Simplify the current_rustc_version macro.

It currently has the syntax
`current_rustc_version!(env!("CFG_RELEASE"))` where the
`env!("CFG_RELEASE")` part looks like a normal expression but it is
actually parsed and processed by the `current_rustc_version` macro.

The documented rationale for this is that you'll find it if you grep for
`env!("CFG_RELEASE")`. But I think that's of very little use -- I would
personally grep for just "CFG_RELEASE" -- and it complicates the macro,
requiring the use of `syn`.

This commit simplifies the macro.

compiler/rustc_macros/src/current_version.rs

@@ -1,37 +1,16 @@
 use proc_macro::TokenStream;
 use proc_macro2::Span;
 use quote::quote;
-use syn::parse::{Parse, ParseStream};
-use syn::{parenthesized, parse_macro_input, LitStr, Token};
 
-pub struct Input {
-    variable: LitStr,
-}
-
-mod kw {
-    syn::custom_keyword!(env);
-}
-
-impl Parse for Input {
-    // Input syntax is `env!("CFG_RELEASE")` to facilitate grepping.
-    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
-        let paren;
-        input.parse::<kw::env>()?;
-        input.parse::<Token![!]>()?;
-        parenthesized!(paren in input);
-        let variable: LitStr = paren.parse()?;
-        Ok(Input { variable })
-    }
-}
-
-pub(crate) fn current_version(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as Input);
-
-    TokenStream::from(match RustcVersion::parse_env_var(&input.variable) {
+pub(crate) fn current_version(_input: TokenStream) -> TokenStream {
+    let env_var = "CFG_RELEASE";
+    TokenStream::from(match RustcVersion::parse_cfg_release(env_var) {
         Ok(RustcVersion { major, minor, patch }) => quote!(
+            // The produced literal has type `rustc_session::RustcVersion`.
             Self { major: #major, minor: #minor, patch: #patch }
         ),
-        Err(err) => syn::Error::new(Span::call_site(), err).into_compile_error(),
+        Err(err) => syn::Error::new(Span::call_site(), format!("{env_var} env var: {err}"))
+            .into_compile_error(),
     })
 }
 
@@ -42,8 +21,8 @@ struct RustcVersion {
 }
 
 impl RustcVersion {
-    fn parse_env_var(env_var: &LitStr) -> Result<Self, Box<dyn std::error::Error>> {
-        let value = proc_macro::tracked_env::var(env_var.value())?;
+    fn parse_cfg_release(env_var: &str) -> Result<Self, Box<dyn std::error::Error>> {
+        let value = proc_macro::tracked_env::var(env_var)?;
         Self::parse_str(&value)
             .ok_or_else(|| format!("failed to parse rustc version: {:?}", value).into())
     }