Fix an ICE parsing a malformed attribute.

Fixes rust-lang#104620.

Author: nnethercote

compiler/rustc_ast/src/attr/mod.rs

@@ -618,9 +618,12 @@ impl MetaItemKind {
             }) => MetaItemKind::list_from_tokens(tokens.clone()),
             AttrArgs::Delimited(..) => None,
             AttrArgs::Eq(_, AttrArgsEq::Ast(expr)) => match expr.kind {
-                ast::ExprKind::Lit(token_lit) => Some(MetaItemKind::NameValue(
-                    Lit::from_token_lit(token_lit, expr.span).expect("token_lit in from_attr_args"),
-                )),
+                ast::ExprKind::Lit(token_lit) => {
+                    // Turn failures to `None`, we'll get parse errors elsewhere.
+                    Lit::from_token_lit(token_lit, expr.span)
+                        .ok()
+                        .map(|lit| MetaItemKind::NameValue(lit))
+                }
                 _ => None,
             },
             AttrArgs::Eq(_, AttrArgsEq::Hir(lit)) => Some(MetaItemKind::NameValue(lit.clone())),

src/test/ui/parser/issue-104620.rs

@@ -0,0 +1,4 @@
+#![feature(rustc_attrs)]
+
+#![rustc_dummy=5z] //~ ERROR unexpected expression: `5z`
+fn main() {}

src/test/ui/parser/issue-104620.stderr

@@ -0,0 +1,8 @@
+error: unexpected expression: `5z`
+  --> $DIR/issue-104620.rs:3:16
+   |
+LL | #![rustc_dummy=5z]
+   |                ^^
+
+error: aborting due to previous error
+