Improve position named arguments lint underline and formatting names

For named arguments used as implicit position arguments, underline both
the opening curly brace and either:
* if there is formatting, the next character (which will either be the
  closing curl brace or the `:` denoting the start of formatting args)
* if there is no formatting, the entire arg span (important if there is
  whitespace like `{  }`)

This should make it more obvious where the named argument should be.

Additionally, in the lint message, emit the formatting argument names
without a dollar sign to avoid potentially confusion.

Fixes #99907

3 files changed, 85 insertions, 32 deletions

diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 082c7893426..e6d514e76d2 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -16,6 +16,7 @@ use smallvec::SmallVec;
 
 use rustc_lint_defs::builtin::NAMED_ARGUMENTS_USED_POSITIONALLY;
 use rustc_lint_defs::{BufferedEarlyLint, BuiltinLintDiagnostics, LintId};
+use rustc_parse_format::Count;
 use std::borrow::Cow;
 use std::collections::hash_map::Entry;
 
@@ -57,26 +58,47 @@ struct PositionalNamedArg {
     replacement: Symbol,
     /// The span for the positional named argument (so the lint can point a message to it)
     positional_named_arg_span: Span,
+    has_formatting: bool,
 }
 
 impl PositionalNamedArg {
-    /// Determines what span to replace with the name of the named argument
-    fn get_span_to_replace(&self, cx: &Context<'_, '_>) -> Option<Span> {
+    /// Determines:
+    /// 1) span to be replaced with the name of the named argument and
+    /// 2) span to be underlined for error messages
+    fn get_positional_arg_spans(&self, cx: &Context<'_, '_>) -> (Option<Span>, Option<Span>) {
         if let Some(inner_span) = &self.inner_span_to_replace {
-            return Some(
-                cx.fmtsp.from_inner(InnerSpan { start: inner_span.start, end: inner_span.end }),
-            );
+            let span =
+                cx.fmtsp.from_inner(InnerSpan { start: inner_span.start, end: inner_span.end });
+            (Some(span), Some(span))
         } else if self.ty == PositionalNamedArgType::Arg {
-            // In the case of a named argument whose position is implicit, there will not be a span
-            // to replace. Instead, we insert the name after the `{`, which is the first character
-            // of arg_span.
-            return cx
-                .arg_spans
-                .get(self.cur_piece)
-                .map(|arg_span| arg_span.with_lo(arg_span.lo() + BytePos(1)).shrink_to_lo());
+            // In the case of a named argument whose position is implicit, if the argument *has*
+            // formatting, there will not be a span to replace. Instead, we insert the name after
+            // the `{`, which will be the first character of arg_span. If the argument does *not*
+            // have formatting, there may or may not be a span to replace. This is because
+            // whitespace is allowed in arguments without formatting (such as `format!("{  }", 1);`)
+            // but is not allowed in arguments with formatting (an error will be generated in cases
+            // like `format!("{ :1.1}", 1.0f32);`.
+            // For the message span, if there is formatting, we want to use the opening `{` and the
+            // next character, which will the `:` indicating the start of formatting. If there is
+            // not any formatting, we want to underline the entire span.
+            if self.has_formatting {
+                cx.arg_spans.get(self.cur_piece).map_or((None, None), |arg_span| {
+                    (
+                        Some(arg_span.with_lo(arg_span.lo() + BytePos(1)).shrink_to_lo()),
+                        Some(arg_span.with_hi(arg_span.lo() + BytePos(2))),
+                    )
+                })
+            } else {
+                cx.arg_spans.get(self.cur_piece).map_or((None, None), |arg_span| {
+                    let replace_start = arg_span.lo() + BytePos(1);
+                    let replace_end = arg_span.hi() - BytePos(1);
+                    let to_replace = arg_span.with_lo(replace_start).with_hi(replace_end);
+                    (Some(to_replace), Some(*arg_span))
+                })
+            }
+        } else {
+            (None, None)
         }
-
-        None
     }
 }
 
@@ -117,10 +139,18 @@ impl PositionalNamedArgsLint {
         cur_piece: usize,
         inner_span_to_replace: Option<rustc_parse_format::InnerSpan>,
         names: &FxHashMap<Symbol, (usize, Span)>,
+        has_formatting: bool,
     ) {
         let start_of_named_args = total_args_length - names.len();
         if current_positional_arg >= start_of_named_args {
-            self.maybe_push(format_argument_index, ty, cur_piece, inner_span_to_replace, names)
+            self.maybe_push(
+                format_argument_index,
+                ty,
+                cur_piece,
+                inner_span_to_replace,
+                names,
+                has_formatting,
+            )
         }
     }
 
@@ -134,6 +164,7 @@ impl PositionalNamedArgsLint {
         cur_piece: usize,
         inner_span_to_replace: Option<rustc_parse_format::InnerSpan>,
         names: &FxHashMap<Symbol, (usize, Span)>,
+        has_formatting: bool,
     ) {
         let named_arg = names
             .iter()
@@ -156,6 +187,7 @@ impl PositionalNamedArgsLint {
                 inner_span_to_replace,
                 replacement,
                 positional_named_arg_span,
+                has_formatting,
             });
         }
     }
@@ -414,6 +446,9 @@ impl<'a, 'b> Context<'a, 'b> {
                     PositionalNamedArgType::Precision,
                 );
 
+                let has_precision = arg.format.precision != Count::CountImplied;
+                let has_width = arg.format.width != Count::CountImplied;
+
                 // argument second, if it's an implicit positional parameter
                 // it's written second, so it should come after width/precision.
                 let pos = match arg.position {
@@ -426,6 +461,7 @@ impl<'a, 'b> Context<'a, 'b> {
                             self.curpiece,
                             arg_end,
                             &self.names,
+                            has_precision || has_width,
                         );
 
                         Exact(i)
@@ -439,6 +475,7 @@ impl<'a, 'b> Context<'a, 'b> {
                             self.curpiece,
                             None,
                             &self.names,
+                            has_precision || has_width,
                         );
                         Exact(i)
                     }
@@ -529,6 +566,7 @@ impl<'a, 'b> Context<'a, 'b> {
                     self.curpiece,
                     *inner_span,
                     &self.names,
+                    true,
                 );
                 self.verify_arg_type(Exact(i), Count);
             }
@@ -1150,24 +1188,22 @@ pub fn expand_format_args_nl<'cx>(
 
 fn create_lints_for_named_arguments_used_positionally(cx: &mut Context<'_, '_>) {
     for named_arg in &cx.unused_names_lint.positional_named_args {
-        let arg_span = named_arg.get_span_to_replace(cx);
+        let (position_sp_to_replace, position_sp_for_msg) = named_arg.get_positional_arg_spans(cx);
 
         let msg = format!("named argument `{}` is not used by name", named_arg.replacement);
-        let replacement = match named_arg.ty {
-            PositionalNamedArgType::Arg => named_arg.replacement.to_string(),
-            _ => named_arg.replacement.to_string() + "$",
-        };
 
         cx.ecx.buffered_early_lint.push(BufferedEarlyLint {
             span: MultiSpan::from_span(named_arg.positional_named_arg_span),
             msg: msg.clone(),
             node_id: ast::CRATE_NODE_ID,
             lint_id: LintId::of(&NAMED_ARGUMENTS_USED_POSITIONALLY),
-            diagnostic: BuiltinLintDiagnostics::NamedArgumentUsedPositionally(
-                arg_span,
-                named_arg.positional_named_arg_span,
-                replacement,
-            ),
+            diagnostic: BuiltinLintDiagnostics::NamedArgumentUsedPositionally {
+                position_sp_to_replace,
+                position_sp_for_msg,
+                named_arg_sp: named_arg.positional_named_arg_span,
+                named_arg_name: named_arg.replacement.to_string(),
+                is_formatting_arg: named_arg.ty != PositionalNamedArgType::Arg,
+            },
         });
     }
 }
diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 04ac50f1d48..b95fc341db6 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -856,13 +856,18 @@ pub trait LintContext: Sized {
                         Applicability::MachineApplicable,
                     );
                 },
-                BuiltinLintDiagnostics::NamedArgumentUsedPositionally(positional_arg, named_arg, name) => {
-                    db.span_label(named_arg, "this named argument is only referred to by position in formatting string");
-                    if let Some(positional_arg) = positional_arg {
-                        let msg = format!("this formatting argument uses named argument `{}` by position", name);
-                        db.span_label(positional_arg, msg);
+                BuiltinLintDiagnostics::NamedArgumentUsedPositionally{ position_sp_to_replace, position_sp_for_msg, named_arg_sp, named_arg_name, is_formatting_arg} => {
+                    db.span_label(named_arg_sp, "this named argument is referred to by position in formatting string");
+                    if let Some(positional_arg_for_msg) = position_sp_for_msg {
+                        let msg = format!("this formatting argument uses named argument `{}` by position", named_arg_name);
+                        db.span_label(positional_arg_for_msg, msg);
+                    }
+
+                    if let Some(positional_arg_to_replace) = position_sp_to_replace {
+                        let name = if is_formatting_arg { named_arg_name + "$" } else { named_arg_name };
+
                         db.span_suggestion_verbose(
-                            positional_arg,
+                            positional_arg_to_replace,
                             "use the named argument by name to avoid ambiguity",
                             name,
                             Applicability::MaybeIncorrect,
diff --git a/compiler/rustc_lint_defs/src/lib.rs b/compiler/rustc_lint_defs/src/lib.rs
index 4fd57ed8533..6acbe97a7a1 100644
--- a/compiler/rustc_lint_defs/src/lib.rs
+++ b/compiler/rustc_lint_defs/src/lib.rs
@@ -467,7 +467,19 @@ pub enum BuiltinLintDiagnostics {
         /// If true, the lifetime will be fully elided.
         use_span: Option<(Span, bool)>,
     },
-    NamedArgumentUsedPositionally(Option<Span>, Span, String),
+    NamedArgumentUsedPositionally {
+        /// Span where the named argument is used by position and will be replaced with the named
+        /// argument name
+        position_sp_to_replace: Option<Span>,
+        /// Span where the named argument is used by position and is used for lint messages
+        position_sp_for_msg: Option<Span>,
+        /// Span where the named argument's name is (so we know where to put the warning message)
+        named_arg_sp: Span,
+        /// String containing the named arguments name
+        named_arg_name: String,
+        /// Indicates if the named argument is used as a width/precision for formatting
+        is_formatting_arg: bool,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the