Implemented '-W type-limits' (#3833)

src/rustc/middle/lint.rs

@@ -54,6 +54,7 @@ enum lint {
     deprecated_pattern,
     non_camel_case_types,
     structural_records,
+	type_limits,
 
     managed_heap_memory,
     owned_heap_memory,
@@ -186,6 +187,11 @@ fn get_lint_dict() -> lint_dict {
            desc: ~"allow legacy modes",
            default: forbid}),
 
+		(~"type_limits",
+		 @{lint: type_limits,
+		   desc: ~"comparisons made useless by limits of the types involved",
+		   default: warn})
+
         /* FIXME(#3266)--make liveness warnings lintable
         (~"unused_variable",
          @{lint: unused_variable,
@@ -397,6 +403,7 @@ fn check_item(i: @ast::item, cx: ty::ctxt) {
     check_item_heap(cx, i);
     check_item_structural_records(cx, i);
     check_item_deprecated_modes(cx, i);
+	check_item_type_limits(cx, i);
 }
 
 // Take a visitor, and modify it so that it will not proceed past subitems.
@@ -430,6 +437,108 @@ fn check_item_while_true(cx: ty::ctxt, it: @ast::item) {
     visit::visit_item(it, (), visit);
 }
 
+fn check_item_type_limits(cx: ty::ctxt, it: @ast::item) {
+	pure fn is_valid<T: cmp::Ord>(binop: ast::binop, v: T, min: T, max: T) -> bool {
+		match binop {
+			ast::lt => v <= max,
+			ast::le => v < max,
+			ast::gt => v >= min,
+			ast::ge => v > min,
+			ast::eq | ast::ne => v >= min && v <= max,
+			_ => fail
+		}
+	}
+
+	pure fn rev_binop(binop: ast::binop) -> ast::binop {
+		match binop {
+			ast::lt => ast::ge,
+			ast::le => ast::gt,
+			ast::gt => ast::le,
+			ast::ge => ast::lt,
+			_ => binop
+		}
+	}
+
+	fn check_limits(cx: ty::ctxt, binop: ast::binop, l: &ast::expr,
+					r: &ast::expr) -> bool {
+		let (lit, expr, swap) = match (l.node, r.node) {
+			(ast::expr_lit(_), _) => (l, r, true),
+			(_, ast::expr_lit(_)) => (r, l, false),
+			_ => return true
+		};
+		// Normalize the binop so that the literal is always on the RHS in
+		// the comparison
+		let norm_binop = if (swap) {
+			rev_binop(binop)
+		} else {
+			binop
+		};
+		match ty::get(ty::expr_ty(cx, @*expr)).sty {
+			ty::ty_int(int_ty) => {
+				let (min, max): (i64, i64) = match int_ty {
+					ast::ty_i =>	(int::min_value as i64,	int::max_value as i64),
+					ast::ty_char =>	(u32::min_value as i64,	u32::max_value as i64),
+					ast::ty_i8 =>	(i8::min_value  as i64,	i8::max_value  as i64),
+					ast::ty_i16 =>	(i16::min_value as i64,	i16::max_value as i64),
+					ast::ty_i32 =>	(i32::min_value as i64,	i32::max_value as i64),
+					ast::ty_i64 =>	(i64::min_value, 		i64::max_value)
+				};
+				let lit_val: i64 = match lit.node {
+					ast::expr_lit(@li) => match li.node {
+						ast::lit_int(v, _) => v,
+						ast::lit_uint(v, _) => v as i64,
+						ast::lit_int_unsuffixed(v) => v,
+						_ => return true
+					},
+					_ => fail
+				};
+				is_valid(norm_binop, lit_val, min, max)
+			}
+			ty::ty_uint(uint_ty) => {
+				let (min, max): (u64, u64) = match uint_ty {
+					ast::ty_u =>	(uint::min_value as u64, uint::max_value as u64),
+					ast::ty_u8 =>	(u8::min_value   as u64, u8::max_value   as u64),
+					ast::ty_u16 =>	(u16::min_value  as u64, u16::max_value  as u64),
+					ast::ty_u32 =>	(u32::min_value  as u64, u32::max_value  as u64),
+					ast::ty_u64 =>	(u64::min_value, 		 u64::max_value)
+				};
+				let lit_val: u64 = match lit.node {
+					ast::expr_lit(@li) => match li.node {
+						ast::lit_int(v, _) => v as u64,
+						ast::lit_uint(v, _) => v,
+						ast::lit_int_unsuffixed(v) => v as u64,
+						_ => return true
+					},
+					_ => fail
+				};
+				is_valid(norm_binop, lit_val, min, max)
+			}
+			_ => true
+		}
+	}
+
+	let visit = item_stopping_visitor(visit::mk_simple_visitor(@{
+		visit_expr: fn@(e: @ast::expr) {
+			match e.node {
+				ast::expr_binary(binop, @l, @r) => {
+					match binop {
+						ast::eq | ast::lt | ast::le | ast::ne | ast::ge | ast::gt => {
+							if !check_limits(cx, binop, &l, &r) {
+								cx.sess.span_lint(type_limits, e.id, it.id, e.span,
+									~"comparison is useless due to type limits");
+							}
+						}
+						_ => ()
+					}
+				}
+				_ => ()
+			}
+		},
+		.. *visit::default_simple_visitor()
+	}));
+	visit::visit_item(it, (), visit);
+}
+
 fn check_item_structural_records(cx: ty::ctxt, it: @ast::item) {
     let visit = item_stopping_visitor(visit::mk_simple_visitor(@{
         visit_expr: fn@(e: @ast::expr) {