make panictry! private to libsyntax

This commit completely removes usage of the `panictry!` macro from
outside libsyntax. The macro causes parse errors to be fatal, so using
it in libsyntax_ext caused parse failures *within* a syntax extension to
be fatal, which is probably not intended.

Furthermore, this commit adds spans to diagnostics emitted by empty
extensions if they were missing, à la #56491.

src/libsyntax_ext/assert.rs

@@ -1,9 +1,11 @@
-use syntax::ast::*;
+use errors::DiagnosticBuilder;
+use syntax::ast::{self, *};
 use syntax::source_map::Spanned;
 use syntax::ext::base::*;
 use syntax::ext::build::AstBuilder;
 use syntax::parse::token;
 use syntax::print::pprust;
+use syntax::ptr::P;
 use syntax::symbol::Symbol;
 use syntax::tokenstream::{TokenStream, TokenTree};
 use syntax_pos::{Span, DUMMY_SP};
@@ -13,33 +15,18 @@ pub fn expand_assert<'cx>(
     sp: Span,
     tts: &[TokenTree],
 ) -> Box<dyn MacResult + 'cx> {
-    let mut parser = cx.new_parser_from_tts(tts);
-
-    if parser.token == token::Eof {
-        cx.struct_span_err(sp, "macro requires a boolean expression as an argument")
-            .span_label(sp, "boolean expression required")
-            .emit();
-        return DummyResult::expr(sp);
-    }
-
-    let cond_expr = panictry!(parser.parse_expr());
-    let custom_msg_args = if parser.eat(&token::Comma) {
-        let ts = parser.parse_tokens();
-        if !ts.is_empty() {
-            Some(ts)
-        } else {
-            None
+    let Assert { cond_expr, custom_message } = match parse_assert(cx, sp, tts) {
+        Ok(assert) => assert,
+        Err(mut err) => {
+            err.emit();
+            return DummyResult::expr(sp);
         }
-    } else {
-        None
     };
 
     let sp = sp.apply_mark(cx.current_expansion.mark);
     let panic_call = Mac_ {
         path: Path::from_ident(Ident::new(Symbol::intern("panic"), sp)),
-        tts: if let Some(ts) = custom_msg_args {
-            ts.into()
-        } else {
+        tts: custom_message.unwrap_or_else(|| {
             TokenStream::from(TokenTree::Token(
                 DUMMY_SP,
                 token::Literal(
@@ -49,8 +36,8 @@ pub fn expand_assert<'cx>(
                     ))),
                     None,
                 ),
-            )).into()
-        },
+            ))
+        }).into(),
         delim: MacDelimiter::Parenthesis,
     };
     let if_expr = cx.expr_if(
@@ -67,3 +54,36 @@ pub fn expand_assert<'cx>(
     );
     MacEager::expr(if_expr)
 }
+
+struct Assert {
+    cond_expr: P<ast::Expr>,
+    custom_message: Option<TokenStream>,
+}
+
+fn parse_assert<'a>(
+    cx: &mut ExtCtxt<'a>,
+    sp: Span,
+    tts: &[TokenTree]
+) -> Result<Assert, DiagnosticBuilder<'a>> {
+    let mut parser = cx.new_parser_from_tts(tts);
+
+    if parser.token == token::Eof {
+        let mut err = cx.struct_span_err(sp, "macro requires a boolean expression as an argument");
+        err.span_label(sp, "boolean expression required");
+        return Err(err);
+    }
+
+    Ok(Assert {
+        cond_expr: parser.parse_expr()?,
+        custom_message: if parser.eat(&token::Comma) {
+            let ts = parser.parse_tokens();
+            if !ts.is_empty() {
+                Some(ts)
+            } else {
+                None
+            }
+        } else {
+            None
+        },
+    })
+}