Fix off by one error and add ui test.

compiler/rustc_builtin_macros/src/format.rs

@@ -485,7 +485,7 @@ impl<'a, 'b> Context<'a, 'b> {
             if let Some(span) = fmt.width_span {
                 let span = self.fmtsp.from_inner(InnerSpan::new(span.start, span.end));
                 match fmt.width {
-                    parse::CountIsParam(pos) if pos > self.num_args() => {
+                    parse::CountIsParam(pos) if pos >= self.num_args() => {
                         e.span_label(
                             span,
                             &format!(

src/test/ui/fmt/ifmt-bad-arg.rs

@@ -86,6 +86,9 @@ tenth number: {}",
     println!("{:foo}", 1); //~ ERROR unknown format trait `foo`
     println!("{5} {:4$} {6:7$}", 1);
     //~^ ERROR invalid reference to positional arguments 4, 5, 6 and 7 (there is 1 argument)
+    let foo = 1;
+    println!("{foo:0$}");
+    //~^ ERROR invalid reference to positional argument 0 (no arguments were given)
 
     // We used to ICE here because we tried to unconditionally access the first argument, which
     // doesn't exist.

src/test/ui/fmt/ifmt-bad-arg.stderr

@@ -251,8 +251,19 @@ LL |     println!("{5} {:4$} {6:7$}", 1);
    = note: positional arguments are zero-based
    = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html
 
+error: invalid reference to positional argument 0 (no arguments were given)
+  --> $DIR/ifmt-bad-arg.rs:90:15
+   |
+LL |     println!("{foo:0$}");
+   |               ^^^^^--^
+   |                    |
+   |                    this width flag expects an `usize` argument at position 0, but no arguments were given
+   |
+   = note: positional arguments are zero-based
+   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html
+
 error: 2 positional arguments in format string, but no arguments were given
-  --> $DIR/ifmt-bad-arg.rs:92:15
+  --> $DIR/ifmt-bad-arg.rs:95:15
    |
 LL |     println!("{:.*}");
    |               ^^--^
@@ -328,7 +339,7 @@ LL |     pub fn from_usize(x: &usize) -> ArgumentV1<'_> {
    |            ^^^^^^^^^^
    = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 36 previous errors
+error: aborting due to 37 previous errors
 
 Some errors have detailed explanations: E0308, E0425.
 For more information about an error, try `rustc --explain E0308`.