ_ｓ = "rust2";
+   |         ^^^^^
+
+error: aborting due to 2 previous errors