Auto merge of #99263 - compiler-errors:issue-99261, r=jyn514

Fix ICE in `named_arguments_used_positionally` lint

Fixes #99261
Fixes #99289
Fixes #99284
Fixes #99273
Fixes #99297
Fixes #99271

This match pattern:

```
 FormatSpec { width: Count::CountIsName(s, _), .. }
| FormatSpec { precision: Count::CountIsName(s, _), .. }
```

does not account for when both `width` and `precision` are both `Count::CountIsName`, so split the check for these two fields into two separate `if let`.

Also, remove any future potential for ICEs by removing the index operator altogether.

---

It is still suspicious that this indexing was broken and caused the ICE, as opposed to just causing a spurious lint message.

cc `@PrestonFrom,` who may be familiar with this code because of implementing the lint this touches, perhaps you'd like to look into why named arguments in `FormatSpec.precision` seem to have indices that don't correspond to a span in `Context.arg_spans`?

Edit: Opened #99265 to track a (related?) incorrect argument indexing issue.

compiler/rustc_builtin_macros/src/format.rs

@@ -16,7 +16,7 @@
 
 use rustc_lint_defs::builtin::NAMED_ARGUMENTS_USED_POSITIONALLY;
 use rustc_lint_defs::{BufferedEarlyLint, BuiltinLintDiagnostics, LintId};
-use rustc_parse_format::{Count, FormatSpec};
+use rustc_parse_format::Count;
 use std::borrow::Cow;
 use std::collections::hash_map::Entry;
 
@@ -985,20 +985,19 @@ fn lint_named_arguments_used_positionally(
                 }
                 _ => {}
             };
-            match a.format {
-                FormatSpec { width: Count::CountIsName(s, _), .. }
-                | FormatSpec { precision: Count::CountIsName(s, _), .. } => {
-                    used_argument_names.insert(s);
-                }
-                _ => {}
-            };
+            if let Count::CountIsName(s, _) = a.format.width {
+                used_argument_names.insert(s);
+            }
+            if let Count::CountIsName(s, _) = a.format.precision {
+                used_argument_names.insert(s);
+            }
         }
     }
 
     for (symbol, (index, span)) in names {
         if !used_argument_names.contains(symbol.as_str()) {
             let msg = format!("named argument `{}` is not used by name", symbol.as_str());
-            let arg_span = cx.arg_spans[index];
+            let arg_span = cx.arg_spans.get(index).copied();
             cx.ecx.buffered_early_lint.push(BufferedEarlyLint {
                 span: MultiSpan::from_span(span),
                 msg: msg.clone(),

compiler/rustc_lint/src/context.rs

@@ -858,15 +858,16 @@ fn lookup_with_diagnostics(
                 },
                 BuiltinLintDiagnostics::NamedArgumentUsedPositionally(positional_arg, named_arg, name) => {
                     db.span_label(named_arg, "this named argument is only referred to by position in formatting string");
-                    let msg = format!("this formatting argument uses named argument `{}` by position", name);
-                    db.span_label(positional_arg, msg);
-                    db.span_suggestion_verbose(
-                        positional_arg,
-                        "use the named argument by name to avoid ambiguity",
-                        format!("{{{}}}", name),
-                        Applicability::MaybeIncorrect,
-                    );
-
+                    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);
+                            db.span_suggestion_verbose(
+                            positional_arg,
+                            "use the named argument by name to avoid ambiguity",
+                            format!("{{{}}}", name),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
                 }
             }
             // Rewrap `db`, and pass control to the user.

compiler/rustc_lint_defs/src/lib.rs

@@ -467,7 +467,7 @@ pub enum BuiltinLintDiagnostics {
         /// If true, the lifetime will be fully elided.
         use_span: Option<(Span, bool)>,
     },
-    NamedArgumentUsedPositionally(Span, Span, String),
+    NamedArgumentUsedPositionally(Option<Span>, Span, String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the

src/test/ui/macros/issue-99261.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+#![deny(named_arguments_used_positionally)]
+
+fn main() {
+    let value: f64 = 314.15926;
+    let digits_before_decimal = 1;
+    let digits_after_decimal = 2;
+    let width = digits_before_decimal + 1 + digits_after_decimal;
+
+    println!(
+        "{value:0>width$.digits_after_decimal$}",
+        value = value,
+        width = width,
+        digits_after_decimal = digits_after_decimal,
+    );
+}