Auto merge of #78088 - fusion-engineering-forks:panic-fmt-lint, r=estebank

Add lint for panic!("{}")

This adds a lint that warns about `panic!("{}")`.

`panic!(msg)` invocations with a single argument use their argument as panic payload literally, without using it as a format string. The same holds for `assert!(expr, msg)`.

This lints checks if `msg` is a string literal (after expansion), and warns in case it contained braces. It suggests to insert `"{}", ` to use the message literally, or to add arguments to use it as a format string.

![image](https://user-images.githubusercontent.com/783247/96643867-79eb1080-1328-11eb-8d4e-a5586837c70a.png)

This lint is also a good starting point for adding warnings about `panic!(not_a_string)` later, once [`panic_any()`](https://github.com/rust-lang/rust/pull/74622) becomes a stable alternative.

22 files changed, 362 insertions, 165 deletions

diff --git a/Cargo.lock b/Cargo.lock
index b5abab7ba96..3dba4f935e5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3831,6 +3831,7 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_middle",
+ "rustc_parse_format",
  "rustc_session",
  "rustc_span",
  "rustc_target",
diff --git a/compiler/rustc_builtin_macros/src/assert.rs b/compiler/rustc_builtin_macros/src/assert.rs
index 5bfd8a2bf56..bb6d3f6a007 100644
--- a/compiler/rustc_builtin_macros/src/assert.rs
+++ b/compiler/rustc_builtin_macros/src/assert.rs
@@ -1,8 +1,8 @@
 use rustc_errors::{Applicability, DiagnosticBuilder};
 
 use rustc_ast::ptr::P;
-use rustc_ast::token::{self, TokenKind};
-use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
+use rustc_ast::token;
+use rustc_ast::tokenstream::{DelimSpan, TokenStream};
 use rustc_ast::{self as ast, *};
 use rustc_ast_pretty::pprust;
 use rustc_expand::base::*;
@@ -26,31 +26,39 @@ pub fn expand_assert<'cx>(
     // `core::panic` and `std::panic` are different macros, so we use call-site
     // context to pick up whichever is currently in scope.
     let sp = cx.with_call_site_ctxt(sp);
-    let tokens = custom_message.unwrap_or_else(|| {
-        TokenStream::from(TokenTree::token(
-            TokenKind::lit(
-                token::Str,
+
+    let panic_call = if let Some(tokens) = custom_message {
+        // Pass the custom message to panic!().
+        cx.expr(
+            sp,
+            ExprKind::MacCall(MacCall {
+                path: Path::from_ident(Ident::new(sym::panic, sp)),
+                args: P(MacArgs::Delimited(
+                    DelimSpan::from_single(sp),
+                    MacDelimiter::Parenthesis,
+                    tokens,
+                )),
+                prior_type_ascription: None,
+            }),
+        )
+    } else {
+        // Pass our own message directly to $crate::panicking::panic(),
+        // because it might contain `{` and `}` that should always be
+        // passed literally.
+        cx.expr_call_global(
+            sp,
+            cx.std_path(&[sym::panicking, sym::panic]),
+            vec![cx.expr_str(
+                DUMMY_SP,
                 Symbol::intern(&format!(
                     "assertion failed: {}",
                     pprust::expr_to_string(&cond_expr).escape_debug()
                 )),
-                None,
-            ),
-            DUMMY_SP,
-        ))
-    });
-    let args = P(MacArgs::Delimited(DelimSpan::from_single(sp), MacDelimiter::Parenthesis, tokens));
-    let panic_call = MacCall {
-        path: Path::from_ident(Ident::new(sym::panic, sp)),
-        args,
-        prior_type_ascription: None,
+            )],
+        )
     };
-    let if_expr = cx.expr_if(
-        sp,
-        cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)),
-        cx.expr(sp, ExprKind::MacCall(panic_call)),
-        None,
-    );
+    let if_expr =
+        cx.expr_if(sp, cx.expr(sp, ExprKind::Unary(UnOp::Not, cond_expr)), panic_call, None);
     MacEager::expr(if_expr)
 }
 
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index a074af0189a..66463eeb907 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -1173,7 +1173,8 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
         mbe::TokenTree::MetaVar(_, name) => format!("${}", name),
         mbe::TokenTree::MetaVarDecl(_, name, kind) => format!("${}:{}", name, kind),
         _ => panic!(
-            "unexpected mbe::TokenTree::{{Sequence or Delimited}} \
+            "{}",
+            "unexpected mbe::TokenTree::{Sequence or Delimited} \
              in follow set checker"
         ),
     }
diff --git a/compiler/rustc_lint/Cargo.toml b/compiler/rustc_lint/Cargo.toml
index 760a8e385d6..c56eb09b634 100644
--- a/compiler/rustc_lint/Cargo.toml
+++ b/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" }
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 24bfdad970a..81549be4b09 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -55,6 +55,7 @@ mod levels;
 mod methods;
 mod non_ascii_idents;
 mod nonstandard_style;
+mod panic_fmt;
 mod passes;
 mod redundant_semicolon;
 mod traits;
@@ -80,6 +81,7 @@ use internal::*;
 use methods::*;
 use non_ascii_idents::*;
 use nonstandard_style::*;
+use panic_fmt::PanicFmt;
 use redundant_semicolon::*;
 use traits::*;
 use types::*;
@@ -166,6 +168,7 @@ macro_rules! late_lint_passes {
                 ClashingExternDeclarations: ClashingExternDeclarations::new(),
                 DropTraitConstraints: DropTraitConstraints,
                 TemporaryCStringAsPtr: TemporaryCStringAsPtr,
+                PanicFmt: PanicFmt,
             ]
         );
     };
diff --git a/compiler/rustc_lint/src/panic_fmt.rs b/compiler/rustc_lint/src/panic_fmt.rs
new file mode 100644
index 00000000000..0d2b20989b0
--- /dev/null
+++ b/compiler/rustc_lint/src/panic_fmt.rs
@@ -0,0 +1,150 @@
+use crate::{LateContext, LateLintPass, LintContext};
+use rustc_ast as ast;
+use rustc_errors::{pluralize, Applicability};
+use rustc_hir as hir;
+use rustc_middle::ty;
+use rustc_parse_format::{ParseMode, Parser, Piece};
+use rustc_span::{sym, InnerSpan};
+
+declare_lint! {
+    /// The `panic_fmt` lint detects `panic!("..")` with `{` or `}` in the string literal.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,no_run
+    /// panic!("{}");
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// `panic!("{}")` panics with the message `"{}"`, as a `panic!()` invocation
+    /// with a single argument does not use `format_args!()`.
+    /// A future edition of Rust will interpret this string as format string,
+    /// which would break this.
+    PANIC_FMT,
+    Warn,
+    "detect braces in single-argument panic!() invocations",
+    report_in_external_macro
+}
+
+declare_lint_pass!(PanicFmt => [PANIC_FMT]);
+
+impl<'tcx> LateLintPass<'tcx> for PanicFmt {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
+        if let hir::ExprKind::Call(f, [arg]) = &expr.kind {
+            if let &ty::FnDef(def_id, _) = cx.typeck_results().expr_ty(f).kind() {
+                if Some(def_id) == cx.tcx.lang_items().begin_panic_fn()
+                    || Some(def_id) == cx.tcx.lang_items().panic_fn()
+                {
+                    check_panic(cx, f, arg);
+                }
+            }
+        }
+    }
+}
+
+fn check_panic<'tcx>(cx: &LateContext<'tcx>, f: &'tcx hir::Expr<'tcx>, arg: &'tcx hir::Expr<'tcx>) {
+    if let hir::ExprKind::Lit(lit) = &arg.kind {
+        if let ast::LitKind::Str(sym, _) = lit.node {
+            let mut expn = f.span.ctxt().outer_expn_data();
+            if let Some(id) = expn.macro_def_id {
+                if cx.tcx.is_diagnostic_item(sym::std_panic_macro, id)
+                    || cx.tcx.is_diagnostic_item(sym::core_panic_macro, id)
+                {
+                    let fmt = sym.as_str();
+                    if !fmt.contains(&['{', '}'][..]) {
+                        return;
+                    }
+
+                    let fmt_span = arg.span.source_callsite();
+
+                    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.clone(), 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] {
+                        let parent = expn.call_site.ctxt().outer_expn_data();
+                        if parent
+                            .macro_def_id
+                            .map_or(false, |id| cx.tcx.is_diagnostic_item(assert, id))
+                        {
+                            expn = parent;
+                        }
+                    }
+
+                    if n_arguments > 0 && fmt_parser.errors.is_empty() {
+                        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 edition");
+                            if expn.call_site.contains(arg.span) {
+                                l.span_suggestion(
+                                    arg.span.shrink_to_hi(),
+                                    &format!("add the missing argument{}", pluralize!(n_arguments)),
+                                    ", ...".into(),
+                                    Applicability::HasPlaceholders,
+                                );
+                                l.span_suggestion(
+                                    arg.span.shrink_to_lo(),
+                                    "or add a \"{}\" format string to use the message literally",
+                                    "\"{}\", ".into(),
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                            l.emit();
+                        });
+                    } else {
+                        let brace_spans: Option<Vec<_>> = snippet
+                            .filter(|s| s.starts_with('"') || s.starts_with("r#"))
+                            .map(|s| {
+                                s.char_indices()
+                                    .filter(|&(_, c)| c == '{' || c == '}')
+                                    .map(|(i, _)| {
+                                        fmt_span.from_inner(InnerSpan { start: i, end: i + 1 })
+                                    })
+                                    .collect()
+                            });
+                        let msg = match &brace_spans {
+                            Some(v) if v.len() == 1 => "panic message contains a brace",
+                            _ => "panic message contains braces",
+                        };
+                        cx.struct_span_lint(PANIC_FMT, brace_spans.unwrap_or(vec![expn.call_site]), |lint| {
+                            let mut l = lint.build(msg);
+                            l.note("this message is not used as a format string, but will be in a future Rust edition");
+                            if expn.call_site.contains(arg.span) {
+                                l.span_suggestion(
+                                    arg.span.shrink_to_lo(),
+                                    "add a \"{}\" format string to use the message literally",
+                                    "\"{}\", ".into(),
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                            l.emit();
+                        });
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_passes/src/diagnostic_items.rs b/compiler/rustc_passes/src/diagnostic_items.rs
index 0f4aa72d5c4..5a087c41f58 100644
--- a/compiler/rustc_passes/src/diagnostic_items.rs
+++ b/compiler/rustc_passes/src/diagnostic_items.rs
@@ -113,6 +113,10 @@ fn collect<'tcx>(tcx: TyCtxt<'tcx>) -> FxHashMap<Symbol, DefId> {
         }
     }
 
+    for m in tcx.hir().krate().exported_macros {
+        collector.observe_item(m.attrs, m.hir_id);
+    }
+
     collector.items
 }
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 3a2a3adce35..338ff005995 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -267,6 +267,7 @@ symbols! {
         asm,
         assert,
         assert_inhabited,
+        assert_macro,
         assert_receiver_is_total_eq,
         assert_uninit_valid,
         assert_zero_valid,
@@ -393,6 +394,7 @@ symbols! {
         copysignf64,
         core,
         core_intrinsics,
+        core_panic_macro,
         cosf32,
         cosf64,
         crate_id,
@@ -416,6 +418,7 @@ symbols! {
         dead_code,
         dealloc,
         debug,
+        debug_assert_macro,
         debug_assertions,
         debug_struct,
         debug_trait,
@@ -789,6 +792,7 @@ symbols! {
         panic_runtime,
         panic_str,
         panic_unwind,
+        panicking,
         param_attrs,
         parent_trait,
         partial_cmp,
@@ -1064,6 +1068,7 @@ symbols! {
         staticlib,
         std,
         std_inject,
+        std_panic_macro,
         stmt,
         stmt_expr_attributes,
         stop_after_dataflow,
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index fe3eff04b4a..0416a7614a3 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -2,6 +2,7 @@
 #[macro_export]
 #[allow_internal_unstable(core_panic, const_caller_location)]
 #[stable(feature = "core", since = "1.6.0")]
+#[cfg_attr(not(bootstrap), rustc_diagnostic_item = "core_panic_macro")]
 macro_rules! panic {
     () => (
         $crate::panic!("explicit panic")
@@ -162,6 +163,7 @@ macro_rules! assert_ne {
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), rustc_diagnostic_item = "debug_assert_macro")]
 macro_rules! debug_assert {
     ($($arg:tt)*) => (if $crate::cfg!(debug_assertions) { $crate::assert!($($arg)*); })
 }
@@ -1215,6 +1217,8 @@ pub(crate) mod builtin {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_builtin_macro]
     #[macro_export]
+    #[cfg_attr(not(bootstrap), rustc_diagnostic_item = "assert_macro")]
+    #[allow_internal_unstable(core_panic)]
     macro_rules! assert {
         ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
         ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};
diff --git a/library/core/tests/nonzero.rs b/library/core/tests/nonzero.rs
index ca449b4350e..b66c482c5e5 100644
--- a/library/core/tests/nonzero.rs
+++ b/library/core/tests/nonzero.rs
@@ -85,7 +85,7 @@ fn test_match_option_string() {
     let five = "Five".to_string();
     match Some(five) {
         Some(s) => assert_eq!(s, "Five"),
-        None => panic!("unexpected None while matching on Some(String { ... })"),
+        None => panic!("{}", "unexpected None while matching on Some(String { ... })"),
     }
 }
 
diff --git a/library/std/src/macros.rs b/library/std/src/macros.rs
index 57649d6f8f2..de072e83dfc 100644
--- a/library/std/src/macros.rs
+++ b/library/std/src/macros.rs
@@ -8,6 +8,7 @@
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow_internal_unstable(libstd_sys_internals)]
+#[cfg_attr(not(any(bootstrap, test)), rustc_diagnostic_item = "std_panic_macro")]
 macro_rules! panic {
     () => ({ $crate::panic!("explicit panic") });
     ($msg:expr $(,)?) => ({ $crate::rt::begin_panic($msg) });
diff --git a/src/test/mir-opt/inst_combine_deref.do_not_miscompile.InstCombine.diff b/src/test/mir-opt/inst_combine_deref.do_not_miscompile.InstCombine.diff
index 23c18bde226..dac9ec3b443 100644
--- a/src/test/mir-opt/inst_combine_deref.do_not_miscompile.InstCombine.diff
+++ b/src/test/mir-opt/inst_combine_deref.do_not_miscompile.InstCombine.diff
@@ -10,7 +10,7 @@
       let mut _8: bool;                    // in scope 0 at $DIR/inst_combine_deref.rs:60:5: 60:23
       let mut _9: bool;                    // in scope 0 at $DIR/inst_combine_deref.rs:60:13: 60:21
       let mut _10: i32;                    // in scope 0 at $DIR/inst_combine_deref.rs:60:13: 60:15
-      let mut _11: !;                      // in scope 0 at $SRC_DIR/std/src/macros.rs:LL:COL
+      let mut _11: !;                      // in scope 0 at $DIR/inst_combine_deref.rs:60:5: 60:23
       scope 1 {
           debug x => _1;                   // in scope 1 at $DIR/inst_combine_deref.rs:55:9: 55:10
           let _2: i32;                     // in scope 1 at $DIR/inst_combine_deref.rs:56:9: 56:10
@@ -69,11 +69,11 @@
       }
   
       bb2: {
-          StorageLive(_11);                // scope 4 at $SRC_DIR/std/src/macros.rs:LL:COL
-          begin_panic::<&str>(const "assertion failed: *y == 99"); // scope 4 at $SRC_DIR/std/src/macros.rs:LL:COL
+          StorageLive(_11);                // scope 4 at $DIR/inst_combine_deref.rs:60:5: 60:23
+          core::panicking::panic(const "assertion failed: *y == 99"); // scope 4 at $DIR/inst_combine_deref.rs:60:5: 60:23
                                            // mir::Constant
-                                           // + span: $SRC_DIR/std/src/macros.rs:LL:COL
-                                           // + literal: Const { ty: fn(&str) -> ! {std::rt::begin_panic::<&str>}, val: Value(Scalar(<ZST>)) }
+                                           // + span: $DIR/inst_combine_deref.rs:60:5: 60:23
+                                           // + literal: Const { ty: fn(&'static str) -> ! {core::panicking::panic}, val: Value(Scalar(<ZST>)) }
                                            // ty::Const
                                            // + ty: &str
                                            // + val: Value(Slice { data: Allocation { bytes: [97, 115, 115, 101, 114, 116, 105, 111, 110, 32, 102, 97, 105, 108, 101, 100, 58, 32, 42, 121, 32, 61, 61, 32, 57, 57], relocations: Relocations(SortedMap { data: [] }), init_mask: InitMask { blocks: [67108863], len: Size { raw: 26 } }, size: Size { raw: 26 }, align: Align { pow2: 0 }, mutability: Not, extra: () }, start: 0, end: 26 })
diff --git a/src/test/ui/auxiliary/fancy-panic.rs b/src/test/ui/auxiliary/fancy-panic.rs
new file mode 100644
index 00000000000..e5a25a171fb
--- /dev/null
+++ b/src/test/ui/auxiliary/fancy-panic.rs
@@ -0,0 +1,6 @@
+#[macro_export]
+macro_rules! fancy_panic {
+    ($msg:expr) => {
+        panic!($msg)
+    };
+}
diff --git a/src/test/ui/fmt/format-args-capture.rs b/src/test/ui/fmt/format-args-capture.rs
index 9348bb46dfe..4e3fa9a3c58 100644
--- a/src/test/ui/fmt/format-args-capture.rs
+++ b/src/test/ui/fmt/format-args-capture.rs
@@ -31,6 +31,7 @@ fn panic_with_single_argument_does_not_get_formatted() {
     // RFC #2795 suggests that this may need to change so that captured arguments are formatted.
     // For stability reasons this will need to part of an edition change.
 
+    #[allow(panic_fmt)]
     let msg = std::panic::catch_unwind(|| {
         panic!("{foo}");
     }).unwrap_err();
diff --git a/src/test/ui/macros/macro-comma-behavior-rpass.rs b/src/test/ui/macros/macro-comma-behavior-rpass.rs
index 32cf59294e7..e5e656de6fa 100644
--- a/src/test/ui/macros/macro-comma-behavior-rpass.rs
+++ b/src/test/ui/macros/macro-comma-behavior-rpass.rs
@@ -57,6 +57,7 @@ fn writeln_1arg() {
 //
 // (Example: Issue #48042)
 #[test]
+#[allow(panic_fmt)]
 fn to_format_or_not_to_format() {
     // ("{}" is the easiest string to test because if this gets
     // sent to format_args!, it'll simply fail to compile.
diff --git a/src/test/ui/panic-brace.rs b/src/test/ui/panic-brace.rs
new file mode 100644
index 00000000000..754dcc287d0
--- /dev/null
+++ b/src/test/ui/panic-brace.rs
@@ -0,0 +1,31 @@
+// build-pass (FIXME(62277): should be check-pass)
+// aux-build:fancy-panic.rs
+
+extern crate fancy_panic;
+
+const C: &str = "abc {}";
+static S: &str = "{bla}";
+
+#[allow(unreachable_code)]
+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} {test} bla");
+    //~^ WARN panic message contains unused formatting placeholders
+    debug_assert!(false, "{{}} bla"); //~ WARN panic message contains braces
+    panic!(C); // No warning (yet)
+    panic!(S); // No warning (yet)
+    panic!(concat!("{", "}")); //~ WARN panic message contains an unused formatting placeholder
+    panic!(concat!("{", "{")); //~ WARN panic message contains braces
+
+    fancy_panic::fancy_panic!("test {} 123");
+    //~^ WARN panic message contains an unused formatting placeholder
+
+    // Check that the lint only triggers for std::panic and core::panic,
+    // not any panic macro:
+    macro_rules! panic {
+        ($e:expr) => ();
+    }
+    panic!("{}"); // OK
+}
diff --git a/src/test/ui/panic-brace.stderr b/src/test/ui/panic-brace.stderr
new file mode 100644
index 00000000000..93808891c3c
--- /dev/null
+++ b/src/test/ui/panic-brace.stderr
@@ -0,0 +1,107 @@
+warning: panic message contains a brace
+  --> $DIR/panic-brace.rs:11:29
+   |
+LL |     panic!("here's a brace: {");
+   |                             ^
+   |
+   = note: `#[warn(panic_fmt)]` on by default
+   = note: this message is not used as a format string, but will be in a future Rust edition
+help: add a "{}" format string to use the message literally
+   |
+LL |     panic!("{}", "here's a brace: {");
+   |            ^^^^^
+
+warning: panic message contains a brace
+  --> $DIR/panic-brace.rs:12:31
+   |
+LL |     std::panic!("another one: }");
+   |                               ^
+   |
+   = note: this message is not used as a format string, but will be in a future Rust edition
+help: add a "{}" format string to use the message literally
+   |
+LL |     std::panic!("{}", "another one: }");
+   |                 ^^^^^
+
+warning: panic message contains an unused formatting placeholder
+  --> $DIR/panic-brace.rs:13:25
+   |
+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 edition
+help: add the missing argument
+   |
+LL |     core::panic!("Hello {}", ...);
+   |                            ^^^^^
+help: or add a "{}" format string to use the message literally
+   |
+LL |     core::panic!("{}", "Hello {}");
+   |                  ^^^^^
+
+warning: panic message contains unused formatting placeholders
+  --> $DIR/panic-brace.rs:14:21
+   |
+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 edition
+help: add the missing arguments
+   |
+LL |     assert!(false, "{:03x} {test} bla", ...);
+   |                                       ^^^^^
+help: or add a "{}" format string to use the message literally
+   |
+LL |     assert!(false, "{}", "{:03x} {test} bla");
+   |                    ^^^^^
+
+warning: panic message contains braces
+  --> $DIR/panic-brace.rs:16:27
+   |
+LL |     debug_assert!(false, "{{}} bla");
+   |                           ^^^^
+   |
+   = note: this message is not used as a format string, but will be in a future Rust edition
+help: add a "{}" format string to use the message literally
+   |
+LL |     debug_assert!(false, "{}", "{{}} bla");
+   |                          ^^^^^
+
+warning: panic message contains an unused formatting placeholder
+  --> $DIR/panic-brace.rs:19:12
+   |
+LL |     panic!(concat!("{", "}"));
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: this message is not used as a format string when given without arguments, but will be in a future Rust edition
+help: add the missing argument
+   |
+LL |     panic!(concat!("{", "}"), ...);
+   |                             ^^^^^
+help: or add a "{}" format string to use the message literally
+   |
+LL |     panic!("{}", concat!("{", "}"));
+   |            ^^^^^
+
+warning: panic message contains braces
+  --> $DIR/panic-brace.rs:20:5
+   |
+LL |     panic!(concat!("{", "{"));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this message is not used as a format string, but will be in a future Rust edition
+help: add a "{}" format string to use the message literally
+   |
+LL |     panic!("{}", concat!("{", "{"));
+   |            ^^^^^
+
+warning: panic message contains an unused formatting placeholder
+  --> $DIR/panic-brace.rs:22:37
+   |
+LL |     fancy_panic::fancy_panic!("test {} 123");
+   |                                     ^^
+   |
+   = note: this message is not used as a format string when given without arguments, but will be in a future Rust edition
+
+warning: 8 warnings emitted
+
diff --git a/src/tools/clippy/clippy_lints/src/assertions_on_constants.rs b/src/tools/clippy/clippy_lints/src/assertions_on_constants.rs
index a2ccb0369c4..a52f0997d43 100644
--- a/src/tools/clippy/clippy_lints/src/assertions_on_constants.rs
+++ b/src/tools/clippy/clippy_lints/src/assertions_on_constants.rs
@@ -129,8 +129,11 @@ fn match_assert_with_message<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>)
         if let ExprKind::Block(ref block, _) = arms[0].body.kind;
         if block.stmts.is_empty();
         if let Some(block_expr) = &block.expr;
-        if let ExprKind::Block(ref inner_block, _) = block_expr.kind;
-        if let Some(begin_panic_call) = &inner_block.expr;
+        // inner block is optional. unwarp it if it exists, or use the expression as is otherwise.
+        if let Some(begin_panic_call) = match block_expr.kind {
+            ExprKind::Block(ref inner_block, _) => &inner_block.expr,
+            _ => &block.expr,
+        };
         // function call
         if let Some(args) = match_panic_call(cx, begin_panic_call);
         if args.len() == 1;
diff --git a/src/tools/clippy/clippy_lints/src/lib.rs b/src/tools/clippy/clippy_lints/src/lib.rs
index 126852df502..19bf67d80c4 100644
--- a/src/tools/clippy/clippy_lints/src/lib.rs
+++ b/src/tools/clippy/clippy_lints/src/lib.rs
@@ -788,7 +788,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &overflow_check_conditional::OVERFLOW_CHECK_CONDITIONAL,
         &panic_in_result_fn::PANIC_IN_RESULT_FN,
         &panic_unimplemented::PANIC,
-        &panic_unimplemented::PANIC_PARAMS,
         &panic_unimplemented::TODO,
         &panic_unimplemented::UNIMPLEMENTED,
         &panic_unimplemented::UNREACHABLE,
@@ -1499,7 +1498,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&open_options::NONSENSICAL_OPEN_OPTIONS),
         LintId::of(&option_env_unwrap::OPTION_ENV_UNWRAP),
         LintId::of(&overflow_check_conditional::OVERFLOW_CHECK_CONDITIONAL),
-        LintId::of(&panic_unimplemented::PANIC_PARAMS),
         LintId::of(&partialeq_ne_impl::PARTIALEQ_NE_IMPL),
         LintId::of(&precedence::PRECEDENCE),
         LintId::of(&ptr::CMP_NULL),
@@ -1666,7 +1664,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&non_copy_const::DECLARE_INTERIOR_MUTABLE_CONST),
         LintId::of(&non_expressive_names::JUST_UNDERSCORES_AND_DIGITS),
         LintId::of(&non_expressive_names::MANY_SINGLE_CHAR_NAMES),
-        LintId::of(&panic_unimplemented::PANIC_PARAMS),
         LintId::of(&ptr::CMP_NULL),
         LintId::of(&ptr::PTR_ARG),
         LintId::of(&ptr_eq::PTR_EQ),
diff --git a/src/tools/clippy/clippy_lints/src/panic_unimplemented.rs b/src/tools/clippy/clippy_lints/src/panic_unimplemented.rs
index 3d888fe7325..8b10d071647 100644
--- a/src/tools/clippy/clippy_lints/src/panic_unimplemented.rs
+++ b/src/tools/clippy/clippy_lints/src/panic_unimplemented.rs
@@ -1,31 +1,11 @@
-use crate::utils::{is_direct_expn_of, is_expn_of, match_panic_call, span_lint};
+use crate::utils::{is_expn_of, match_panic_call, span_lint};
 use if_chain::if_chain;
-use rustc_ast::ast::LitKind;
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::Expr;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::Span;
 
 declare_clippy_lint! {
-    /// **What it does:** Checks for missing parameters in `panic!`.
-    ///
-    /// **Why is this bad?** Contrary to the `format!` family of macros, there are
-    /// two forms of `panic!`: if there are no parameters given, the first argument
-    /// is not a format string and used literally. So while `format!("{}")` will
-    /// fail to compile, `panic!("{}")` will not.
-    ///
-    /// **Known problems:** None.
-    ///
-    /// **Example:**
-    /// ```no_run
-    /// panic!("This `panic!` is probably missing a parameter there: {}");
-    /// ```
-    pub PANIC_PARAMS,
-    style,
-    "missing parameters in `panic!` calls"
-}
-
-declare_clippy_lint! {
     /// **What it does:** Checks for usage of `panic!`.
     ///
     /// **Why is this bad?** `panic!` will stop the execution of the executable
@@ -89,11 +69,11 @@ declare_clippy_lint! {
     "`unreachable!` should not be present in production code"
 }
 
-declare_lint_pass!(PanicUnimplemented => [PANIC_PARAMS, UNIMPLEMENTED, UNREACHABLE, TODO, PANIC]);
+declare_lint_pass!(PanicUnimplemented => [UNIMPLEMENTED, UNREACHABLE, TODO, PANIC]);
 
 impl<'tcx> LateLintPass<'tcx> for PanicUnimplemented {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if let Some(params) = match_panic_call(cx, expr) {
+        if let Some(_) = match_panic_call(cx, expr) {
             let span = get_outer_span(expr);
             if is_expn_of(expr.span, "unimplemented").is_some() {
                 span_lint(
@@ -113,7 +93,6 @@ impl<'tcx> LateLintPass<'tcx> for PanicUnimplemented {
                 );
             } else if is_expn_of(expr.span, "panic").is_some() {
                 span_lint(cx, PANIC, span, "`panic` should not be present in production code");
-                match_panic(params, expr, cx);
             }
         }
     }
@@ -132,20 +111,3 @@ fn get_outer_span(expr: &Expr<'_>) -> Span {
         }
     }
 }
-
-fn match_panic(params: &[Expr<'_>], expr: &Expr<'_>, cx: &LateContext<'_>) {
-    if_chain! {
-        if let ExprKind::Lit(ref lit) = params[0].kind;
-        if is_direct_expn_of(expr.span, "panic").is_some();
-        if let LitKind::Str(ref string, _) = lit.node;
-        let string = string.as_str().replace("{{", "").replace("}}", "");
-        if let Some(par) = string.find('{');
-        if string[par..].contains('}');
-        if params[0].span.source_callee().is_none();
-        if params[0].span.lo() != params[0].span.hi();
-        then {
-            span_lint(cx, PANIC_PARAMS, params[0].span,
-                      "you probably are missing some parameter in your format string");
-        }
-    }
-}
diff --git a/src/tools/clippy/tests/ui/panic.rs b/src/tools/clippy/tests/ui/panic.rs
deleted file mode 100644
index 6e004aa9a92..00000000000
--- a/src/tools/clippy/tests/ui/panic.rs
+++ /dev/null
@@ -1,61 +0,0 @@
-#![warn(clippy::panic_params)]
-#![allow(clippy::assertions_on_constants)]
-fn missing() {
-    if true {
-        panic!("{}");
-    } else if false {
-        panic!("{:?}");
-    } else {
-        assert!(true, "here be missing values: {}");
-    }
-
-    panic!("{{{this}}}");
-}
-
-fn ok_single() {
-    panic!("foo bar");
-}
-
-fn ok_inner() {
-    // Test for #768
-    assert!("foo bar".contains(&format!("foo {}", "bar")));
-}
-
-fn ok_multiple() {
-    panic!("{}", "This is {ok}");
-}
-
-fn ok_bracket() {
-    match 42 {
-        1337 => panic!("{so is this"),
-        666 => panic!("so is this}"),
-        _ => panic!("}so is that{"),
-    }
-}
-
-const ONE: u32 = 1;
-
-fn ok_nomsg() {
-    assert!({ 1 == ONE });
-    assert!(if 1 == ONE { ONE == 1 } else { false });
-}
-
-fn ok_escaped() {
-    panic!("{{ why should this not be ok? }}");
-    panic!(" or {{ that ?");
-    panic!(" or }} this ?");
-    panic!(" {or {{ that ?");
-    panic!(" }or }} this ?");
-    panic!("{{ test }");
-    panic!("{case }}");
-}
-
-fn main() {
-    missing();
-    ok_single();
-    ok_multiple();
-    ok_bracket();
-    ok_inner();
-    ok_nomsg();
-    ok_escaped();
-}
diff --git a/src/tools/clippy/tests/ui/panic.stderr b/src/tools/clippy/tests/ui/panic.stderr
deleted file mode 100644
index 1f8ff8ccf55..00000000000
--- a/src/tools/clippy/tests/ui/panic.stderr
+++ /dev/null
@@ -1,28 +0,0 @@
-error: you probably are missing some parameter in your format string
-  --> $DIR/panic.rs:5:16
-   |
-LL |         panic!("{}");
-   |                ^^^^
-   |
-   = note: `-D clippy::panic-params` implied by `-D warnings`
-
-error: you probably are missing some parameter in your format string
-  --> $DIR/panic.rs:7:16
-   |
-LL |         panic!("{:?}");
-   |                ^^^^^^
-
-error: you probably are missing some parameter in your format string
-  --> $DIR/panic.rs:9:23
-   |
-LL |         assert!(true, "here be missing values: {}");
-   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: you probably are missing some parameter in your format string
-  --> $DIR/panic.rs:12:12
-   |
-LL |     panic!("{{{this}}}");
-   |            ^^^^^^^^^^^^
-
-error: aborting due to 4 previous errors
-