Auto merge of #62008 - ia0:issues_61053, r=petrochenkov

Add meta-variable checks in macro definitions

This is an implementation of #61053. It is not sound (some errors are not reported) and not complete (reports may not be actual errors). This is due to the possibility to define macros in macros in indirect ways. See module documentation of `macro_check` for more details.

What remains to be done:
- [x] Migrate from an error to an allow-by-default lint.
- [x] Add more comments in particular for the handling of nested macros.
- [x] Add more tests if needed.
- [x] Try to avoid cloning too much (one idea is to use lists on the stack).
- [ ] Run crater with deny-by-default lint (measure rate of false positives).
    - [ ] Remove extra commit for deny-by-default lint
- [x] Create a PR to remove the old `question_mark_macro_sep` lint #62160

src/librustc/lint/builtin.rs

@@ -362,6 +362,12 @@ pub mod parser {
         Warn,
         "ill-formed attribute inputs that were previously accepted and used in practice"
     }
+
+    declare_lint! {
+        pub META_VARIABLE_MISUSE,
+        Allow,
+        "possible meta-variable misuse at macro definition"
+    }
 }
 
 declare_lint! {
@@ -448,6 +454,7 @@ declare_lint_pass! {
         MACRO_USE_EXTERN_CRATE,
         MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
         parser::ILL_FORMED_ATTRIBUTE_INPUT,
+        parser::META_VARIABLE_MISUSE,
         DEPRECATED_IN_FUTURE,
         AMBIGUOUS_ASSOCIATED_ITEMS,
         NESTED_IMPL_TRAIT,

src/librustc/lint/mod.rs

@@ -27,7 +27,7 @@ use crate::hir::def_id::{CrateNum, LOCAL_CRATE};
 use crate::hir::intravisit;
 use crate::hir;
 use crate::lint::builtin::BuiltinLintDiagnostics;
-use crate::lint::builtin::parser::ILL_FORMED_ATTRIBUTE_INPUT;
+use crate::lint::builtin::parser::{ILL_FORMED_ATTRIBUTE_INPUT, META_VARIABLE_MISUSE};
 use crate::session::{Session, DiagnosticMessageId};
 use crate::ty::TyCtxt;
 use crate::ty::query::Providers;
@@ -82,6 +82,7 @@ impl Lint {
     pub fn from_parser_lint_id(lint_id: BufferedEarlyLintId) -> &'static Self {
         match lint_id {
             BufferedEarlyLintId::IllFormedAttributeInput => ILL_FORMED_ATTRIBUTE_INPUT,
+            BufferedEarlyLintId::MetaVariableMisuse => META_VARIABLE_MISUSE,
         }
     }
 

src/libsyntax/early_buffered_lints.rs

@@ -10,6 +10,7 @@ use syntax_pos::MultiSpan;
 /// passed to `rustc::lint::Lint::from_parser_lint_id` to get a `rustc::lint::Lint`.
 pub enum BufferedEarlyLintId {
     IllFormedAttributeInput,
+    MetaVariableMisuse,
 }
 
 /// Stores buffered lint info which can later be passed to `librustc`.