Implement checks for meta-variables in macros

src/librustc/lint/builtin.rs

@@ -353,6 +353,12 @@ 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! {
@@ -439,6 +445,7 @@ 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;
@@ -81,6 +81,7 @@ 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`.