Implement #507

[rust.git] / src / misc.rs

diff --git a/src/misc.rs b/src/misc.rs
index b5c3d0c514f109a3e680f8a31a5dee35cee2014b..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.
 ///
@@ -316,3 +316,87 @@ fn check_pat(&mut self, cx: &LateContext, pat: &Pat) {
         }
     }
 }
+
+
+/// **What it does:** This lint checks for the use of bindings with a single leading underscore
+///
+/// **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:** None
+///
+/// **Example**:
+/// ```
+/// let _x = 0;
+/// let y = _x + 1; // Here we are using `_x`, even though it has a leading underscore.
+///                 // We should rename `_x` to `x`
+/// ```
+declare_lint!(pub USED_UNDERSCORE_BINDING, Warn,
+              "using a binding which is prefixed with an underscore");
+
+#[derive(Copy, Clone)]
+pub struct UsedUnderscoreBinding;
+
+impl LintPass for UsedUnderscoreBinding {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(USED_UNDERSCORE_BINDING)
+    }
+}
+
+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()
+                                .expect("path should always have at least one segment")
+                                .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 bang macro
+                && is_used(cx, expr)
+            },
+            ExprField(_, spanned) => {
+                let name = spanned.node.as_str();
+                name.chars().next() == Some('_')
+                && name.chars().skip(1).next() != Some('_')
+            },
+            _ => false
+        };
+        if needs_lint {
+            cx.span_lint(USED_UNDERSCORE_BINDING, expr.span,
+                         "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,
+            }
+        })
+    })
+}