From e2509ddbe69a56bb1f4a56b46946b2a706d5aabe Mon Sep 17 00:00:00 2001
From: Veikka Tuominen <git@vexu.eu>
Date: Fri, 2 Dec 2022 20:16:47 +0200
Subject: [PATCH] AstGen: add error for invalid string comparisons

These operations are allowed because the string literals are just
pointers but they produce unexpected results. These errors prevent
beginners from shooting themselves in the foot while still allowing
advanced users to circumvent them if they desire to do so.

Closes #8290
---
 src/AstGen.zig                                | 13 +++++++++
 .../compile_errors/invalid_compare_string.zig | 29 +++++++++++++++++++
 test/cases/compile_errors/switch_on_slice.zig |  2 +-
 3 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 test/cases/compile_errors/invalid_compare_string.zig

diff --git a/src/AstGen.zig b/src/AstGen.zig
index b18d550347..1ec0cfa78f 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -5628,6 +5628,14 @@ fn simpleBinOp(
     const tree = astgen.tree;
     const node_datas = tree.nodes.items(.data);
 
+    if (op_inst_tag == .cmp_neq or op_inst_tag == .cmp_eq) {
+        const node_tags = tree.nodes.items(.tag);
+        const str = if (op_inst_tag == .cmp_eq) "==" else "!=";
+        if (node_tags[node_datas[node].lhs] == .string_literal or
+            node_tags[node_datas[node].rhs] == .string_literal)
+            return astgen.failNode(node, "cannot compare strings with {s}", .{str});
+    }
+
     const lhs = try reachableExpr(gz, scope, .{ .rl = .none }, node_datas[node].lhs, node);
     var line: u32 = undefined;
     var column: u32 = undefined;
@@ -6625,6 +6633,11 @@ fn switchExpr(
             continue;
         }
 
+        for (case.ast.values) |val| {
+            if (node_tags[val] == .string_literal)
+                return astgen.failNode(val, "cannot switch on strings", .{});
+        }
+
         if (case.ast.values.len == 1 and node_tags[case.ast.values[0]] != .switch_range) {
             scalar_cases_len += 1;
         } else {
diff --git a/test/cases/compile_errors/invalid_compare_string.zig b/test/cases/compile_errors/invalid_compare_string.zig
new file mode 100644
index 0000000000..a5c7f041a5
--- /dev/null
+++ b/test/cases/compile_errors/invalid_compare_string.zig
@@ -0,0 +1,29 @@
+comptime {
+    var a = "foo";
+    if (a == "foo") unreachable;
+}
+comptime {
+    var a = "foo";
+    if (a == ("foo")) unreachable; // intentionally allow
+}
+comptime {
+    var a = "foo";
+    switch (a) {
+        "foo" => unreachable,
+        else => {},
+    }
+}
+comptime {
+    var a = "foo";
+    switch (a) {
+        ("foo") => unreachable, // intentionally allow
+        else => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:11: error: cannot compare strings with ==
+// :12:9: error: cannot switch on strings
diff --git a/test/cases/compile_errors/switch_on_slice.zig b/test/cases/compile_errors/switch_on_slice.zig
index b4644b132c..c2b28f67ab 100644
--- a/test/cases/compile_errors/switch_on_slice.zig
+++ b/test/cases/compile_errors/switch_on_slice.zig
@@ -1,7 +1,7 @@
 pub export fn entry() void {
     var a: [:0]const u8 = "foo";
     switch (a) {
-        "--version", "version" => unreachable,
+        ("--version"), ("version") => unreachable,
         else => {},
     }
 }