Implement #507

Make `used_underscore_binding` lint compatible with MacroAttributes
expansions. TODO: Add a good test for this.

src/misc.rs

@@ -3,7 +3,7 @@ use syntax::ptr::P;
 use rustc_front::hir::*;
 use reexport::*;
 use rustc_front::util::{is_comparison_binop, binop_to_string};
-use syntax::codemap::{Span, Spanned};
+use syntax::codemap::{Span, Spanned, ExpnFormat};
 use rustc_front::intravisit::FnKind;
 use rustc::middle::ty;
 use rustc::middle::const_eval::ConstVal::Float;
@@ -11,7 +11,7 @@ use rustc::middle::const_eval::eval_const_expr_partial;
 use rustc::middle::const_eval::EvalHint::ExprTypeChecked;
 
 use utils::{get_item_name, match_path, snippet, get_parent_expr, span_lint};
-use utils::{span_help_and_lint, in_external_macro, walk_ptrs_ty, is_integer_literal};
+use utils::{span_help_and_lint, walk_ptrs_ty, is_integer_literal};
 
 /// **What it does:** This lint checks for function arguments and let bindings denoted as `ref`. It is `Warn` by default.
 ///
@@ -345,6 +345,9 @@ impl LintPass for UsedUnderscoreBinding {
 
 impl LateLintPass for UsedUnderscoreBinding {
     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
+        if in_attributes_expansion(cx, expr) { // Don't lint things expanded by #[derive(...)], etc
+            return;
+        }
         let needs_lint = match expr.node {
             ExprPath(_, ref path) => {
                 let ident = path.segments.last()
@@ -352,7 +355,7 @@ impl LateLintPass for UsedUnderscoreBinding {
                                 .identifier;
                 ident.name.as_str().chars().next() == Some('_') //starts with '_'
                 && ident.name.as_str().chars().skip(1).next() != Some('_') //doesn't start with "__"
-                && ident.name != ident.unhygienic_name //not in macro
+                && ident.name != ident.unhygienic_name //not in bang macro
                 && is_used(cx, expr)
             },
             ExprField(_, spanned) => {
@@ -362,9 +365,6 @@ impl LateLintPass for UsedUnderscoreBinding {
             },
             _ => false
         };
-        if in_external_macro(cx, expr.span) {
-            return
-        }
         if needs_lint {
             cx.span_lint(USED_UNDERSCORE_BINDING, expr.span,
                          "used binding which is prefixed with an underscore. A leading underscore \
@@ -373,6 +373,8 @@ impl LateLintPass for UsedUnderscoreBinding {
     }
 }
 
+/// Heuristic to see if an expression is used. Should be compatible with `unused_variables`'s idea
+/// of what it means for an expression to be "used".
 fn is_used(cx: &LateContext, expr: &Expr) -> bool {
     if let Some(ref parent) = get_parent_expr(cx, expr) {
         match parent.node {
@@ -385,3 +387,16 @@ fn is_used(cx: &LateContext, expr: &Expr) -> bool {
         true
     }
 }
+
+/// Test whether an expression is in a macro expansion (e.g. something generated by #[derive(...)]
+/// or the like)
+fn in_attributes_expansion(cx: &LateContext, expr: &Expr) -> bool {
+    cx.sess().codemap().with_expn_info(expr.span.expn_id, |info_opt| {
+        info_opt.map_or(false, |info| {
+            match info.callee.format {
+                ExpnFormat::MacroAttribute(_) => true,
+                _ => false,
+            }
+        })
+    })
+}

tests/compile-fail/used_underscore_binding.rs

@@ -9,7 +9,15 @@ fn prefix_underscore(_foo: u32) -> u32 {
 
 /// Test that we lint even if the use is within a macro expansion
 fn in_macro(_foo: u32) {
-    println!("{}", _foo); // doesn't warn, nut should #507
+    println!("{}", _foo); //~ ERROR used binding which is prefixed with an underscore
+}
+
+// TODO: This doesn't actually correctly test this. Need to find a #[derive(...)] which sets off
+// the lint if the `in_attributes_expansion` test isn't there
+/// Test that we do not lint for unused underscores in a MacroAttribute expansion
+#[derive(Clone)]
+struct MacroAttributesTest {
+    _foo: u32,
 }
 
 // Struct for testing use of fields prefixed with an underscore
@@ -68,6 +76,7 @@ fn non_variables() {
 
 fn main() {
     let foo = 0u32;
+    let _ = MacroAttributesTest{_foo: 0};
     // tests of unused_underscore lint
     let _ = prefix_underscore(foo);
     in_macro(foo);