Rollup merge of #61615 - petrochenkov:errlit, r=matklad

syntax: Treat error literals in more principled way

Free them from their character literal origins.

I actually tried to remove `LitKind::Err` entirely (by converting it into `ExprKind::Err` immediately), and it caused no diagnostic regressions in the test suite.
However, I'd still want to use error literals as general purpose error tokens some day, so I kept them.

The downside of having `LitKind::Err` in addition to `ExprKind::Err` is that every time you want to do something with `ExprKind::Err` you need to make sure that `ExprKind::Lit(LitKind::Err)` is treated in the same way.
Fortunately, this usually happens automatically because both literals and errors are "leaf" expressions, however this PR does fix a couple of inconsistencies between them.

Addresses https://github.com/rust-lang/rust/pull/60679#discussion_r282640663 in a way

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libsyntax/parse/literal.rs b/src/libsyntax/parse/literal.rs
index 7d5356ffe4d..467ad6ccfbe 100644
--- a/src/libsyntax/parse/literal.rs
+++ b/src/libsyntax/parse/literal.rs
@@ -311,7 +311,11 @@ impl<'a> Parser<'a> {
                 let (lit, span) = (token.expect_lit(), token.span);
                 self.bump();
                 err.report(&self.sess.span_diagnostic, lit, span);
-                let lit = token::Lit::new(token::Err, lit.symbol, lit.suffix);
+                // Pack possible quotes and prefixes from the original literal into
+                // the error literal's symbol so they can be pretty-printed faithfully.
+                let suffixless_lit = token::Lit::new(lit.kind, lit.symbol, None);
+                let symbol = Symbol::intern(&pprust::literal_to_string(suffixless_lit));
+                let lit = token::Lit::new(token::Err, symbol, lit.suffix);
                 Lit::from_lit_token(lit, span).map_err(|_| unreachable!())
             }
         }