Implement #507

[rust.git] / src / misc.rs

diff --git a/src/misc.rs b/src/misc.rs
index 3b093027f481943ee137a343d237b5f79a0b3ef5..139dfde36815a71e82e1d2ba39445ff15482cc24 100644 (file)
--- a/src/misc.rs
+++ b/src/misc.rs
@@ -3,15 +3,15 @@
 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;
 use rustc::middle::const_eval::eval_const_expr_partial;
 use rustc::middle::const_eval::EvalHint::ExprTypeChecked;
 
-use utils::{get_item_name, match_path, snippet, span_lint, walk_ptrs_ty, is_integer_literal};
-use utils::span_help_and_lint;
+use utils::{get_item_name, match_path, snippet, get_parent_expr, span_lint};
+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.
 ///
@@ -323,16 +323,7 @@ fn check_pat(&mut self, cx: &LateContext, pat: &Pat) {
 /// **Why is this bad?** A single leading underscore is usually used to indicate that a binding
 /// will not be used. Using such a binding breaks this expectation.
 ///
-/// **Known problems:** This lint's idea of a "used" variable is not quite the same as in the
-/// built-in `unused_variables` lint. For example, in the following code
-/// ```
-/// fn foo(y: u32) -> u32) {
-///     let _x = 1;
-///     _x +=1;
-///     y
-/// }
-/// ```
-/// _x will trigger both the `unused_variables` lint and the `used_underscore_binding` lint.
+/// **Known problems:** None
 ///
 /// **Example**:
 /// ```
@@ -354,6 +345,9 @@ fn get_lints(&self) -> LintArray {
 
 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()
@@ -361,8 +355,8 @@ fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
                                 .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
-                && cx.tcx.def_map.borrow().contains_key(&expr.id) //local variable
+                && ident.name != ident.unhygienic_name //not in bang macro
+                && is_used(cx, expr)
             },
             ExprField(_, spanned) => {
                 let name = spanned.node.as_str();
@@ -373,8 +367,36 @@ fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
         };
         if needs_lint {
             cx.span_lint(USED_UNDERSCORE_BINDING, expr.span,
-                         "used binding which is prefixed with an underscore. A leading underscore\
+                         "used binding which is prefixed with an underscore. A leading underscore \
                           signals that a binding will not be used.");
         }
     }
 }
+
+/// 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 {
+            ExprAssign(_, ref rhs) => **rhs == *expr,
+            ExprAssignOp(_, _, ref rhs) => **rhs == *expr,
+            _ => is_used(cx, &parent)
+        }
+    }
+    else {
+        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,
+            }
+        })
+    })
+}