Parse the format string for the panic_fmt lint for better warnings.

Cargo.lock

@@ -3831,6 +3831,7 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_middle",
+ "rustc_parse_format",
  "rustc_session",
  "rustc_span",
  "rustc_target",

compiler/rustc_lint/Cargo.toml

@@ -20,3 +20,4 @@ rustc_feature = { path = "../rustc_feature" }
 rustc_index = { path = "../rustc_index" }
 rustc_session = { path = "../rustc_session" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
+rustc_parse_format = { path = "../rustc_parse_format" }

compiler/rustc_lint/src/panic_fmt.rs

@@ -3,6 +3,7 @@
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_middle::ty;
+use rustc_parse_format::{ParseMode, Parser, Piece};
 use rustc_span::sym;
 
 declare_lint! {
@@ -52,13 +53,28 @@ fn check_panic<'tcx>(cx: &LateContext<'tcx>, f: &'tcx hir::Expr<'tcx>, arg: &'tc
                 if cx.tcx.is_diagnostic_item(sym::std_panic_macro, id)
                     || cx.tcx.is_diagnostic_item(sym::core_panic_macro, id)
                 {
-                    let s = sym.as_str();
+                    let fmt = sym.as_str();
-                    if !s.contains(&['{', '}'][..]) {
+                    if !fmt.contains(&['{', '}'][..]) {
                         return;
                     }
-                    let s = s.replace("{{", "").replace("}}", "");
+
-                    let looks_like_placeholder =
+                    let fmt_span = arg.span.source_callsite();
-                        s.find('{').map_or(false, |i| s[i + 1..].contains('}'));
+
+                    let (snippet, style) =
+                        match cx.sess().parse_sess.source_map().span_to_snippet(fmt_span) {
+                            Ok(snippet) => {
+                                // Count the number of `#`s between the `r` and `"`.
+                                let style = snippet.strip_prefix('r').and_then(|s| s.find('"'));
+                                (Some(snippet), style)
+                            }
+                            Err(_) => (None, None),
+                        };
+
+                    let mut fmt_parser =
+                        Parser::new(fmt.as_ref(), style, snippet, false, ParseMode::Format);
+                    let n_arguments =
+                        (&mut fmt_parser).filter(|a| matches!(a, Piece::NextArgument(_))).count();
+
                     // Unwrap another level of macro expansion if this panic!()
                     // was expanded from assert!() or debug_assert!().
                     for &assert in &[sym::assert_macro, sym::debug_assert_macro] {
@@ -70,15 +86,23 @@ fn check_panic<'tcx>(cx: &LateContext<'tcx>, f: &'tcx hir::Expr<'tcx>, arg: &'tc
                             expn = parent;
                         }
                     }
-                    if looks_like_placeholder {
+
-                        cx.struct_span_lint(PANIC_FMT, arg.span.source_callsite(), |lint| {
+                    if n_arguments > 0 && fmt_parser.errors.is_empty() {
-                            let mut l = lint.build("panic message contains an unused formatting placeholder");
+                        let arg_spans: Vec<_> = match &fmt_parser.arg_places[..] {
+                            [] => vec![fmt_span],
+                            v => v.iter().map(|span| fmt_span.from_inner(*span)).collect(),
+                        };
+                        cx.struct_span_lint(PANIC_FMT, arg_spans, |lint| {
+                            let mut l = lint.build(match n_arguments {
+                                1 => "panic message contains an unused formatting placeholder",
+                                _ => "panic message contains unused formatting placeholders",
+                            });
                             l.note("this message is not used as a format string when given without arguments, but will be in a future Rust version");
                             if expn.call_site.contains(arg.span) {
                                 l.span_suggestion(
                                     arg.span.shrink_to_hi(),
                                     "add the missing argument(s)",
-                                    ", argument".into(),
+                                    ", ...".into(),
                                     Applicability::HasPlaceholders,
                                 );
                                 l.span_suggestion(

src/test/ui/panic-brace.rs

@@ -5,6 +5,6 @@ fn main() {
     panic!("here's a brace: {"); //~ WARN panic message contains a brace
     std::panic!("another one: }"); //~ WARN panic message contains a brace
     core::panic!("Hello {}"); //~ WARN panic message contains an unused formatting placeholder
-    assert!(false, "{:03x} bla"); //~ WARN panic message contains an unused formatting placeholder
+    assert!(false, "{:03x} {test} bla"); //~ WARN panic message contains unused formatting placeholders
     debug_assert!(false, "{{}} bla"); //~ WARN panic message contains a brace
 }

src/test/ui/panic-brace.stderr

@@ -24,35 +24,35 @@ LL |     std::panic!("{}", "another one: }");
    |                 ^^^^^
 
 warning: panic message contains an unused formatting placeholder
-  --> $DIR/panic-brace.rs:7:18
+  --> $DIR/panic-brace.rs:7:25
    |
 LL |     core::panic!("Hello {}");
-   |                  ^^^^^^^^^^
+   |                         ^^
    |
    = note: this message is not used as a format string when given without arguments, but will be in a future Rust version
 help: add the missing argument(s)
    |
-LL |     core::panic!("Hello {}", argument);
+LL |     core::panic!("Hello {}", ...);
-   |                            ^^^^^^^^^^
+   |                            ^^^^^
 help: or add a "{}" format string to use the message literally
    |
 LL |     core::panic!("{}", "Hello {}");
    |                  ^^^^^
 
-warning: panic message contains an unused formatting placeholder
+warning: panic message contains unused formatting placeholders
-  --> $DIR/panic-brace.rs:8:20
+  --> $DIR/panic-brace.rs:8:21
    |
-LL |     assert!(false, "{:03x} bla");
+LL |     assert!(false, "{:03x} {test} bla");
    |                     ^^^^^^ ^^^^^^
    |
    = note: this message is not used as a format string when given without arguments, but will be in a future Rust version
 help: add the missing argument(s)
    |
-LL |     assert!(false, "{:03x} bla", argument);
+LL |     assert!(false, "{:03x} {test} bla", ...);
-   |                                ^^^^^^^^^^
+   |                                       ^^^^^
 help: or add a "{}" format string to use the message literally
    |
-LL |     assert!(false, "{}", "{:03x} bla");
+LL |     assert!(false, "{}", "{:03x} {test} bla");
    |                    ^^^^^
 
 warning: panic message contains a brace