Add support for $crate to Ident

Author: Daniel Bloom

library/proc_macro/src/bridge/symbol.rs

@@ -33,7 +33,7 @@ impl Symbol {
     /// Validates and normalizes before converting it to a symbol.
     pub(crate) fn new_ident(string: &str, is_raw: bool) -> Self {
         // Fast-path: check if this is a valid ASCII identifier
-        if Self::is_valid_ascii_ident(string.as_bytes()) {
+        if Self::is_valid_ascii_ident(string.as_bytes()) || string == "$crate" {
             if is_raw && !Self::can_be_raw(string) {
                 panic!("`{}` cannot be a raw identifier", string);
             }
@@ -79,7 +79,7 @@ impl Symbol {
     // Mimics the behavior of `Symbol::can_be_raw` from `rustc_span`
     fn can_be_raw(string: &str) -> bool {
         match string {
-            "_" | "super" | "self" | "Self" | "crate" => false,
+            "_" | "super" | "self" | "Self" | "crate" | "$crate" => false,
             _ => true,
         }
     }

tests/ui/proc-macro/auxiliary/mixed-site-span.rs

@@ -17,7 +17,11 @@ pub fn proc_macro_rules(input: TokenStream) -> TokenStream {
             TokenTree::from(single_quote),
             id("label_use"),
         ].iter().cloned().collect();
+        let dollar_crate = id("$crate");
         quote!(
+            use $dollar_crate::proc_macro_rules as _; // OK
+            type A = $dollar_crate::ItemUse; // ERROR
+
             struct $item_def;
             let $local_def = 0;
 
@@ -29,7 +33,8 @@ pub fn proc_macro_rules(input: TokenStream) -> TokenStream {
         let mut dollar_crate = input.into_iter().next().unwrap();
         dollar_crate.set_span(Span::mixed_site());
         quote!(
-            type A = $dollar_crate::ItemUse;
+            use $dollar_crate::proc_macro_rules as _; // OK
+            type A = $dollar_crate::ItemUse; // ERROR
         )
     }
 }

tests/ui/proc-macro/mixed-site-span.rs

@@ -11,7 +11,8 @@ fn main() {
     'label_use: loop {
         let local_use = 1;
         proc_macro_rules!();
-        //~^ ERROR use of undeclared label `'label_use`
+        //~^ ERROR cannot find type `ItemUse` in crate `$crate`
+        //~| ERROR use of undeclared label `'label_use`
         //~| ERROR cannot find value `local_use` in this scope
         ItemDef; // OK
         local_def; //~ ERROR cannot find value `local_def` in this scope

tests/ui/proc-macro/mixed-site-span.stderr

@@ -6,6 +6,18 @@ LL |         proc_macro_rules!();
    |
    = note: this error originates in the macro `proc_macro_rules` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error[E0412]: cannot find type `ItemUse` in crate `$crate`
+  --> $DIR/mixed-site-span.rs:13:9
+   |
+LL |         proc_macro_rules!();
+   |         ^^^^^^^^^^^^^^^^^^^ not found in `$crate`
+   |
+   = note: this error originates in the macro `proc_macro_rules` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider importing this struct
+   |
+LL + use ItemUse;
+   |
+
 error[E0425]: cannot find value `local_use` in this scope
   --> $DIR/mixed-site-span.rs:13:9
    |
@@ -15,20 +27,20 @@ LL |         proc_macro_rules!();
    = note: this error originates in the macro `proc_macro_rules` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `local_def` in this scope
-  --> $DIR/mixed-site-span.rs:17:9
+  --> $DIR/mixed-site-span.rs:18:9
    |
 LL |         local_def;
    |         ^^^^^^^^^ help: a local variable with a similar name exists: `local_use`
 
 error[E0412]: cannot find type `ItemUse` in crate `$crate`
-  --> $DIR/mixed-site-span.rs:24:1
+  --> $DIR/mixed-site-span.rs:25:1
    |
 LL | pass_dollar_crate!();
    | ^^^^^^^^^^^^^^^^^^^^ not found in `$crate`
    |
    = note: this error originates in the macro `proc_macro_rules` which comes from the expansion of the macro `pass_dollar_crate` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0412, E0425, E0426.
 For more information about an error, try `rustc --explain E0412`.