From 540b52931aeeee7c72c73361118d07ab986cf048 Mon Sep 17 00:00:00 2001
From: Jarred Sumner <jarred@jarredsumner.com>
Date: Tue, 8 Jun 2021 20:42:29 -0700
Subject: [PATCH] Improve error message when std.fmt.format is missing
 arguments

Use fmt in fmt so the number in the error message is fmt'd
---
 doc/langref.html.in     | 2 +-
 lib/std/fmt.zig         | 7 ++++++-
 test/compile_errors.zig | 8 ++++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/doc/langref.html.in b/doc/langref.html.in
index c84885074e..2378d0f0ba 100644
--- a/doc/langref.html.in
+++ b/doc/langref.html.in
@@ -6262,7 +6262,7 @@ pub fn printValue(self: *Writer, value: anytype) !void {
       <p>
       And now, what happens if we give too many arguments to {#syntax#}printf{#endsyntax#}?
       </p>
-      {#code_begin|test_err|Unused arguments#}
+      {#code_begin|test_err|Unused argument in "here is a string: '{s}' here is a number: {}#}
 const print = @import("std").debug.print;
 
 const a_number: i32 = 1234;
diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index 07c076a248..81b505c952 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -359,7 +359,12 @@ pub fn format(
     }
 
     if (comptime arg_state.hasUnusedArgs()) {
-        @compileError("Unused arguments");
+        const missing_count = arg_state.args_len - @popCount(ArgSetType, arg_state.used_args);
+        switch (missing_count) {
+            0 => unreachable,
+            1 => @compileError("Unused argument in \"" ++ fmt ++ "\""),
+            else => @compileError((comptime comptimePrint("{d}", .{missing_count})) ++ " unused arguments in \"" ++ fmt ++ "\""),
+        }
     }
 }
 
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index f0ab776eb6..65c7de6dec 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -2,6 +2,14 @@ const tests = @import("tests.zig");
 const std = @import("std");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add("std.fmt error for unused arguments",
+        \\pub fn main() !void {
+        \\    @import("std").debug.print("{d} {d} {d} {d} {d}", .{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15});
+        \\}
+    , &.{
+        \\error: 10 unused arguments in "{d} {d} {d} {d} {d}"
+    });
+
     cases.add("lazy pointer with undefined element type",
         \\export fn foo() void {
         \\    comptime var T: type = undefined;