From 62187b12c202c1ada2569211da090d16950c3640 Mon Sep 17 00:00:00 2001
From: miam-miam100 <49870539+miam-miam100@users.noreply.github.com>
Date: Tue, 19 Jul 2022 22:25:26 +0100
Subject: [PATCH 1/2] Add diagnostic width span when '0$' is used as width.

---
 compiler/rustc_parse_format/src/lib.rs   |  3 ++-
 compiler/rustc_parse_format/src/tests.rs | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
index f6fa19030acb9..6e7553f5e496b 100644
--- a/compiler/rustc_parse_format/src/lib.rs
+++ b/compiler/rustc_parse_format/src/lib.rs
@@ -572,9 +572,10 @@ impl<'a> Parser<'a> {
             // '0' flag and then an ill-formatted format string with just a '$'
             // and no count, but this is better if we instead interpret this as
             // no '0' flag and '0$' as the width instead.
-            if self.consume('$') {
+            if let Some(end) = self.consume_pos('$') {
                 spec.width = CountIsParam(0);
                 havewidth = true;
+                spec.width_span = Some(self.to_span_index(end - 1).to(self.to_span_index(end + 1)));
             } else {
                 spec.flags |= 1 << (FlagSignAwareZeroPad as u32);
             }
diff --git a/compiler/rustc_parse_format/src/tests.rs b/compiler/rustc_parse_format/src/tests.rs
index c9667922ee7c3..9c305b4996a13 100644
--- a/compiler/rustc_parse_format/src/tests.rs
+++ b/compiler/rustc_parse_format/src/tests.rs
@@ -178,6 +178,23 @@ fn format_counts() {
             },
         })],
     );
+    same(
+        "{1:0$.10x}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(1),
+            format: FormatSpec {
+                fill: None,
+                align: AlignUnknown,
+                flags: 0,
+                precision: CountIs(10),
+                width: CountIsParam(0),
+                precision_span: None,
+                width_span: Some(InnerSpan::new(4, 6)),
+                ty: "x",
+                ty_span: None,
+            },
+        })],
+    );
     same(
         "{:.*x}",
         &[NextArgument(Argument {

From f8dfc4bf350bad5140a7d1e7a9030b91dbdc4ce4 Mon Sep 17 00:00:00 2001
From: miam-miam100 <49870539+miam-miam100@users.noreply.github.com>
Date: Wed, 20 Jul 2022 13:40:45 +0100
Subject: [PATCH 2/2] Fix off by one error and add ui test.

---
 compiler/rustc_builtin_macros/src/format.rs |  2 +-
 src/test/ui/fmt/ifmt-bad-arg.rs             |  3 +++
 src/test/ui/fmt/ifmt-bad-arg.stderr         | 15 +++++++++++++--
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 3b7e2102ffa9b..bce466921d0ff 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/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!(
diff --git a/src/test/ui/fmt/ifmt-bad-arg.rs b/src/test/ui/fmt/ifmt-bad-arg.rs
index b3e54ed32aac7..84f4cc7f4ccd8 100644
--- a/src/test/ui/fmt/ifmt-bad-arg.rs
+++ b/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.
diff --git a/src/test/ui/fmt/ifmt-bad-arg.stderr b/src/test/ui/fmt/ifmt-bad-arg.stderr
index d181fe14107e7..5439ee173985b 100644
--- a/src/test/ui/fmt/ifmt-bad-arg.stderr
+++ b/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`.