Rollup merge of rust-lang#87264 - mystor:expand_literal, r=petrochenkov

proc_macro: Add an expand_expr method to TokenStream

This feature is aimed at giving proc macros access to powers similar to those used by builtin macros such as `format_args!` or `concat!`. These macros are able to accept macros in place of string literal parameters, such as the format string, as they perform recursive macro expansion while being expanded.

This can be especially useful in many cases thanks to helper macros like `concat!`, `stringify!` and `include_str!` which are often used to construct string literals at compile-time in user code.

For now, this method only allows expanding macros which produce literals, although more expressions will be supported before the method is stabilized.

In earlier versions of this PR, this method exclusively returned `Literal`, and spans on returned literals were stripped of expansion context before being returned to be as conservative as possible about permission leakage. The method's naming has been generalized to eventually support arbitrary expressions, and the context stripping has been removed (rust-lang#87264 (comment)), which should allow for more general APIs like "format_args_implicits" (rust-lang#67984) to be supported as well.

## API Surface

```rust
impl TokenStream {
    pub fn expand_expr(&self) -> Result<Expr, ExpandError>;
}

#[non_exhaustive]
pub struct ExpandError;

impl Debug for ExpandError { ... }
impl Display for ExpandError { ... }
impl Error for ExpandError {}
impl !Send for ExpandError {}
impl !Sync for ExpandError {}
```

compiler/rustc_expand/src/proc_macro.rs

@@ -24,8 +24,9 @@ impl base::ProcMacro for BangProcMacro {
         span: Span,
         input: TokenStream,
     ) -> Result<TokenStream, ErrorReported> {
+        let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
         let server = proc_macro_server::Rustc::new(ecx);
-        self.client.run(&EXEC_STRATEGY, server, input, ecx.ecfg.proc_macro_backtrace).map_err(|e| {
+        self.client.run(&EXEC_STRATEGY, server, input, proc_macro_backtrace).map_err(|e| {
             let mut err = ecx.struct_span_err(span, "proc macro panicked");
             if let Some(s) = e.as_str() {
                 err.help(&format!("message: {}", s));
@@ -48,9 +49,10 @@ impl base::AttrProcMacro for AttrProcMacro {
         annotation: TokenStream,
         annotated: TokenStream,
     ) -> Result<TokenStream, ErrorReported> {
+        let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
         let server = proc_macro_server::Rustc::new(ecx);
         self.client
-            .run(&EXEC_STRATEGY, server, annotation, annotated, ecx.ecfg.proc_macro_backtrace)
+            .run(&EXEC_STRATEGY, server, annotation, annotated, proc_macro_backtrace)
             .map_err(|e| {
                 let mut err = ecx.struct_span_err(span, "custom attribute panicked");
                 if let Some(s) = e.as_str() {
@@ -97,19 +99,19 @@ impl MultiItemModifier for ProcMacroDerive {
             nt_to_tokenstream(&item, &ecx.sess.parse_sess, CanSynthesizeMissingTokens::No)
         };
 
+        let proc_macro_backtrace = ecx.ecfg.proc_macro_backtrace;
         let server = proc_macro_server::Rustc::new(ecx);
-        let stream =
-            match self.client.run(&EXEC_STRATEGY, server, input, ecx.ecfg.proc_macro_backtrace) {
-                Ok(stream) => stream,
-                Err(e) => {
-                    let mut err = ecx.struct_span_err(span, "proc-macro derive panicked");
-                    if let Some(s) = e.as_str() {
-                        err.help(&format!("message: {}", s));
-                    }
-                    err.emit();
-                    return ExpandResult::Ready(vec![]);
+        let stream = match self.client.run(&EXEC_STRATEGY, server, input, proc_macro_backtrace) {
+            Ok(stream) => stream,
+            Err(e) => {
+                let mut err = ecx.struct_span_err(span, "proc-macro derive panicked");
+                if let Some(s) = e.as_str() {
+                    err.help(&format!("message: {}", s));
                 }
-            };
+                err.emit();
+                return ExpandResult::Ready(vec![]);
+            }
+        };
 
         let error_count_before = ecx.sess.parse_sess.span_diagnostic.err_count();
         let mut parser =