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

@@ -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 {

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

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 => {},
     }
 }