Merge pull request #202 from birkenfeld/unit_cmp

new lint: comparing unit types (fixes #201)

README.md

@@ -45,6 +45,7 @@ string_add           | allow   | using `x + ..` where x is a `String`; suggests
 string_add_assign    | allow   | using `x = x + ..` where x is a `String`; suggests using `push_str()` instead
 string_to_string     | warn    | calling `String.to_string()` which is a no-op
 toplevel_ref_arg     | warn    | a function argument is declared `ref` (i.e. `fn foo(ref x: u8)`, but not `fn foo((ref x, ref y): (u8, u8))`)
+unit_cmp             | warn    | comparing unit values (which is always `true` or `false`, respectively)
 zero_width_space     | deny    | using a zero-width space in a string literal, which is confusing
 
 To use, add the following lines to your Cargo.toml:

src/lib.rs

@@ -64,6 +64,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box returns::ReturnPass as LintPassObject);
     reg.register_lint_pass(box methods::MethodsPass as LintPassObject);
     reg.register_lint_pass(box types::LetPass as LintPassObject);
+    reg.register_lint_pass(box types::UnitCmp as LintPassObject);
     reg.register_lint_pass(box loops::LoopsPass as LintPassObject);
     reg.register_lint_pass(box lifetimes::LifetimePass as LintPassObject);
     reg.register_lint_pass(box ranges::StepByZero as LintPassObject);
@@ -105,6 +106,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         types::BOX_VEC,
         types::LET_UNIT_VALUE,
         types::LINKEDLIST,
+        types::UNIT_CMP,
         unicode::NON_ASCII_LITERAL,
         unicode::ZERO_WIDTH_SPACE,
     ]);

src/types.rs

@@ -1,6 +1,7 @@
 use rustc::lint::*;
 use syntax::ast;
 use syntax::ast::*;
+use syntax::ast_util::{is_comparison_binop, binop_to_string};
 use syntax::ptr::P;
 use rustc::middle::ty;
 use syntax::codemap::ExpnInfo;
@@ -107,3 +108,31 @@ impl LintPass for LetPass {
             |info| check_let_unit(cx, decl, info));
     }
 }
+
+declare_lint!(pub UNIT_CMP, Warn,
+              "comparing unit values (which is always `true` or `false`, respectively)");
+
+#[allow(missing_copy_implementations)]
+pub struct UnitCmp;
+
+impl LintPass for UnitCmp {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(UNIT_CMP)
+    }
+
+    fn check_expr(&mut self, cx: &Context, expr: &Expr) {
+        if let ExprBinary(ref cmp, ref left, _) = expr.node {
+            let op = cmp.node;
+            let sty = &cx.tcx.expr_ty(left).sty;
+            if *sty == ty::TyTuple(vec![]) && is_comparison_binop(op) {
+                let result = match op {
+                    BiEq | BiLe | BiGe => "true",
+                    _ => "false"
+                };
+                span_lint(cx, UNIT_CMP, expr.span, &format!(
+                    "{}-comparison of unit values detected. This will always be {}",
+                    binop_to_string(op), result));
+            }
+        }
+    }
+}

tests/compile-fail/unit_cmp.rs

@@ -0,0 +1,17 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(unit_cmp)]
+
+fn main() {
+    // this is fine
+    if true == false {
+    }
+
+    // this warns
+    if { true; } == { false; } {  //~ERROR ==-comparison of unit values detected. This will always be true
+    }
+
+    if { true; } > { false; } {  //~ERROR >-comparison of unit values detected. This will always be false
+    }
+}