Auto merge of #6821 - Jarcho:write_literal_suggestion, r=flip1995

Write literal suggestion

fixes: #6768

changelog: Add suggestion to `write_literal` and `print_literal` lints
changelog: Change `use_debug` to point only at the format string

6 files changed, 454 insertions, 115 deletions

diff --git a/clippy_lints/src/write.rs b/clippy_lints/src/write.rs
index fd3e5a7ce91..12a47a6b703 100644
--- a/clippy_lints/src/write.rs
+++ b/clippy_lints/src/write.rs
@@ -1,19 +1,19 @@
 use std::borrow::Cow;
-use std::ops::Range;
+use std::iter;
+use std::ops::{Deref, Range};
 
 use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg, span_lint_and_then};
-use clippy_utils::source::snippet_with_applicability;
-use if_chain::if_chain;
-use rustc_ast::ast::{Expr, ExprKind, ImplKind, Item, ItemKind, LitKind, MacCall, StrLit, StrStyle};
-use rustc_ast::token;
+use clippy_utils::source::{snippet_opt, snippet_with_applicability};
+use rustc_ast::ast::{Expr, ExprKind, ImplKind, Item, ItemKind, MacCall, Path, StrLit, StrStyle};
+use rustc_ast::token::{self, LitKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::Applicability;
 use rustc_lexer::unescape::{self, EscapeError};
 use rustc_lint::{EarlyContext, EarlyLintPass};
 use rustc_parse::parser;
 use rustc_session::{declare_tool_lint, impl_lint_pass};
-use rustc_span::symbol::kw;
-use rustc_span::{sym, BytePos, Span};
+use rustc_span::symbol::{kw, Symbol};
+use rustc_span::{sym, BytePos, Span, DUMMY_SP};
 
 declare_clippy_lint! {
     /// **What it does:** This lint warns when you use `println!("")` to
@@ -354,7 +354,120 @@ fn newline_span(fmtstr: &StrLit) -> Span {
     sp.with_lo(newline_sp_hi - newline_sp_len).with_hi(newline_sp_hi)
 }
 
+/// Stores a list of replacement spans for each argument, but only if all the replacements used an
+/// empty format string.
+#[derive(Default)]
+struct SimpleFormatArgs {
+    unnamed: Vec<Vec<Span>>,
+    named: Vec<(Symbol, Vec<Span>)>,
+}
+impl SimpleFormatArgs {
+    fn get_unnamed(&self) -> impl Iterator<Item = &[Span]> {
+        self.unnamed.iter().map(|x| match x.as_slice() {
+            // Ignore the dummy span added from out of order format arguments.
+            [DUMMY_SP] => &[],
+            x => x,
+        })
+    }
+
+    fn get_named(&self, n: &Path) -> &[Span] {
+        self.named.iter().find(|x| *n == x.0).map_or(&[], |x| x.1.as_slice())
+    }
+
+    fn push(&mut self, arg: rustc_parse_format::Argument<'_>, span: Span) {
+        use rustc_parse_format::{
+            AlignUnknown, ArgumentImplicitlyIs, ArgumentIs, ArgumentNamed, CountImplied, FormatSpec,
+        };
+
+        const SIMPLE: FormatSpec<'_> = FormatSpec {
+            fill: None,
+            align: AlignUnknown,
+            flags: 0,
+            precision: CountImplied,
+            precision_span: None,
+            width: CountImplied,
+            width_span: None,
+            ty: "",
+            ty_span: None,
+        };
+
+        match arg.position {
+            ArgumentIs(n) | ArgumentImplicitlyIs(n) => {
+                if self.unnamed.len() <= n {
+                    // Use a dummy span to mark all unseen arguments.
+                    self.unnamed.resize_with(n, || vec![DUMMY_SP]);
+                    if arg.format == SIMPLE {
+                        self.unnamed.push(vec![span]);
+                    } else {
+                        self.unnamed.push(Vec::new());
+                    }
+                } else {
+                    let args = &mut self.unnamed[n];
+                    match (args.as_mut_slice(), arg.format == SIMPLE) {
+                        // A non-empty format string has been seen already.
+                        ([], _) => (),
+                        // Replace the dummy span, if it exists.
+                        ([dummy @ DUMMY_SP], true) => *dummy = span,
+                        ([_, ..], true) => args.push(span),
+                        ([_, ..], false) => *args = Vec::new(),
+                    }
+                }
+            },
+            ArgumentNamed(n) => {
+                if let Some(x) = self.named.iter_mut().find(|x| x.0 == n) {
+                    match x.1.as_slice() {
+                        // A non-empty format string has been seen already.
+                        [] => (),
+                        [_, ..] if arg.format == SIMPLE => x.1.push(span),
+                        [_, ..] => x.1 = Vec::new(),
+                    }
+                } else if arg.format == SIMPLE {
+                    self.named.push((n, vec![span]));
+                } else {
+                    self.named.push((n, Vec::new()));
+                }
+            },
+        };
+    }
+}
+
 impl Write {
+    /// Parses a format string into a collection of spans for each argument. This only keeps track
+    /// of empty format arguments. Will also lint usages of debug format strings outside of debug
+    /// impls.
+    fn parse_fmt_string(&self, cx: &EarlyContext<'_>, str_lit: &StrLit) -> Option<SimpleFormatArgs> {
+        use rustc_parse_format::{ParseMode, Parser, Piece};
+
+        let str_sym = str_lit.symbol_unescaped.as_str();
+        let style = match str_lit.style {
+            StrStyle::Cooked => None,
+            StrStyle::Raw(n) => Some(n as usize),
+        };
+
+        let mut parser = Parser::new(&str_sym, style, snippet_opt(cx, str_lit.span), false, ParseMode::Format);
+        let mut args = SimpleFormatArgs::default();
+
+        while let Some(arg) = parser.next() {
+            let arg = match arg {
+                Piece::String(_) => continue,
+                Piece::NextArgument(arg) => arg,
+            };
+            let span = parser
+                .arg_places
+                .last()
+                .map_or(DUMMY_SP, |&x| str_lit.span.from_inner(x));
+
+            if !self.in_debug_impl && arg.format.ty == "?" {
+                // FIXME: modify rustc's fmt string parser to give us the current span
+                span_lint(cx, USE_DEBUG, span, "use of `Debug`-based formatting");
+            }
+
+            args.push(arg, span);
+        }
+
+        parser.errors.is_empty().then(move || args)
+    }
+
     /// Checks the arguments of `print[ln]!` and `write[ln]!` calls. It will return a tuple of two
     /// `Option`s. The first `Option` of the tuple is the macro's format string. It includes
     /// the contents of the string, whether it's a raw string, and the span of the literal in the
@@ -376,111 +489,96 @@ impl Write {
     /// ```
     #[allow(clippy::too_many_lines)]
     fn check_tts<'a>(&self, cx: &EarlyContext<'a>, tts: TokenStream, is_write: bool) -> (Option<StrLit>, Option<Expr>) {
-        use rustc_parse_format::{
-            AlignUnknown, ArgumentImplicitlyIs, ArgumentIs, ArgumentNamed, CountImplied, FormatSpec, ParseMode, Parser,
-            Piece,
-        };
-
         let mut parser = parser::Parser::new(&cx.sess.parse_sess, tts, false, None);
-        let mut expr: Option<Expr> = None;
-        if is_write {
-            expr = match parser.parse_expr().map_err(|mut err| err.cancel()) {
-                Ok(p) => Some(p.into_inner()),
-                Err(_) => return (None, None),
-            };
-            // might be `writeln!(foo)`
-            if parser.expect(&token::Comma).map_err(|mut err| err.cancel()).is_err() {
-                return (None, expr);
+        let expr = if is_write {
+            match parser
+                .parse_expr()
+                .map(rustc_ast::ptr::P::into_inner)
+                .map_err(|mut e| e.cancel())
+            {
+                // write!(e, ...)
+                Ok(p) if parser.eat(&token::Comma) => Some(p),
+                // write!(e) or error
+                e => return (None, e.ok()),
             }
-        }
+        } else {
+            None
+        };
 
         let fmtstr = match parser.parse_str_lit() {
             Ok(fmtstr) => fmtstr,
             Err(_) => return (None, expr),
         };
-        let tmp = fmtstr.symbol.as_str();
-        let mut args = vec![];
-        let mut fmt_parser = Parser::new(&tmp, None, None, false, ParseMode::Format);
-        while let Some(piece) = fmt_parser.next() {
-            if !fmt_parser.errors.is_empty() {
-                return (None, expr);
-            }
-            if let Piece::NextArgument(arg) = piece {
-                if !self.in_debug_impl && arg.format.ty == "?" {
-                    // FIXME: modify rustc's fmt string parser to give us the current span
-                    span_lint(cx, USE_DEBUG, parser.prev_token.span, "use of `Debug`-based formatting");
-                }
-                args.push(arg);
-            }
-        }
+
+        let args = match self.parse_fmt_string(cx, &fmtstr) {
+            Some(args) => args,
+            None => return (Some(fmtstr), expr),
+        };
+
         let lint = if is_write { WRITE_LITERAL } else { PRINT_LITERAL };
-        let mut idx = 0;
+        let mut unnamed_args = args.get_unnamed();
         loop {
-            const SIMPLE: FormatSpec<'_> = FormatSpec {
-                fill: None,
-                align: AlignUnknown,
-                flags: 0,
-                precision: CountImplied,
-                precision_span: None,
-                width: CountImplied,
-                width_span: None,
-                ty: "",
-                ty_span: None,
-            };
             if !parser.eat(&token::Comma) {
                 return (Some(fmtstr), expr);
             }
+
+            let comma_span = parser.prev_token.span;
             let token_expr = if let Ok(expr) = parser.parse_expr().map_err(|mut err| err.cancel()) {
                 expr
             } else {
                 return (Some(fmtstr), None);
             };
-            match &token_expr.kind {
-                ExprKind::Lit(lit) if !matches!(lit.kind, LitKind::Int(..) | LitKind::Float(..)) => {
-                    let mut all_simple = true;
-                    let mut seen = false;
-                    for arg in &args {
-                        match arg.position {
-                            ArgumentImplicitlyIs(n) | ArgumentIs(n) => {
-                                if n == idx {
-                                    all_simple &= arg.format == SIMPLE;
-                                    seen = true;
-                                }
-                            },
-                            ArgumentNamed(_) => {},
-                        }
-                    }
-                    if all_simple && seen {
-                        span_lint(cx, lint, token_expr.span, "literal with an empty format string");
-                    }
-                    idx += 1;
+            let (fmt_spans, lit) = match &token_expr.kind {
+                ExprKind::Lit(lit) => (unnamed_args.next().unwrap_or(&[]), lit),
+                ExprKind::Assign(lhs, rhs, _) => match (&lhs.kind, &rhs.kind) {
+                    (ExprKind::Path(_, p), ExprKind::Lit(lit)) => (args.get_named(p), lit),
+                    _ => continue,
                 },
-                ExprKind::Assign(lhs, rhs, _) => {
-                    if_chain! {
-                        if let ExprKind::Lit(ref lit) = rhs.kind;
-                        if !matches!(lit.kind, LitKind::Int(..) | LitKind::Float(..));
-                        if let ExprKind::Path(_, p) = &lhs.kind;
-                        then {
-                            let mut all_simple = true;
-                            let mut seen = false;
-                            for arg in &args {
-                                match arg.position {
-                                    ArgumentImplicitlyIs(_) | ArgumentIs(_) => {},
-                                    ArgumentNamed(name) => {
-                                        if *p == name {
-                                            seen = true;
-                                            all_simple &= arg.format == SIMPLE;
-                                        }
-                                    },
-                                }
-                            }
-                            if all_simple && seen {
-                                span_lint(cx, lint, rhs.span, "literal with an empty format string");
-                            }
-                        }
-                    }
+                _ => {
+                    unnamed_args.next();
+                    continue;
+                },
+            };
+
+            let replacement: String = match lit.token.kind {
+                LitKind::Integer | LitKind::Float | LitKind::Err => continue,
+                LitKind::StrRaw(_) | LitKind::ByteStrRaw(_) if matches!(fmtstr.style, StrStyle::Raw(_)) => {
+                    lit.token.symbol.as_str().replace("{", "{{").replace("}", "}}")
                 },
-                _ => idx += 1,
+                LitKind::Str | LitKind::ByteStr if matches!(fmtstr.style, StrStyle::Cooked) => {
+                    lit.token.symbol.as_str().replace("{", "{{").replace("}", "}}")
+                },
+                LitKind::StrRaw(_) | LitKind::Str | LitKind::ByteStrRaw(_) | LitKind::ByteStr => continue,
+                LitKind::Byte | LitKind::Char => match &*lit.token.symbol.as_str() {
+                    "\"" if matches!(fmtstr.style, StrStyle::Cooked) => "\\\"",
+                    "\"" if matches!(fmtstr.style, StrStyle::Raw(0)) => continue,
+                    "\\\\" if matches!(fmtstr.style, StrStyle::Raw(_)) => "\\",
+                    "\\'" => "'",
+                    "{" => "{{",
+                    "}" => "}}",
+                    x if matches!(fmtstr.style, StrStyle::Raw(_)) && x.starts_with('\\') => continue,
+                    x => x,
+                }
+                .into(),
+                LitKind::Bool => lit.token.symbol.as_str().deref().into(),
+            };
+
+            if !fmt_spans.is_empty() {
+                span_lint_and_then(
+                    cx,
+                    lint,
+                    token_expr.span,
+                    "literal with an empty format string",
+                    |diag| {
+                        diag.multipart_suggestion(
+                            "try this",
+                            iter::once((comma_span.to(token_expr.span), String::new()))
+                                .chain(fmt_spans.iter().cloned().zip(iter::repeat(replacement)))
+                                .collect(),
+                            Applicability::MachineApplicable,
+                        );
+                    },
+                );
             }
         }
     }
diff --git a/tests/ui/print.stderr b/tests/ui/print.stderr
index 208d9532628..1754c418381 100644
--- a/tests/ui/print.stderr
+++ b/tests/ui/print.stderr
@@ -1,8 +1,8 @@
 error: use of `Debug`-based formatting
-  --> $DIR/print.rs:11:19
+  --> $DIR/print.rs:11:20
    |
 LL |         write!(f, "{:?}", 43.1415)
-   |                   ^^^^^^
+   |                    ^^^^
    |
    = note: `-D clippy::use-debug` implied by `-D warnings`
 
@@ -33,10 +33,10 @@ LL |     print!("Hello {:?}", "World");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: use of `Debug`-based formatting
-  --> $DIR/print.rs:28:12
+  --> $DIR/print.rs:28:19
    |
 LL |     print!("Hello {:?}", "World");
-   |            ^^^^^^^^^^^^
+   |                   ^^^^
 
 error: use of `print!`
   --> $DIR/print.rs:30:5
@@ -45,10 +45,10 @@ LL |     print!("Hello {:#?}", "#orld");
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: use of `Debug`-based formatting
-  --> $DIR/print.rs:30:12
+  --> $DIR/print.rs:30:19
    |
 LL |     print!("Hello {:#?}", "#orld");
-   |            ^^^^^^^^^^^^^
+   |                   ^^^^^
 
 error: aborting due to 8 previous errors
 
diff --git a/tests/ui/print_literal.stderr b/tests/ui/print_literal.stderr
index e284aece236..54a4084c89e 100644
--- a/tests/ui/print_literal.stderr
+++ b/tests/ui/print_literal.stderr
@@ -5,66 +5,120 @@ LL |     print!("Hello {}", "world");
    |                        ^^^^^^^
    |
    = note: `-D clippy::print-literal` implied by `-D warnings`
+help: try this
+   |
+LL |     print!("Hello world");
+   |                   ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:26:36
    |
 LL |     println!("Hello {} {}", world, "world");
    |                                    ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("Hello {} world", world);
+   |                        ^^^^^       --
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:27:26
    |
 LL |     println!("Hello {}", "world");
    |                          ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("Hello world");
+   |                     ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:32:25
    |
 LL |     println!("{0} {1}", "hello", "world");
    |                         ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("hello {1}", "world");
+   |               ^^^^^    --
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:32:34
    |
 LL |     println!("{0} {1}", "hello", "world");
    |                                  ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("{0} world", "hello");
+   |                   ^^^^^         --
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:33:25
    |
 LL |     println!("{1} {0}", "hello", "world");
    |                         ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("{1} hello", "world");
+   |                   ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/print_literal.rs:33:34
    |
 LL |     println!("{1} {0}", "hello", "world");
    |                                  ^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("world {0}", "hello");
+   |               ^^^^^             --
 
 error: literal with an empty format string
-  --> $DIR/print_literal.rs:36:35
+  --> $DIR/print_literal.rs:36:29
    |
 LL |     println!("{foo} {bar}", foo = "hello", bar = "world");
-   |                                   ^^^^^^^
+   |                             ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("hello {bar}", bar = "world");
+   |               ^^^^^      --
 
 error: literal with an empty format string
-  --> $DIR/print_literal.rs:36:50
+  --> $DIR/print_literal.rs:36:44
    |
 LL |     println!("{foo} {bar}", foo = "hello", bar = "world");
-   |                                                  ^^^^^^^
+   |                                            ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("{foo} world", foo = "hello");
+   |                     ^^^^^               --
 
 error: literal with an empty format string
-  --> $DIR/print_literal.rs:37:35
+  --> $DIR/print_literal.rs:37:29
    |
 LL |     println!("{bar} {foo}", foo = "hello", bar = "world");
-   |                                   ^^^^^^^
+   |                             ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("{bar} hello", bar = "world");
+   |                     ^^^^^--
 
 error: literal with an empty format string
-  --> $DIR/print_literal.rs:37:50
+  --> $DIR/print_literal.rs:37:44
    |
 LL |     println!("{bar} {foo}", foo = "hello", bar = "world");
-   |                                                  ^^^^^^^
+   |                                            ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     println!("world {foo}", foo = "hello");
+   |               ^^^^^                     --
 
 error: aborting due to 11 previous errors
 
diff --git a/tests/ui/write_literal.stderr b/tests/ui/write_literal.stderr
index e54d89ecf29..507a78e8280 100644
--- a/tests/ui/write_literal.stderr
+++ b/tests/ui/write_literal.stderr
@@ -5,66 +5,120 @@ LL |     write!(&mut v, "Hello {}", "world");
    |                                ^^^^^^^
    |
    = note: `-D clippy::write-literal` implied by `-D warnings`
+help: try this
+   |
+LL |     write!(&mut v, "Hello world");
+   |                           ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:31:44
    |
 LL |     writeln!(&mut v, "Hello {} {}", world, "world");
    |                                            ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "Hello {} world", world);
+   |                                ^^^^^       --
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:32:34
    |
 LL |     writeln!(&mut v, "Hello {}", "world");
    |                                  ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "Hello world");
+   |                             ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:37:33
    |
 LL |     writeln!(&mut v, "{0} {1}", "hello", "world");
    |                                 ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "hello {1}", "world");
+   |                       ^^^^^    --
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:37:42
    |
 LL |     writeln!(&mut v, "{0} {1}", "hello", "world");
    |                                          ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "{0} world", "hello");
+   |                           ^^^^^         --
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:38:33
    |
 LL |     writeln!(&mut v, "{1} {0}", "hello", "world");
    |                                 ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "{1} hello", "world");
+   |                           ^^^^^--
 
 error: literal with an empty format string
   --> $DIR/write_literal.rs:38:42
    |
 LL |     writeln!(&mut v, "{1} {0}", "hello", "world");
    |                                          ^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "world {0}", "hello");
+   |                       ^^^^^             --
 
 error: literal with an empty format string
-  --> $DIR/write_literal.rs:41:43
+  --> $DIR/write_literal.rs:41:37
    |
 LL |     writeln!(&mut v, "{foo} {bar}", foo = "hello", bar = "world");
-   |                                           ^^^^^^^
+   |                                     ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "hello {bar}", bar = "world");
+   |                       ^^^^^      --
 
 error: literal with an empty format string
-  --> $DIR/write_literal.rs:41:58
+  --> $DIR/write_literal.rs:41:52
    |
 LL |     writeln!(&mut v, "{foo} {bar}", foo = "hello", bar = "world");
-   |                                                          ^^^^^^^
+   |                                                    ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "{foo} world", foo = "hello");
+   |                             ^^^^^               --
 
 error: literal with an empty format string
-  --> $DIR/write_literal.rs:42:43
+  --> $DIR/write_literal.rs:42:37
    |
 LL |     writeln!(&mut v, "{bar} {foo}", foo = "hello", bar = "world");
-   |                                           ^^^^^^^
+   |                                     ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "{bar} hello", bar = "world");
+   |                             ^^^^^--
 
 error: literal with an empty format string
-  --> $DIR/write_literal.rs:42:58
+  --> $DIR/write_literal.rs:42:52
    |
 LL |     writeln!(&mut v, "{bar} {foo}", foo = "hello", bar = "world");
-   |                                                          ^^^^^^^
+   |                                                    ^^^^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "world {foo}", foo = "hello");
+   |                       ^^^^^                     --
 
 error: aborting due to 11 previous errors
 
diff --git a/tests/ui/write_literal_2.rs b/tests/ui/write_literal_2.rs
new file mode 100644
index 00000000000..f341e8215e1
--- /dev/null
+++ b/tests/ui/write_literal_2.rs
@@ -0,0 +1,27 @@
+#![allow(unused_must_use)]
+#![warn(clippy::write_literal)]
+
+use std::io::Write;
+
+fn main() {
+    let mut v = Vec::new();
+
+    writeln!(&mut v, "{}", "{hello}");
+    writeln!(&mut v, r"{}", r"{hello}");
+    writeln!(&mut v, "{}", '\'');
+    writeln!(&mut v, "{}", '"');
+    writeln!(&mut v, r"{}", '"'); // don't lint
+    writeln!(&mut v, r"{}", '\'');
+    writeln!(
+        &mut v,
+        "some {}",
+        "hello \
+        world!"
+    );
+    writeln!(
+        &mut v,
+        "some {}\
+        {} \\ {}",
+        "1", "2", "3",
+    );
+}
diff --git a/tests/ui/write_literal_2.stderr b/tests/ui/write_literal_2.stderr
new file mode 100644
index 00000000000..5b488358011
--- /dev/null
+++ b/tests/ui/write_literal_2.stderr
@@ -0,0 +1,106 @@
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:9:28
+   |
+LL |     writeln!(&mut v, "{}", "{hello}");
+   |                            ^^^^^^^^^
+   |
+   = note: `-D clippy::write-literal` implied by `-D warnings`
+help: try this
+   |
+LL |     writeln!(&mut v, "{{hello}}");
+   |                       ^^^^^^^^^--
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:10:29
+   |
+LL |     writeln!(&mut v, r"{}", r"{hello}");
+   |                             ^^^^^^^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, r"{{hello}}");
+   |                        ^^^^^^^^^--
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:11:28
+   |
+LL |     writeln!(&mut v, "{}", '/'');
+   |                            ^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "'");
+   |                       ^--
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:12:28
+   |
+LL |     writeln!(&mut v, "{}", '"');
+   |                            ^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, "/"");
+   |                       ^^--
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:14:29
+   |
+LL |     writeln!(&mut v, r"{}", '/'');
+   |                             ^^^^
+   |
+help: try this
+   |
+LL |     writeln!(&mut v, r"'");
+   |                        ^--
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:18:9
+   |
+LL | /         "hello /
+LL | |         world!"
+   | |_______________^
+   |
+help: try this
+   |
+LL |         "some hello /
+LL |         world!"
+   |
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:25:9
+   |
+LL |         "1", "2", "3",
+   |         ^^^
+   |
+help: try this
+   |
+LL |         "some 1{} / {}", "2", "3",
+   |               ^        --
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:25:14
+   |
+LL |         "1", "2", "3",
+   |              ^^^
+   |
+help: try this
+   |
+LL |         2 / {}",
+LL |         "1", "3",
+   |
+
+error: literal with an empty format string
+  --> $DIR/write_literal_2.rs:25:19
+   |
+LL |         "1", "2", "3",
+   |                   ^^^
+   |
+help: try this
+   |
+LL |         {} / 3",
+LL |         "1", "2",
+   |
+
+error: aborting due to 9 previous errors
+