On `format!()` arg count mismatch provide extra info

When positional width and precision formatting flags are present in a
formatting string that has an argument count mismatch, provide extra
information pointing at them making it easiser to understand where the
problem may lay:

```
error: 4 positional arguments in format string, but there are 3 arguments
  --> $DIR/ifmt-bad-arg.rs:78:15
   |
LL |     println!("{} {:.*} {}", 1, 3.2, 4);
   |               ^^ ^^--^ ^^      --- this parameter corresponds to the precision flag
   |                    |
   |                    this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html

error: 4 positional arguments in format string, but there are 3 arguments
  --> $DIR/ifmt-bad-arg.rs:81:15
   |
LL |     println!("{} {:07$.*} {}", 1, 3.2, 4);
   |               ^^ ^^-----^ ^^      --- this parameter corresponds to the precision flag
   |                    |  |
   |                    |  this precision flag adds an extra required argument at position 1, which is why there are 4 arguments expected
   |                    this width flag expects an `usize` argument at position 7, but there are 3 arguments
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html

error: 3 positional arguments in format string, but there are 3 arguments
  --> $DIR/ifmt-bad-arg.rs:84:15
   |
LL |     println!("{} {:07$} {}", 1, 3.2, 4);
   |               ^^ ^^---^ ^^
   |                    |
   |                    this width flag expects an `usize` argument at position 7, but there are 3 arguments
   |
   = note: positional arguments are zero-based
   = note: for information about formatting flags, visit https://doc.rust-lang.org/std/fmt/index.html
```

1 files changed, 92 insertions, 15 deletions

diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index e53660b6568..aa8fc70d806 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -109,6 +109,8 @@ struct Context<'a, 'b> {
     invalid_refs: Vec<(usize, usize)>,
     /// Spans of all the formatting arguments, in order.
     arg_spans: Vec<Span>,
+    /// All the formatting arguments that have formatting flags set, in order for diagnostics.
+    arg_with_formatting: Vec<parse::FormatSpec<'a>>,
     /// Whether this formatting string is a literal or it comes from a macro.
     is_literal: bool,
 }
@@ -279,14 +281,20 @@ impl<'a, 'b> Context<'a, 'b> {
             .iter()
             .map(|(r, pos)| (r.to_string(), self.arg_spans.get(*pos)));
 
+        let mut zero_based_note = false;
+
         if self.names.is_empty() && !numbered_position_args {
+            let count = self.pieces.len() + self.arg_with_formatting
+                .iter()
+                .filter(|fmt| fmt.precision_span.is_some())
+                .count();
             e = self.ecx.mut_span_err(
                 sp,
                 &format!(
                     "{} positional argument{} in format string, but {}",
-                         self.pieces.len(),
-                         if self.pieces.len() > 1 { "s" } else { "" },
-                    self.describe_num_args()
+                    count,
+                    if count > 1 { "s" } else { "" },
+                    self.describe_num_args(),
                 ),
             );
         } else {
@@ -317,9 +325,70 @@ impl<'a, 'b> Context<'a, 'b> {
                 &format!("invalid reference to positional {} ({})",
                          arg_list,
                          self.describe_num_args()));
-            e.note("positional arguments are zero-based");
+            zero_based_note = true;
         };
 
+        for fmt in &self.arg_with_formatting {
+            if let Some(span) = fmt.precision_span {
+                let span = self.fmtsp.from_inner(span);
+                match fmt.precision {
+                    parse::CountIsParam(pos) if pos > self.args.len() => {
+                        e.span_label(span, &format!(
+                            "this precision flag expects an `usize` argument at position {}, \
+                             but {}",
+                            pos,
+                            self.describe_num_args(),
+                        ));
+                        zero_based_note = true;
+                    }
+                    parse::CountIsParam(pos) => {
+                        let count = self.pieces.len() + self.arg_with_formatting
+                            .iter()
+                            .filter(|fmt| fmt.precision_span.is_some())
+                            .count();
+                        e.span_label(span, &format!(
+                            "this precision flag adds an extra required argument at position {}, \
+                             which is why there {} expected",
+                            pos,
+                            if count == 1 {
+                                "is 1 argument".to_string()
+                            } else {
+                                format!("are {} arguments", count)
+                            },
+                        ));
+                        e.span_label(
+                            self.args[pos].span,
+                            "this parameter corresponds to the precision flag",
+                        );
+                        zero_based_note = true;
+                    }
+                    _ => {}
+                }
+            }
+            if let Some(span) = fmt.width_span {
+                let span = self.fmtsp.from_inner(span);
+                match fmt.width {
+                    parse::CountIsParam(pos) if pos > self.args.len() => {
+                        e.span_label(span, &format!(
+                            "this width flag expects an `usize` argument at position {}, \
+                             but {}",
+                            pos,
+                            self.describe_num_args(),
+                        ));
+                        zero_based_note = true;
+                    }
+                    _ => {}
+                }
+            }
+        }
+        if zero_based_note {
+            e.note("positional arguments are zero-based");
+        }
+        if !self.arg_with_formatting.is_empty() {
+            e.note("for information about formatting flags, visit \
+                    https://doc.rust-lang.org/std/fmt/index.html");
+        }
+
         e.emit();
     }
 
@@ -435,10 +504,11 @@ impl<'a, 'b> Context<'a, 'b> {
 
     /// Builds a static `rt::Argument` from a `parse::Piece` or append
     /// to the `literal` string.
-    fn build_piece(&mut self,
-                   piece: &parse::Piece<'_>,
-                   arg_index_consumed: &mut Vec<usize>)
-                   -> Option<P<ast::Expr>> {
+    fn build_piece(
+        &mut self,
+        piece: &parse::Piece<'a>,
+        arg_index_consumed: &mut Vec<usize>,
+    ) -> Option<P<ast::Expr>> {
         let sp = self.macsp;
         match *piece {
             parse::String(s) => {
@@ -496,7 +566,9 @@ impl<'a, 'b> Context<'a, 'b> {
                         align: parse::AlignUnknown,
                         flags: 0,
                         precision: parse::CountImplied,
+                        precision_span: None,
                         width: parse::CountImplied,
+                        width_span: None,
                         ty: arg.format.ty,
                     },
                 };
@@ -506,6 +578,9 @@ impl<'a, 'b> Context<'a, 'b> {
                 let pos_simple =
                     arg.position.index() == simple_arg.position.index();
 
+                if arg.format.precision_span.is_some() || arg.format.width_span.is_some() {
+                    self.arg_with_formatting.push(arg.format); //'liself.fmtsp.from_inner(span));
+                }
                 if !pos_simple || arg.format != simple_arg.format || fill != ' ' {
                     self.all_pieces_simple = false;
                 }
@@ -530,7 +605,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 let path = self.ecx.path_global(sp, Context::rtpath(self.ecx, "FormatSpec"));
                 let fmt = self.ecx.expr_struct(
                     sp,
-                                         path,
+                    path,
                     vec![
                         self.ecx.field_imm(sp, self.ecx.ident_of("fill"), fill),
                         self.ecx.field_imm(sp, self.ecx.ident_of("align"), align),
@@ -657,12 +732,13 @@ impl<'a, 'b> Context<'a, 'b> {
         self.ecx.expr_call_global(self.macsp, path, fn_args)
     }
 
-    fn format_arg(ecx: &ExtCtxt<'_>,
-                  macsp: Span,
-                  mut sp: Span,
-                  ty: &ArgumentType,
-                  arg: ast::Ident)
-                  -> P<ast::Expr> {
+    fn format_arg(
+        ecx: &ExtCtxt<'_>,
+        macsp: Span,
+        mut sp: Span,
+        ty: &ArgumentType,
+        arg: ast::Ident,
+    ) -> P<ast::Expr> {
         sp = sp.apply_mark(ecx.current_expansion.id);
         let arg = ecx.expr_ident(sp, arg);
         let trait_ = match *ty {
@@ -941,6 +1017,7 @@ pub fn expand_preparsed_format_args(
         fmtsp: fmt.span,
         invalid_refs: Vec::new(),
         arg_spans,
+        arg_with_formatting: Vec::new(),
         is_literal,
     };