Auto merge of #7442 - camsteffen:format-args, r=xFrednet

Refactor `format_args!` expansion parsing

Introduces `FormatExpn::parse` and `FormatArgsExpn::parse`. Motivated by rust-lang/rust#83302, so I only have to change Clippy in one place. Fixed an FP along the way.

I also allowed `needless_bool` in macros because I often want to do `if_chain! { .. then { true } else { false } }`.

changelog: Fix false positive in `useless_format` when some text is appended or prepended to a single string with some useless formatting params
changelog: Allow `needless_bool` in macros

9 files changed, 238 insertions, 265 deletions

diff --git a/clippy_lints/src/explicit_write.rs b/clippy_lints/src/explicit_write.rs
index da4936ff25b..66724294804 100644
--- a/clippy_lints/src/explicit_write.rs
+++ b/clippy_lints/src/explicit_write.rs
@@ -1,9 +1,9 @@
-use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
+use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
+use clippy_utils::higher::FormatArgsExpn;
 use clippy_utils::{is_expn_of, match_function_call, paths};
 use if_chain::if_chain;
-use rustc_ast::ast::LitKind;
 use rustc_errors::Applicability;
-use rustc_hir::{BorrowKind, Expr, ExprKind};
+use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::sym;
@@ -34,29 +34,26 @@ impl<'tcx> LateLintPass<'tcx> for ExplicitWrite {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         if_chain! {
             // match call to unwrap
-            if let ExprKind::MethodCall(unwrap_fun, _, unwrap_args, _) = expr.kind;
+            if let ExprKind::MethodCall(unwrap_fun, _, [write_call], _) = expr.kind;
             if unwrap_fun.ident.name == sym::unwrap;
             // match call to write_fmt
-            if !unwrap_args.is_empty();
-            if let ExprKind::MethodCall(write_fun, _, write_args, _) =
-                unwrap_args[0].kind;
+            if let ExprKind::MethodCall(write_fun, _, [write_recv, write_arg], _) = write_call.kind;
             if write_fun.ident.name == sym!(write_fmt);
             // match calls to std::io::stdout() / std::io::stderr ()
-            if !write_args.is_empty();
-            if let Some(dest_name) = if match_function_call(cx, &write_args[0], &paths::STDOUT).is_some() {
+            if let Some(dest_name) = if match_function_call(cx, write_recv, &paths::STDOUT).is_some() {
                 Some("stdout")
-            } else if match_function_call(cx, &write_args[0], &paths::STDERR).is_some() {
+            } else if match_function_call(cx, write_recv, &paths::STDERR).is_some() {
                 Some("stderr")
             } else {
                 None
             };
+            if let Some(format_args) = FormatArgsExpn::parse(write_arg);
             then {
-                let write_span = unwrap_args[0].span;
                 let calling_macro =
                     // ordering is important here, since `writeln!` uses `write!` internally
-                    if is_expn_of(write_span, "writeln").is_some() {
+                    if is_expn_of(write_call.span, "writeln").is_some() {
                         Some("writeln")
-                    } else if is_expn_of(write_span, "write").is_some() {
+                    } else if is_expn_of(write_call.span, "write").is_some() {
                         Some("write")
                     } else {
                         None
@@ -70,82 +67,40 @@ impl<'tcx> LateLintPass<'tcx> for ExplicitWrite {
                 // We need to remove the last trailing newline from the string because the
                 // underlying `fmt::write` function doesn't know whether `println!` or `print!` was
                 // used.
-                if let Some(mut write_output) = write_output_string(write_args) {
+                let (used, sugg_mac) = if let Some(macro_name) = calling_macro {
+                    (
+                        format!("{}!({}(), ...)", macro_name, dest_name),
+                        macro_name.replace("write", "print"),
+                    )
+                } else {
+                    (
+                        format!("{}().write_fmt(...)", dest_name),
+                        "print".into(),
+                    )
+                };
+                let msg = format!("use of `{}.unwrap()`", used);
+                if let [write_output] = *format_args.format_string_symbols {
+                    let mut write_output = write_output.to_string();
                     if write_output.ends_with('\n') {
                         write_output.pop();
                     }
 
-                    if let Some(macro_name) = calling_macro {
-                        span_lint_and_sugg(
-                            cx,
-                            EXPLICIT_WRITE,
-                            expr.span,
-                            &format!(
-                                "use of `{}!({}(), ...).unwrap()`",
-                                macro_name,
-                                dest_name
-                            ),
-                            "try this",
-                            format!("{}{}!(\"{}\")", prefix, macro_name.replace("write", "print"), write_output.escape_default()),
-                            Applicability::MachineApplicable
-                        );
-                    } else {
-                        span_lint_and_sugg(
-                            cx,
-                            EXPLICIT_WRITE,
-                            expr.span,
-                            &format!("use of `{}().write_fmt(...).unwrap()`", dest_name),
-                            "try this",
-                            format!("{}print!(\"{}\")", prefix, write_output.escape_default()),
-                            Applicability::MachineApplicable
-                        );
-                    }
+                    let sugg = format!("{}{}!(\"{}\")", prefix, sugg_mac, write_output.escape_default());
+                    span_lint_and_sugg(
+                        cx,
+                        EXPLICIT_WRITE,
+                        expr.span,
+                        &msg,
+                        "try this",
+                        sugg,
+                        Applicability::MachineApplicable
+                    );
                 } else {
                     // We don't have a proper suggestion
-                    if let Some(macro_name) = calling_macro {
-                        span_lint(
-                            cx,
-                            EXPLICIT_WRITE,
-                            expr.span,
-                            &format!(
-                                "use of `{}!({}(), ...).unwrap()`. Consider using `{}{}!` instead",
-                                macro_name,
-                                dest_name,
-                                prefix,
-                                macro_name.replace("write", "print")
-                            )
-                        );
-                    } else {
-                        span_lint(
-                            cx,
-                            EXPLICIT_WRITE,
-                            expr.span,
-                            &format!("use of `{}().write_fmt(...).unwrap()`. Consider using `{}print!` instead", dest_name, prefix),
-                        );
-                    }
+                    let help = format!("consider using `{}{}!` instead", prefix, sugg_mac);
+                    span_lint_and_help(cx, EXPLICIT_WRITE, expr.span, &msg, None, &help);
                 }
-
             }
         }
     }
 }
-
-// Extract the output string from the given `write_args`.
-fn write_output_string(write_args: &[Expr<'_>]) -> Option<String> {
-    if_chain! {
-        // Obtain the string that should be printed
-        if write_args.len() > 1;
-        if let ExprKind::Call(_, output_args) = write_args[1].kind;
-        if !output_args.is_empty();
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, output_string_expr) = output_args[0].kind;
-        if let ExprKind::Array(string_exprs) = output_string_expr.kind;
-        // we only want to provide an automatic suggestion for simple (non-format) strings
-        if string_exprs.len() == 1;
-        if let ExprKind::Lit(ref lit) = string_exprs[0].kind;
-        if let LitKind::Str(ref write_output, _) = lit.node;
-        then {
-            return Some(write_output.to_string())
-        }
-    }
-    None
-}
diff --git a/clippy_lints/src/format.rs b/clippy_lints/src/format.rs
index c2b055ed648..ca3490d8eda 100644
--- a/clippy_lints/src/format.rs
+++ b/clippy_lints/src/format.rs
@@ -1,17 +1,16 @@
-use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::paths;
-use clippy_utils::source::{snippet, snippet_opt};
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::higher::FormatExpn;
+use clippy_utils::last_path_segment;
+use clippy_utils::source::{snippet_opt, snippet_with_applicability};
 use clippy_utils::sugg::Sugg;
-use clippy_utils::ty::is_type_diagnostic_item;
-use clippy_utils::{is_expn_of, last_path_segment, match_def_path, match_function_call};
 use if_chain::if_chain;
-use rustc_ast::ast::LitKind;
 use rustc_errors::Applicability;
-use rustc_hir::{Arm, BorrowKind, Expr, ExprKind, MatchSource, PatKind};
-use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_hir::{BorrowKind, Expr, ExprKind, QPath};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::source_map::Span;
-use rustc_span::sym;
+use rustc_span::symbol::kw;
+use rustc_span::{sym, Span};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for the use of `format!("string literal with no
@@ -44,130 +43,78 @@ declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);
 
 impl<'tcx> LateLintPass<'tcx> for UselessFormat {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        let span = match is_expn_of(expr.span, "format") {
-            Some(s) if !s.from_expansion() => s,
+        let FormatExpn { call_site, format_args } = match FormatExpn::parse(expr) {
+            Some(e) if !e.call_site.from_expansion() => e,
             _ => return,
         };
 
-        // Operate on the only argument of `alloc::fmt::format`.
-        if let Some(sugg) = on_new_v1(cx, expr) {
-            span_useless_format(cx, span, "consider using `.to_string()`", sugg);
-        } else if let Some(sugg) = on_new_v1_fmt(cx, expr) {
-            span_useless_format(cx, span, "consider using `.to_string()`", sugg);
-        }
-    }
-}
-
-fn span_useless_format<T: LintContext>(cx: &T, span: Span, help: &str, mut sugg: String) {
-    let to_replace = span.source_callsite();
-
-    // The callsite span contains the statement semicolon for some reason.
-    let snippet = snippet(cx, to_replace, "..");
-    if snippet.ends_with(';') {
-        sugg.push(';');
-    }
-
-    span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |diag| {
-        diag.span_suggestion(
-            to_replace,
-            help,
-            sugg,
-            Applicability::MachineApplicable, // snippet
-        );
-    });
-}
-
-fn on_argumentv1_new<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, arms: &'tcx [Arm<'_>]) -> Option<String> {
-    if_chain! {
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, format_args) = expr.kind;
-        if let ExprKind::Array(elems) = arms[0].body.kind;
-        if elems.len() == 1;
-        if let Some(args) = match_function_call(cx, &elems[0], &paths::FMT_ARGUMENTV1_NEW);
-        // matches `core::fmt::Display::fmt`
-        if args.len() == 2;
-        if let ExprKind::Path(ref qpath) = args[1].kind;
-        if let Some(did) = cx.qpath_res(qpath, args[1].hir_id).opt_def_id();
-        if match_def_path(cx, did, &paths::DISPLAY_FMT_METHOD);
-        // check `(arg0,)` in match block
-        if let PatKind::Tuple(pats, None) = arms[0].pat.kind;
-        if pats.len() == 1;
-        then {
-            let ty = cx.typeck_results().pat_ty(pats[0]).peel_refs();
-            if *ty.kind() != rustc_middle::ty::Str && !is_type_diagnostic_item(cx, ty, sym::string_type) {
-                return None;
-            }
-            if let ExprKind::Lit(ref lit) = format_args.kind {
-                if let LitKind::Str(ref s, _) = lit.node {
-                    return Some(format!("{:?}.to_string()", s.as_str()));
+        let mut applicability = Applicability::MachineApplicable;
+        if format_args.value_args.is_empty() {
+            if_chain! {
+                if let [e] = &*format_args.format_string_parts;
+                if let ExprKind::Lit(lit) = &e.kind;
+                if let Some(s_src) = snippet_opt(cx, lit.span);
+                then {
+                    // Simulate macro expansion, converting {{ and }} to { and }.
+                    let s_expand = s_src.replace("{{", "{").replace("}}", "}");
+                    let sugg = format!("{}.to_string()", s_expand);
+                    span_useless_format(cx, call_site, sugg, applicability);
                 }
-            } else {
-                let sugg = Sugg::hir(cx, format_args, "<arg>");
-                if let ExprKind::MethodCall(path, _, _, _) = format_args.kind {
-                    if path.ident.name == sym!(to_string) {
-                        return Some(format!("{}", sugg));
-                    }
-                } else if let ExprKind::Binary(..) = format_args.kind {
-                    return Some(format!("{}", sugg));
+            }
+        } else if let [value] = *format_args.value_args {
+            if_chain! {
+                if format_args.format_string_symbols == [kw::Empty];
+                if match cx.typeck_results().expr_ty(value).peel_refs().kind() {
+                    ty::Adt(adt, _) => cx.tcx.is_diagnostic_item(sym::string_type, adt.did),
+                    ty::Str => true,
+                    _ => false,
+                };
+                if format_args.args.iter().all(|e| is_display_arg(e));
+                if format_args.fmt_expr.map_or(true, |e| check_unformatted(e));
+                then {
+                    let is_new_string = match value.kind {
+                        ExprKind::Binary(..) => true,
+                        ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
+                        _ => false,
+                    };
+                    let sugg = if is_new_string {
+                        snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
+                    } else {
+                        let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);
+                        format!("{}.to_string()", sugg.maybe_par())
+                    };
+                    span_useless_format(cx, call_site, sugg, applicability);
                 }
-                return Some(format!("{}.to_string()", sugg.maybe_par()));
             }
-        }
+        };
     }
-    None
 }
 
-fn on_new_v1<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<String> {
-    if_chain! {
-        if let Some(args) = match_function_call(cx, expr, &paths::FMT_ARGUMENTS_NEW_V1);
-        if args.len() == 2;
-        // Argument 1 in `new_v1()`
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, arr) = args[0].kind;
-        if let ExprKind::Array(pieces) = arr.kind;
-        if pieces.len() == 1;
-        if let ExprKind::Lit(ref lit) = pieces[0].kind;
-        if let LitKind::Str(ref s, _) = lit.node;
-        // Argument 2 in `new_v1()`
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, arg1) = args[1].kind;
-        if let ExprKind::Match(matchee, arms, MatchSource::Normal) = arg1.kind;
-        if arms.len() == 1;
-        if let ExprKind::Tup(tup) = matchee.kind;
-        then {
-            // `format!("foo")` expansion contains `match () { () => [], }`
-            if tup.is_empty() {
-                if let Some(s_src) = snippet_opt(cx, lit.span) {
-                    // Simulate macro expansion, converting {{ and }} to { and }.
-                    let s_expand = s_src.replace("{{", "{").replace("}}", "}");
-                    return Some(format!("{}.to_string()", s_expand));
-                }
-            } else if s.as_str().is_empty() {
-                return on_argumentv1_new(cx, &tup[0], arms);
-            }
-        }
+fn span_useless_format(cx: &LateContext<'_>, span: Span, mut sugg: String, mut applicability: Applicability) {
+    // The callsite span contains the statement semicolon for some reason.
+    if snippet_with_applicability(cx, span, "..", &mut applicability).ends_with(';') {
+        sugg.push(';');
     }
-    None
+
+    span_lint_and_sugg(
+        cx,
+        USELESS_FORMAT,
+        span,
+        "useless use of `format!`",
+        "consider using `.to_string()`",
+        sugg,
+        applicability,
+    );
 }
 
-fn on_new_v1_fmt<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<String> {
+fn is_display_arg(expr: &Expr<'_>) -> bool {
     if_chain! {
-        if let Some(args) = match_function_call(cx, expr, &paths::FMT_ARGUMENTS_NEW_V1_FORMATTED);
-        if args.len() == 3;
-        if check_unformatted(&args[2]);
-        // Argument 1 in `new_v1_formatted()`
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, arr) = args[0].kind;
-        if let ExprKind::Array(pieces) = arr.kind;
-        if pieces.len() == 1;
-        if let ExprKind::Lit(ref lit) = pieces[0].kind;
-        if let LitKind::Str(..) = lit.node;
-        // Argument 2 in `new_v1_formatted()`
-        if let ExprKind::AddrOf(BorrowKind::Ref, _, arg1) = args[1].kind;
-        if let ExprKind::Match(matchee, arms, MatchSource::Normal) = arg1.kind;
-        if arms.len() == 1;
-        if let ExprKind::Tup(tup) = matchee.kind;
-        then {
-            return on_argumentv1_new(cx, &tup[0], arms);
-        }
+        if let ExprKind::Call(_, [_, fmt]) = expr.kind;
+        if let ExprKind::Path(QPath::Resolved(_, path)) = fmt.kind;
+        if let [.., t, _] = path.segments;
+        if t.ident.name.as_str() == "Display";
+        then { true } else { false }
     }
-    None
 }
 
 /// Checks if the expression matches
@@ -184,10 +131,9 @@ fn on_new_v1_fmt<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> Option<S
 fn check_unformatted(expr: &Expr<'_>) -> bool {
     if_chain! {
         if let ExprKind::AddrOf(BorrowKind::Ref, _, expr) = expr.kind;
-        if let ExprKind::Array(exprs) = expr.kind;
-        if exprs.len() == 1;
+        if let ExprKind::Array([expr]) = expr.kind;
         // struct `core::fmt::rt::v1::Argument`
-        if let ExprKind::Struct(_, fields, _) = exprs[0].kind;
+        if let ExprKind::Struct(_, fields, _) = expr.kind;
         if let Some(format_field) = fields.iter().find(|f| f.ident.name == sym::format);
         // struct `core::fmt::rt::v1::FormatSpec`
         if let ExprKind::Struct(_, fields, _) = format_field.expr.kind;
diff --git a/clippy_lints/src/methods/expect_fun_call.rs b/clippy_lints/src/methods/expect_fun_call.rs
index 03cb41697d5..f8ee31a00df 100644
--- a/clippy_lints/src/methods/expect_fun_call.rs
+++ b/clippy_lints/src/methods/expect_fun_call.rs
@@ -1,8 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::is_expn_of;
-use clippy_utils::source::{snippet, snippet_with_applicability};
+use clippy_utils::higher::FormatExpn;
+use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::ty::is_type_diagnostic_item;
-use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_lint::LateContext;
@@ -94,27 +93,6 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_span: Spa
         }
     }
 
-    fn generate_format_arg_snippet(
-        cx: &LateContext<'_>,
-        a: &hir::Expr<'_>,
-        applicability: &mut Applicability,
-    ) -> Vec<String> {
-        if_chain! {
-            if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, format_arg) = a.kind;
-            if let hir::ExprKind::Match(format_arg_expr, _, _) = format_arg.kind;
-            if let hir::ExprKind::Tup(format_arg_expr_tup) = format_arg_expr.kind;
-
-            then {
-                format_arg_expr_tup
-                    .iter()
-                    .map(|a| snippet_with_applicability(cx, a.span, "..", applicability).into_owned())
-                    .collect()
-            } else {
-                unreachable!()
-            }
-        }
-    }
-
     fn is_call(node: &hir::ExprKind<'_>) -> bool {
         match node {
             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, expr) => {
@@ -150,36 +128,22 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_span: Spa
     let mut applicability = Applicability::MachineApplicable;
 
     //Special handling for `format!` as arg_root
-    if_chain! {
-        if let hir::ExprKind::Block(block, None) = &arg_root.kind;
-        if block.stmts.len() == 1;
-        if let hir::StmtKind::Local(local) = &block.stmts[0].kind;
-        if let Some(arg_root) = &local.init;
-        if let hir::ExprKind::Call(inner_fun, inner_args) = arg_root.kind;
-        if is_expn_of(inner_fun.span, "format").is_some() && inner_args.len() == 1;
-        if let hir::ExprKind::Call(_, format_args) = &inner_args[0].kind;
-        then {
-            let fmt_spec = &format_args[0];
-            let fmt_args = &format_args[1];
-
-            let mut args = vec![snippet(cx, fmt_spec.span, "..").into_owned()];
-
-            args.extend(generate_format_arg_snippet(cx, fmt_args, &mut applicability));
-
-            let sugg = args.join(", ");
-
-            span_lint_and_sugg(
-                cx,
-                EXPECT_FUN_CALL,
-                span_replace_word,
-                &format!("use of `{}` followed by a function call", name),
-                "try this",
-                format!("unwrap_or_else({} panic!({}))", closure_args, sugg),
-                applicability,
-            );
-
-            return;
-        }
+    if let Some(format_expn) = FormatExpn::parse(arg_root) {
+        let span = match *format_expn.format_args.value_args {
+            [] => format_expn.format_args.format_string_span,
+            [.., last] => format_expn.format_args.format_string_span.to(last.span),
+        };
+        let sugg = snippet_with_applicability(cx, span, "..", &mut applicability);
+        span_lint_and_sugg(
+            cx,
+            EXPECT_FUN_CALL,
+            span_replace_word,
+            &format!("use of `{}` followed by a function call", name),
+            "try this",
+            format!("unwrap_or_else({} panic!({}))", closure_args, sugg),
+            applicability,
+        );
+        return;
     }
 
     let mut arg_root_snippet: Cow<'_, _> = snippet_with_applicability(cx, arg_root.span, "..", &mut applicability);
diff --git a/clippy_lints/src/needless_bool.rs b/clippy_lints/src/needless_bool.rs
index 3b3736fd3a1..780690548e5 100644
--- a/clippy_lints/src/needless_bool.rs
+++ b/clippy_lints/src/needless_bool.rs
@@ -71,6 +71,9 @@ declare_lint_pass!(NeedlessBool => [NEEDLESS_BOOL]);
 impl<'tcx> LateLintPass<'tcx> for NeedlessBool {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
         use self::Expression::{Bool, RetBool};
+        if e.span.from_expansion() {
+            return;
+        }
         if let ExprKind::If(pred, then_block, Some(else_expr)) = e.kind {
             let reduce = |ret, not| {
                 let mut applicability = Applicability::MachineApplicable;
diff --git a/clippy_utils/src/higher.rs b/clippy_utils/src/higher.rs
index 8be36756b33..5ffd88f4140 100644
--- a/clippy_utils/src/higher.rs
+++ b/clippy_utils/src/higher.rs
@@ -5,11 +5,11 @@
 
 use crate::{is_expn_of, match_def_path, paths};
 use if_chain::if_chain;
-use rustc_ast::ast;
+use rustc_ast::ast::{self, LitKind};
 use rustc_hir as hir;
 use rustc_hir::{BorrowKind, Expr, ExprKind, StmtKind, UnOp};
 use rustc_lint::LateContext;
-use rustc_span::source_map::Span;
+use rustc_span::{sym, ExpnKind, Span, Symbol};
 
 /// Converts a hir binary operator to the corresponding `ast` type.
 #[must_use]
@@ -266,3 +266,107 @@ pub fn extract_assert_macro_args<'tcx>(e: &'tcx Expr<'tcx>) -> Option<Vec<&'tcx
     }
     None
 }
+
+/// A parsed `format!` expansion
+pub struct FormatExpn<'tcx> {
+    /// Span of `format!(..)`
+    pub call_site: Span,
+    /// Inner `format_args!` expansion
+    pub format_args: FormatArgsExpn<'tcx>,
+}
+
+impl FormatExpn<'tcx> {
+    /// Parses an expanded `format!` invocation
+    pub fn parse(expr: &'tcx Expr<'tcx>) -> Option<Self> {
+        if_chain! {
+            if let ExprKind::Block(block, _) = expr.kind;
+            if let [stmt] = block.stmts;
+            if let StmtKind::Local(local) = stmt.kind;
+            if let Some(init) = local.init;
+            if let ExprKind::Call(_, [format_args]) = init.kind;
+            let expn_data = expr.span.ctxt().outer_expn_data();
+            if let ExpnKind::Macro { name: sym::format, .. } = expn_data.kind;
+            if let Some(format_args) = FormatArgsExpn::parse(format_args);
+            then {
+                Some(FormatExpn {
+                    call_site: expn_data.call_site,
+                    format_args,
+                })
+            } else {
+                None
+            }
+        }
+    }
+}
+
+/// A parsed `format_args!` expansion
+pub struct FormatArgsExpn<'tcx> {
+    /// Span of the first argument, the format string
+    pub format_string_span: Span,
+    /// Values passed after the format string
+    pub value_args: Vec<&'tcx Expr<'tcx>>,
+
+    /// String literal expressions which represent the format string split by "{}"
+    pub format_string_parts: &'tcx [Expr<'tcx>],
+    /// Symbols corresponding to [`format_string_parts`]
+    pub format_string_symbols: Vec<Symbol>,
+    /// Expressions like `ArgumentV1::new(arg0, Debug::fmt)`
+    pub args: &'tcx [Expr<'tcx>],
+    /// The final argument passed to `Arguments::new_v1_formatted`, if applicable
+    pub fmt_expr: Option<&'tcx Expr<'tcx>>,
+}
+
+impl FormatArgsExpn<'tcx> {
+    /// Parses an expanded `format_args!` or `format_args_nl!` invocation
+    pub fn parse(expr: &'tcx Expr<'tcx>) -> Option<Self> {
+        if_chain! {
+            if let ExpnKind::Macro { name, .. } = expr.span.ctxt().outer_expn_data().kind;
+            let name = name.as_str();
+            if name.ends_with("format_args") || name.ends_with("format_args_nl");
+            if let ExprKind::Call(_, args) = expr.kind;
+            if let Some((strs_ref, args, fmt_expr)) = match args {
+                // Arguments::new_v1
+                [strs_ref, args] => Some((strs_ref, args, None)),
+                // Arguments::new_v1_formatted
+                [strs_ref, args, fmt_expr] => Some((strs_ref, args, Some(fmt_expr))),
+                _ => None,
+            };
+            if let ExprKind::AddrOf(BorrowKind::Ref, _, strs_arr) = strs_ref.kind;
+            if let ExprKind::Array(format_string_parts) = strs_arr.kind;
+            if let Some(format_string_symbols) = format_string_parts
+                .iter()
+                .map(|e| {
+                    if let ExprKind::Lit(lit) = &e.kind {
+                        if let LitKind::Str(symbol, _style) = lit.node {
+                            return Some(symbol);
+                        }
+                    }
+                    None
+                })
+                .collect();
+            if let ExprKind::AddrOf(BorrowKind::Ref, _, args) = args.kind;
+            if let ExprKind::Match(args, [arm], _) = args.kind;
+            if let ExprKind::Tup(value_args) = args.kind;
+            if let Some(value_args) = value_args
+                .iter()
+                .map(|e| match e.kind {
+                    ExprKind::AddrOf(_, _, e) => Some(e),
+                    _ => None,
+                })
+                .collect();
+            if let ExprKind::Array(args) = arm.body.kind;
+            then {
+                Some(FormatArgsExpn {
+                    format_string_span: strs_ref.span,
+                    value_args,
+                    format_string_parts,
+                    format_string_symbols,
+                    args,
+                    fmt_expr,
+                })
+            } else {
+                None
+            }
+        }
+    }
+}
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index 9ebfbd6b423..c960eec3064 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -38,7 +38,6 @@ pub const DEFAULT_TRAIT_METHOD: [&str; 4] = ["core", "default", "Default", "defa
 pub const DEREF_MUT_TRAIT_METHOD: [&str; 5] = ["core", "ops", "deref", "DerefMut", "deref_mut"];
 pub const DEREF_TRAIT_METHOD: [&str; 5] = ["core", "ops", "deref", "Deref", "deref"];
 pub const DIR_BUILDER: [&str; 3] = ["std", "fs", "DirBuilder"];
-pub const DISPLAY_FMT_METHOD: [&str; 4] = ["core", "fmt", "Display", "fmt"];
 pub const DISPLAY_TRAIT: [&str; 3] = ["core", "fmt", "Display"];
 pub const DOUBLE_ENDED_ITERATOR: [&str; 4] = ["core", "iter", "traits", "DoubleEndedIterator"];
 pub const DROP: [&str; 3] = ["core", "mem", "drop"];
@@ -50,9 +49,6 @@ pub const F32_EPSILON: [&str; 4] = ["core", "f32", "<impl f32>", "EPSILON"];
 pub const F64_EPSILON: [&str; 4] = ["core", "f64", "<impl f64>", "EPSILON"];
 pub const FILE: [&str; 3] = ["std", "fs", "File"];
 pub const FILE_TYPE: [&str; 3] = ["std", "fs", "FileType"];
-pub const FMT_ARGUMENTS_NEW_V1: [&str; 4] = ["core", "fmt", "Arguments", "new_v1"];
-pub const FMT_ARGUMENTS_NEW_V1_FORMATTED: [&str; 4] = ["core", "fmt", "Arguments", "new_v1_formatted"];
-pub const FMT_ARGUMENTV1_NEW: [&str; 4] = ["core", "fmt", "ArgumentV1", "new"];
 pub const FROM_FROM: [&str; 4] = ["core", "convert", "From", "from"];
 pub const FROM_ITERATOR: [&str; 5] = ["core", "iter", "traits", "collect", "FromIterator"];
 pub const FROM_ITERATOR_METHOD: [&str; 6] = ["core", "iter", "traits", "collect", "FromIterator", "from_iter"];
diff --git a/tests/ui/explicit_write_non_rustfix.stderr b/tests/ui/explicit_write_non_rustfix.stderr
index 77cadb99bb5..b94ec6403dd 100644
--- a/tests/ui/explicit_write_non_rustfix.stderr
+++ b/tests/ui/explicit_write_non_rustfix.stderr
@@ -1,10 +1,11 @@
-error: use of `writeln!(stderr(), ...).unwrap()`. Consider using `eprintln!` instead
+error: use of `writeln!(stderr(), ...).unwrap()`
   --> $DIR/explicit_write_non_rustfix.rs:7:5
    |
 LL |     writeln!(std::io::stderr(), "foo {}", bar).unwrap();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::explicit-write` implied by `-D warnings`
+   = help: consider using `eprintln!` instead
 
 error: aborting due to previous error
 
diff --git a/tests/ui/format.fixed b/tests/ui/format.fixed
index e4cfb005fd1..5dd64140e81 100644
--- a/tests/ui/format.fixed
+++ b/tests/ui/format.fixed
@@ -69,4 +69,6 @@ fn main() {
     // Wrap it with braces
     let v: Vec<String> = vec!["foo".to_string(), "bar".to_string()];
     let _s: String = (&*v.join("\n")).to_string();
+
+    format!("prepend {:+}", "s");
 }
diff --git a/tests/ui/format.rs b/tests/ui/format.rs
index 683957f0ff0..4599fb5207e 100644
--- a/tests/ui/format.rs
+++ b/tests/ui/format.rs
@@ -71,4 +71,6 @@ fn main() {
     // Wrap it with braces
     let v: Vec<String> = vec!["foo".to_string(), "bar".to_string()];
     let _s: String = format!("{}", &*v.join("\n"));
+
+    format!("prepend {:+}", "s");
 }