diff --git a/src/check_expr.cpp b/src/check_expr.cpp
index c4d7ab8d9..52b731efb 100644
--- a/src/check_expr.cpp
+++ b/src/check_expr.cpp
@@ -8204,14 +8204,69 @@ gb_internal CallArgumentData check_call_arguments_proc_group(CheckerContext *c,
 			}
 		}
 
+		// NOTE(bill): Missing `&`: a container argument passed by value where an overload wants a pointer
+		// to it. Detected only when the element-mismatch diagnosis did not fire. The `&x` suggestion is
+		// still printed below by the existing try-address block.
+		Ast *addr_expr = nullptr;
+		isize addr_index = -1;
+		if (diag_expr == nullptr) {
+			for (Entity *proc : procs) {
+				Type *t = base_type(proc->type);
+				if (t == nullptr || t->kind != Type_Proc || t->Proc.params == nullptr) {
+					continue;
+				}
+				isize n = gb_min(cast(isize)t->Proc.param_count, positional_operands.count);
+				for (isize i = 0; i < n; i++) {
+					Operand src = positional_operands[i];
+					if (src.deferred_untyped_arg || src.mode == Addressing_Invalid || src.type == nullptr) {
+						continue;
+					}
+					Type *dst = t->Proc.params->Tuple.variables[i]->type;
+					if (check_is_assignable_to(c, &src, dst)) {
+						continue;
+					}
+					if (check_is_assignable_to(c, &src, type_deref(dst))) {
+						addr_expr  = src.expr;
+						addr_index = i;
+						break;
+					}
+				}
+				if (addr_expr != nullptr) {
+					break;
+				}
+			}
+		}
+
 		if (diag_expr != nullptr) {
 			gbString es = type_to_string(diag_elem);
-			error(diag_expr, "'%s' expected argument #%td to be of type '%s', got '%s'", expr_name, diag_index+1, es, diag_arg_type);
 			gbString cs = type_to_string(diag_cont);
-			error_line("\t'%s' is the element type of '%s'\n", es, cs);
+			gbString label = gb_string_make(heap_allocator(), "");
+			label = gb_string_append_fmt(label, "expected '%s', found '%s'", es, diag_arg_type);
+			set_caret_label(label);
+			// secondary span under the container argument, explaining where the element type comes from
+			Ast *cont_expr = positional_operands[0].expr;
+			gbString sec = gb_string_make(heap_allocator(), "");
+			sec = gb_string_append_fmt(sec, "'%s', elements are '%s'", cs, es);
+			if (cont_expr != nullptr) {
+				set_caret_secondary(ast_token(cont_expr).pos, ast_end_pos(cont_expr), sec);
+			}
+			error(diag_expr, "mismatched argument #%td in call to '%s'", diag_index+1, expr_name);
+			gb_string_free(sec);
+			gb_string_free(label);
 			gb_string_free(cs);
 			gb_string_free(es);
 			gb_string_free(diag_arg_type);
+		} else if (addr_expr != nullptr) {
+			Operand src = positional_operands[addr_index];
+			gbString ft = type_to_string(src.type);
+			gbString ex = expr_to_string(src.expr);
+			gbString label = gb_string_make(heap_allocator(), "");
+			label = gb_string_append_fmt(label, "expected '^%s', found '%s', pass '&%s'", ft, ft, ex);
+			set_caret_label_vertical(label);
+			error(src.expr, "mismatched argument #%td in call to '%s'", addr_index+1, expr_name);
+			gb_string_free(label);
+			gb_string_free(ex);
+			gb_string_free(ft);
 		} else {
 			error(operand->expr, "No procedures or ambiguous call for procedure group '%s' that match with the given arguments", expr_name);
 			if (positional_operands.count == 0 && named_operands.count == 0) {
diff --git a/src/error.cpp b/src/error.cpp
index 3f7cb4aae..77fef3cea 100644
--- a/src/error.cpp
+++ b/src/error.cpp
@@ -47,6 +47,41 @@ gb_internal bool is_error_muted(void) {
 	return global_error_mute_depth > 0;
 }
 
+// Optional annotations for the next errored source line.
+// All per-thread, so nothing leaks between concurrently-checked files.
+// `show_error_on_line` snapshots and clears them.
+//
+//   primary label, inline:                    primary label, vertical:      secondary span (always vertical):
+//     append(&x, true)                          append(x, {1,2,3})             append(&x, true)
+//                ^~~^ expected '…', found '…'          ^                              ^^  ^~~^ expected '…', found '…'
+//                                                      |                              |
+//                                                      expected '^…', pass '&x'       '[dynamic][3]int', elements are '[3]int'
+gb_thread_local char     global_caret_label_buf[512];      // primary label text
+gb_thread_local bool     global_caret_label_vertical;      // render primary label as | + text below, not inline
+gb_thread_local bool     global_caret_sec_present;         // a secondary span (to the left) is set
+gb_thread_local TokenPos global_caret_sec_pos;
+gb_thread_local TokenPos global_caret_sec_end;
+gb_thread_local char     global_caret_sec_label[512];
+
+gb_internal void set_caret_label(char const *text) {
+	global_caret_label_vertical = false;
+	if (text == nullptr) {
+		global_caret_label_buf[0] = 0;
+	} else {
+		gb_snprintf(global_caret_label_buf, gb_size_of(global_caret_label_buf), "%s", text);
+	}
+}
+gb_internal void set_caret_label_vertical(char const *text) {
+	set_caret_label(text);
+	global_caret_label_vertical = true;
+}
+gb_internal void set_caret_secondary(TokenPos pos, TokenPos end, char const *text) {
+	global_caret_sec_present = text != nullptr;
+	global_caret_sec_pos = pos;
+	global_caret_sec_end = end;
+	gb_snprintf(global_caret_sec_label, gb_size_of(global_caret_sec_label), "%s", text ? text : "");
+}
+
 
 gb_internal void push_error_value(TokenPos const &pos, ErrorValueKind kind = ErrorValue_Error) {
 	GB_ASSERT_MSG(global_error_collector.curr_error_value_set.load() == false, "Possible race condition in error handling system, please report this with an issue");
@@ -308,6 +343,22 @@ gb_internal void terminal_reset_colours(void) {
 
 gb_internal isize show_error_on_line(TokenPos const &pos, TokenPos end) {
 	get_error_value()->end = end;
+
+	// Consume the caret annotations now, so they are cleared even on an early return and can never leak
+	// onto a later error's line.
+	char caret_label[512];
+	gb_snprintf(caret_label, gb_size_of(caret_label), "%s", global_caret_label_buf);
+	bool     caret_label_vertical = global_caret_label_vertical;
+	bool     sec_present = global_caret_sec_present;
+	TokenPos sec_pos = global_caret_sec_pos;
+	TokenPos sec_end = global_caret_sec_end;
+	char sec_label[512];
+	gb_snprintf(sec_label, gb_size_of(sec_label), "%s", global_caret_sec_label);
+	global_caret_label_buf[0] = 0;
+	global_caret_label_vertical = false;
+	global_caret_sec_present = false;
+	global_caret_sec_label[0] = 0;
+
 	if (!show_error_line()) {
 		return -1;
 	}
@@ -512,13 +563,57 @@ gb_internal isize show_error_on_line(TokenPos const &pos, TokenPos end) {
 		error_out(" ...");
 	}
 
-	error_out("\n\t");
-
-	for (i32 i = squiggle_padding; i > 0; i -= 1) {
-		error_out(" ");
+	// Secondary span (drawn to the left of the primary on the same line): compute its padding/width the
+	// same way as the primary's, and only draw it if it sits fully to the left within the window.
+	i32 sec_pad = 0;
+	i32 sec_len = 0;
+	bool draw_sec = sec_present && sec_pos.line == pos.line && sec_end.line == pos.line;
+	if (draw_sec) {
+		i32 sec_start_byte = error_start_index_bytes + (sec_pos.column - pos.column);
+		i32 sec_end_byte   = error_start_index_bytes + (sec_end.column - pos.column);
+		if (window_open_bytes > 0) {
+			sec_pad += 4;
+		}
+		for (i32 i = 0; i < line_length_graphemes; i += 1) {
+			if (graphemes[i].byte_index < window_open_bytes)  continue;
+			if (graphemes[i].byte_index >= sec_start_byte)    break;
+			sec_pad += graphemes[i].width;
+		}
+		for (i32 i = 0; i < line_length_graphemes; i += 1) {
+			if (graphemes[i].byte_index < sec_start_byte) continue;
+			if (graphemes[i].byte_index >= sec_end_byte)  break;
+			sec_len += graphemes[i].width;
+		}
+		if (sec_len < 1) {
+			sec_len = 1;
+		}
+		if (sec_start_byte >= error_start_index_bytes || sec_pad + sec_len > squiggle_padding) {
+			draw_sec = false; // overlaps or is not to the left; skip rather than misalign
+		}
 	}
 
+	error_out("\n\t");
+
+	i32 printed = 0;
 	terminal_set_colours(TerminalStyle_Bold, TerminalColour_Green);
+	if (draw_sec) {
+		for (i32 i = sec_pad - printed; i > 0; i -= 1) {
+			error_out(" ");
+		}
+		printed = sec_pad;
+		error_out("^");
+		for (i32 k = sec_len - 2; k > 0; k -= 1) {
+			error_out("~");
+		}
+		if (sec_len >= 2) {
+			error_out("^");
+		}
+		printed += sec_len;
+	}
+
+	for (i32 i = squiggle_padding - printed; i > 0; i -= 1) {
+		error_out(" ");
+	}
 
 	if (squiggle_length > 0) {
 		error_out("^");
@@ -535,12 +630,46 @@ gb_internal isize show_error_on_line(TokenPos const &pos, TokenPos end) {
 		}
 	}
 
+	if (caret_label[0] != 0 && !caret_label_vertical) {
+		terminal_set_colours(TerminalStyle_Normal, TerminalColour_Grey);
+		error_out(" %s", caret_label);
+	}
+
 	// NOTE(Feoramund): Specifically print a newline, then reset colours,
 	// instead of the other way around. Otherwise the printing mechanism
 	// will collapse the newline for reasons currently beyond my ken.
 	error_out("\n");
 	terminal_reset_colours();
 
+	// A vertical label hangs under its span: `|` then the text, aligned to the span's column. The
+	// secondary span takes priority (its label explains the primary); otherwise a vertical primary label.
+	i32 vpad = -1;
+	char const *vtext = nullptr;
+	if (draw_sec && sec_label[0] != 0) {
+		vpad = sec_pad;
+		vtext = sec_label;
+	} else if (caret_label_vertical && caret_label[0] != 0) {
+		vpad = squiggle_padding;
+		vtext = caret_label;
+	}
+	if (vpad >= 0 && vtext != nullptr && vtext[0] != 0) {
+		error_out("\t");
+		for (i32 i = vpad; i > 0; i -= 1) {
+			error_out(" ");
+		}
+		terminal_set_colours(TerminalStyle_Bold, TerminalColour_Green);
+		error_out("|\n");
+		terminal_reset_colours();
+
+		error_out("\t");
+		for (i32 i = vpad; i > 0; i -= 1) {
+			error_out(" ");
+		}
+		terminal_set_colours(TerminalStyle_Normal, TerminalColour_Grey);
+		error_out("%s\n", vtext);
+		terminal_reset_colours();
+	}
+
 	return squiggle_padding;
 }