Auto merge of #52767 - ljedrz:avoid_format, r=petrochenkov

Prefer to_string() to format!()

Simple benchmarks suggest in some cases it can be faster by even 37%:
```
test converting_f64_long  ... bench:         339 ns/iter (+/- 199)
test converting_f64_short ... bench:         136 ns/iter (+/- 34)
test converting_i32_long  ... bench:          87 ns/iter (+/- 16)
test converting_i32_short ... bench:          87 ns/iter (+/- 49)
test converting_str       ... bench:          54 ns/iter (+/- 15)
test formatting_f64_long  ... bench:         349 ns/iter (+/- 176)
test formatting_f64_short ... bench:         145 ns/iter (+/- 14)
test formatting_i32_long  ... bench:          98 ns/iter (+/- 14)
test formatting_i32_short ... bench:          93 ns/iter (+/- 15)
test formatting_str       ... bench:          86 ns/iter (+/- 23)
```

36 files changed, 95 insertions, 95 deletions

diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index c67787b5b0a..724d3b74190 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -925,7 +925,7 @@ impl<'a> Builder<'a> {
             cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
         }
 
-        cargo.env("RUSTC_VERBOSE", format!("{}", self.verbosity));
+        cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
         // in std, we want to avoid denying warnings for stage 0 as that makes cfg's painful.
         if self.config.deny_warnings && !(mode == Mode::Std && stage == 0) {
diff --git a/src/libcore/tests/str_lossy.rs b/src/libcore/tests/str_lossy.rs
index 69e28256da9..56ef3f070c1 100644
--- a/src/libcore/tests/str_lossy.rs
+++ b/src/libcore/tests/str_lossy.rs
@@ -79,7 +79,7 @@ fn chunks() {
 fn display() {
     assert_eq!(
         "Hello\u{FFFD}\u{FFFD} There\u{FFFD} Goodbye",
-        &format!("{}", Utf8Lossy::from_bytes(b"Hello\xC0\x80 There\xE6\x83 Goodbye")));
+        &Utf8Lossy::from_bytes(b"Hello\xC0\x80 There\xE6\x83 Goodbye").to_string());
 }
 
 #[test]
diff --git a/src/librustc/infer/error_reporting/mod.rs b/src/librustc/infer/error_reporting/mod.rs
index c46492895dd..90248e89fa7 100644
--- a/src/librustc/infer/error_reporting/mod.rs
+++ b/src/librustc/infer/error_reporting/mod.rs
@@ -557,7 +557,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         // Output the lifetimes fot the first type
         let lifetimes = sub.regions()
             .map(|lifetime| {
-                let s = format!("{}", lifetime);
+                let s = lifetime.to_string();
                 if s.is_empty() {
                     "'_".to_string()
                 } else {
@@ -582,7 +582,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 value.0.extend((values.0).0);
                 other_value.0.extend((values.1).0);
             } else {
-                value.push_highlighted(format!("{}", type_arg));
+                value.push_highlighted(type_arg.to_string());
             }
 
             if len > 0 && i != len - 1 {
@@ -716,7 +716,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             mutbl: hir::Mutability,
             s: &mut DiagnosticStyledString,
         ) {
-            let r = &format!("{}", r);
+            let r = &r.to_string();
             s.push_highlighted(format!(
                 "&{}{}{}",
                 r,
@@ -727,7 +727,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     ""
                 }
             ));
-            s.push_normal(format!("{}", ty));
+            s.push_normal(ty.to_string());
         }
 
         match (&t1.sty, &t2.sty) {
@@ -768,7 +768,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     }
 
                     fn lifetime_display(lifetime: Region) -> String {
-                        let s = format!("{}", lifetime);
+                        let s = lifetime.to_string();
                         if s.is_empty() {
                             "'_".to_string()
                         } else {
@@ -863,8 +863,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     // We couldn't find anything in common, highlight everything.
                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
                     (
-                        DiagnosticStyledString::highlighted(format!("{}", t1)),
-                        DiagnosticStyledString::highlighted(format!("{}", t2)),
+                        DiagnosticStyledString::highlighted(t1.to_string()),
+                        DiagnosticStyledString::highlighted(t2.to_string()),
                     )
                 }
             }
@@ -873,12 +873,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             (&ty::TyRef(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
-                values.1.push_normal(format!("{}", t2));
+                values.1.push_normal(t2.to_string());
                 values
             }
             (_, &ty::TyRef(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
-                values.0.push_normal(format!("{}", t1));
+                values.0.push_normal(t1.to_string());
                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
                 values
             }
@@ -902,8 +902,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 } else {
                     // We couldn't find anything in common, highlight everything.
                     (
-                        DiagnosticStyledString::highlighted(format!("{}", t1)),
-                        DiagnosticStyledString::highlighted(format!("{}", t2)),
+                        DiagnosticStyledString::highlighted(t1.to_string()),
+                        DiagnosticStyledString::highlighted(t2.to_string()),
                     )
                 }
             }
@@ -1073,8 +1073,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         }
 
         Some((
-            DiagnosticStyledString::highlighted(format!("{}", exp_found.expected)),
-            DiagnosticStyledString::highlighted(format!("{}", exp_found.found)),
+            DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
+            DiagnosticStyledString::highlighted(exp_found.found.to_string()),
         ))
     }
 
diff --git a/src/librustc/infer/error_reporting/nice_region_error/static_impl_trait.rs b/src/librustc/infer/error_reporting/nice_region_error/static_impl_trait.rs
index f9ec5fa13c9..193f86a3827 100644
--- a/src/librustc/infer/error_reporting/nice_region_error/static_impl_trait.rs
+++ b/src/librustc/infer/error_reporting/nice_region_error/static_impl_trait.rs
@@ -57,7 +57,7 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
                         let lifetime_name = match sup_r {
                             RegionKind::ReFree(FreeRegion {
                                 bound_region: BoundRegion::BrNamed(_, ref name), ..
-                            }) => format!("{}", name),
+                            }) => name.to_string(),
                             _ => "'_".to_owned(),
                         };
                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(return_sp) {
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index f1f826486a5..4bfb4c96497 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -2093,7 +2093,7 @@ impl<'tcx> Debug for Rvalue<'tcx> {
 
                 // When printing regions, add trailing space if necessary.
                 let region = if ppaux::verbose() || ppaux::identify_regions() {
-                    let mut region = format!("{}", region);
+                    let mut region = region.to_string();
                     if region.len() > 0 {
                         region.push(' ');
                     }
diff --git a/src/librustc/session/code_stats.rs b/src/librustc/session/code_stats.rs
index df4060e71e5..1eee6508c59 100644
--- a/src/librustc/session/code_stats.rs
+++ b/src/librustc/session/code_stats.rs
@@ -135,8 +135,8 @@ impl CodeStats {
                 let VariantInfo { ref name, kind: _, align: _, size, ref fields } = *variant_info;
                 let indent = if !struct_like {
                     let name = match name.as_ref() {
-                        Some(name) => format!("{}", name),
-                        None => format!("{}", i),
+                        Some(name) => name.to_owned(),
+                        None => i.to_string(),
                     };
                     println!("print-type-size {}variant `{}`: {} bytes",
                              indent, name, size - discr_size);
diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index 5f8a2208bb0..b99c630edfc 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/src/librustc/traits/error_reporting.rs
@@ -543,7 +543,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     &data.parent_trait_ref);
                 match self.get_parent_trait_ref(&data.parent_code) {
                     Some(t) => Some(t),
-                    None => Some(format!("{}", parent_trait_ref.skip_binder().self_ty())),
+                    None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
                 }
             }
             _ => None,
@@ -797,12 +797,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             ty::TypeVariants::TyTuple(ref tys) => ArgKind::Tuple(
                                 Some(span),
                                 tys.iter()
-                                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
+                                    .map(|ty| ("_".to_owned(), ty.sty.to_string()))
                                     .collect::<Vec<_>>()
                             ),
-                            _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
+                            _ => ArgKind::Arg("_".to_owned(), t.sty.to_string()),
                         }).collect(),
-                    ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
+                    ref sty => vec![ArgKind::Arg("_".to_owned(), sty.to_string())],
                 };
                 if found.len() == expected.len() {
                     self.report_closure_arg_mismatch(span,
@@ -989,7 +989,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }) => {
                 (self.tcx.sess.codemap().def_span(span),
                  fields.iter().map(|field| {
-                     ArgKind::Arg(format!("{}", field.ident), "_".to_string())
+                     ArgKind::Arg(field.ident.to_string(), "_".to_string())
                  }).collect::<Vec<_>>())
             }
             hir::map::NodeStructCtor(ref variant_data) => {
@@ -1152,7 +1152,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     ::rustc_target::spec::abi::Abi::Rust
                 )
             };
-            format!("{}", ty::Binder::bind(sig))
+            ty::Binder::bind(sig).to_string()
         }
 
         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
@@ -1575,10 +1575,10 @@ impl ArgKind {
             ty::TyTuple(ref tys) => ArgKind::Tuple(
                 None,
                 tys.iter()
-                   .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
+                   .map(|ty| ("_".to_owned(), ty.sty.to_string()))
                    .collect::<Vec<_>>()
             ),
-            _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
+            _ => ArgKind::Arg("_".to_owned(), t.sty.to_string()),
         }
     }
 }
diff --git a/src/librustc/ty/item_path.rs b/src/librustc/ty/item_path.rs
index a44962c77b5..c44b7327a08 100644
--- a/src/librustc/ty/item_path.rs
+++ b/src/librustc/ty/item_path.rs
@@ -315,7 +315,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             ty::TyUint(_) |
             ty::TyFloat(_) |
             ty::TyStr => {
-                buffer.push(&format!("{}", self_ty));
+                buffer.push(&self_ty.to_string());
             }
 
             _ => {
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index b0797c31461..4fda3bdca3d 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -225,7 +225,7 @@ impl AssociatedItem {
                 // late-bound regions, and we don't want method signatures to show up
                 // `as for<'r> fn(&'r MyType)`.  Pretty-printing handles late-bound
                 // regions just fine, showing `fn(&MyType)`.
-                format!("{}", tcx.fn_sig(self.def_id).skip_binder())
+                tcx.fn_sig(self.def_id).skip_binder().to_string()
             }
             ty::AssociatedKind::Type => format!("type {};", self.ident),
             ty::AssociatedKind::Existential => format!("existential type {};", self.ident),
diff --git a/src/librustc/util/common.rs b/src/librustc/util/common.rs
index 857cabe18b1..990fbc4bc91 100644
--- a/src/librustc/util/common.rs
+++ b/src/librustc/util/common.rs
@@ -242,7 +242,7 @@ pub fn to_readable_str(mut val: usize) -> String {
         val /= 1000;
 
         if val == 0 {
-            groups.push(format!("{}", group));
+            groups.push(group.to_string());
             break;
         } else {
             groups.push(format!("{:03}", group));
diff --git a/src/librustc_borrowck/borrowck/mod.rs b/src/librustc_borrowck/borrowck/mod.rs
index 0cb4a766e80..ddd0b691aec 100644
--- a/src/librustc_borrowck/borrowck/mod.rs
+++ b/src/librustc_borrowck/borrowck/mod.rs
@@ -697,7 +697,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 let mut err = self.cannot_act_on_moved_value(use_span,
                                                              verb,
                                                              msg,
-                                                             Some(format!("{}", nl)),
+                                                             Some(nl.to_string()),
                                                              Origin::Ast);
                 let need_note = match lp.ty.sty {
                     ty::TypeVariants::TyClosure(id, _) => {
diff --git a/src/librustc_codegen_llvm/back/archive.rs b/src/librustc_codegen_llvm/back/archive.rs
index 9ea6c44502a..b99835cb5c6 100644
--- a/src/librustc_codegen_llvm/back/archive.rs
+++ b/src/librustc_codegen_llvm/back/archive.rs
@@ -149,7 +149,7 @@ impl<'a> ArchiveBuilder<'a> {
         // Ignoring obj file starting with the crate name
         // as simple comparison is not enough - there
         // might be also an extra name suffix
-        let obj_start = format!("{}", name);
+        let obj_start = name.to_owned();
 
         self.add_archive(rlib, move |fname: &str| {
             // Ignore bytecode/metadata files, no matter the name.
diff --git a/src/librustc_codegen_llvm/back/link.rs b/src/librustc_codegen_llvm/back/link.rs
index 13f0c90e885..83ff8bc821c 100644
--- a/src/librustc_codegen_llvm/back/link.rs
+++ b/src/librustc_codegen_llvm/back/link.rs
@@ -811,7 +811,7 @@ fn link_natively(sess: &Session,
                 }
             };
 
-            linker_error.note(&format!("{}", e));
+            linker_error.note(&e.to_string());
 
             if !linker_not_found {
                 linker_error.note(&format!("{:?}", &cmd));
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 29b9990de34..ec53151b3b8 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -530,7 +530,7 @@ fn run_compiler_with_pool<'a>(
     if let Some(err) = input_err {
         // Immediately stop compilation if there was an issue reading
         // the input (for example if the input stream is not UTF-8).
-        sess.err(&format!("{}", err));
+        sess.err(&err.to_string());
         return (Err(CompileIncomplete::Stopped), Some(sess));
     }
 
@@ -1110,7 +1110,7 @@ impl RustcDefaultCalls {
                         cfgs.push(if let Some(value) = value {
                             format!("{}=\"{}\"", name, value)
                         } else {
-                            format!("{}", name)
+                            name.to_string()
                         });
                     }
 
diff --git a/src/librustc_lint/builtin.rs b/src/librustc_lint/builtin.rs
index e6aa7c0d16c..8df6df15511 100644
--- a/src/librustc_lint/builtin.rs
+++ b/src/librustc_lint/builtin.rs
@@ -190,7 +190,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonShorthandFieldPatterns {
                                      fieldpat.span,
                                      &format!("the `{}:` in this pattern is redundant", ident));
                         let subspan = cx.tcx.sess.codemap().span_through_char(fieldpat.span, ':');
-                        err.span_suggestion_short(subspan, "remove this", format!("{}", ident));
+                        err.span_suggestion_short(subspan, "remove this", ident.to_string());
                         err.emit();
                     }
                 }
@@ -701,7 +701,7 @@ impl EarlyLintPass for BadRepr {
                         attr.span,
                         "`repr` attribute isn't configurable with a literal",
                     );
-                    match format!("{}", lit).as_ref() {
+                    match lit.to_string().as_ref() {
                         | "C" | "packed" | "rust" | "transparent"
                         | "u8" | "u16" | "u32" | "u64" | "u128" | "usize"
                         | "i8" | "i16" | "i32" | "i64" | "i128" | "isize" => {
diff --git a/src/librustc_metadata/locator.rs b/src/librustc_metadata/locator.rs
index 42af5db8294..f68bcdd62c6 100644
--- a/src/librustc_metadata/locator.rs
+++ b/src/librustc_metadata/locator.rs
@@ -722,7 +722,7 @@ impl<'a> Context<'a> {
                   root.triple);
             self.rejected_via_triple.push(CrateMismatch {
                 path: libpath.to_path_buf(),
-                got: format!("{}", root.triple),
+                got: root.triple.to_string(),
             });
             return None;
         }
diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index 347bf61480e..fe33dc0a58a 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -86,7 +86,7 @@ impl<'tcx> fmt::Display for BorrowData<'tcx> {
             mir::BorrowKind::Unique => "uniq ",
             mir::BorrowKind::Mut { .. } => "mut ",
         };
-        let region = format!("{}", self.region);
+        let region = self.region.to_string();
         let region = if region.len() > 0 {
             format!("{} ", region)
         } else {
diff --git a/src/librustc_mir/borrow_check/error_reporting.rs b/src/librustc_mir/borrow_check/error_reporting.rs
index e20e608a2cd..e2ac7dde558 100644
--- a/src/librustc_mir/borrow_check/error_reporting.rs
+++ b/src/librustc_mir/borrow_check/error_reporting.rs
@@ -642,7 +642,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 self.append_local_to_string(local, buf)?;
             }
             Place::Static(ref static_) => {
-                buf.push_str(&format!("{}", &self.tcx.item_name(static_.def_id)));
+                buf.push_str(&self.tcx.item_name(static_.def_id).to_string());
             }
             Place::Projection(ref proj) => {
                 match proj.elem {
@@ -766,7 +766,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         let local = &self.mir.local_decls[local_index];
         match local.name {
             Some(name) => {
-                buf.push_str(&format!("{}", name));
+                buf.push_str(&name.to_string());
                 Ok(())
             }
             None => Err(()),
@@ -794,7 +794,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 ProjectionElem::Index(..)
                 | ProjectionElem::ConstantIndex { .. }
                 | ProjectionElem::Subslice { .. } => {
-                    format!("{}", self.describe_field(&proj.base, field))
+                    self.describe_field(&proj.base, field).to_string()
                 }
             },
         }
@@ -808,11 +808,11 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         } else {
             match ty.sty {
                 ty::TyAdt(def, _) => if def.is_enum() {
-                    format!("{}", field.index())
+                    field.index().to_string()
                 } else {
-                    format!("{}", def.non_enum_variant().fields[field.index()].ident)
+                    def.non_enum_variant().fields[field.index()].ident.to_string()
                 },
-                ty::TyTuple(_) => format!("{}", field.index()),
+                ty::TyTuple(_) => field.index().to_string(),
                 ty::TyRef(_, ty, _) | ty::TyRawPtr(ty::TypeAndMut { ty, .. }) => {
                     self.describe_field_from_ty(&ty, field)
                 }
diff --git a/src/librustc_mir/borrow_check/flows.rs b/src/librustc_mir/borrow_check/flows.rs
index 6dfcece3071..90dc96cbd3c 100644
--- a/src/librustc_mir/borrow_check/flows.rs
+++ b/src/librustc_mir/borrow_check/flows.rs
@@ -114,7 +114,7 @@ impl<'b, 'gcx, 'tcx> fmt::Display for Flows<'b, 'gcx, 'tcx> {
             };
             saw_one = true;
             let borrow_data = &self.borrows.operator().borrows()[borrow];
-            s.push_str(&format!("{}", borrow_data));
+            s.push_str(&borrow_data.to_string());
         });
         s.push_str("] ");
 
@@ -126,7 +126,7 @@ impl<'b, 'gcx, 'tcx> fmt::Display for Flows<'b, 'gcx, 'tcx> {
             };
             saw_one = true;
             let borrow_data = &self.borrows.operator().borrows()[borrow];
-            s.push_str(&format!("{}", borrow_data));
+            s.push_str(&borrow_data.to_string());
         });
         s.push_str("] ");
 
@@ -138,7 +138,7 @@ impl<'b, 'gcx, 'tcx> fmt::Display for Flows<'b, 'gcx, 'tcx> {
             };
             saw_one = true;
             let move_path = &self.uninits.operator().move_data().move_paths[mpi_uninit];
-            s.push_str(&format!("{}", move_path));
+            s.push_str(&move_path.to_string());
         });
         s.push_str("] ");
 
diff --git a/src/librustc_mir/borrow_check/move_errors.rs b/src/librustc_mir/borrow_check/move_errors.rs
index 215ade5bd4d..103f431d4ba 100644
--- a/src/librustc_mir/borrow_check/move_errors.rs
+++ b/src/librustc_mir/borrow_check/move_errors.rs
@@ -307,7 +307,7 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                             err.span_suggestion(
                                 span,
                                 "consider removing this dereference operator",
-                                format!("{}", &snippet[1..]),
+                                (&snippet[1..]).to_owned(),
                             );
                         }
                         _ => {
diff --git a/src/librustc_traits/lowering.rs b/src/librustc_traits/lowering.rs
index cf612585776..a3c24f8af22 100644
--- a/src/librustc_traits/lowering.rs
+++ b/src/librustc_traits/lowering.rs
@@ -523,7 +523,7 @@ impl<'a, 'tcx> ClauseDumper<'a, 'tcx> {
                             Clause::Implies(program_clause) => program_clause,
                             Clause::ForAll(program_clause) => program_clause.skip_binder(),
                         };
-                        format!("{}", program_clause)
+                        program_clause.to_string()
                     })
                     .collect();
 
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 2cfeb251392..4cde171f1bf 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -2838,7 +2838,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     ty::TyFnDef(..) => {
                         let ptr_ty = self.tcx.mk_fn_ptr(arg_ty.fn_sig(self.tcx));
                         let ptr_ty = self.resolve_type_vars_if_possible(&ptr_ty);
-                        variadic_error(tcx.sess, arg.span, arg_ty, &format!("{}", ptr_ty));
+                        variadic_error(tcx.sess, arg.span, arg_ty, &ptr_ty.to_string());
                     }
                     _ => {}
                 }
diff --git a/src/librustc_typeck/outlives/mod.rs b/src/librustc_typeck/outlives/mod.rs
index 5801a6ada3f..74ef62e0c63 100644
--- a/src/librustc_typeck/outlives/mod.rs
+++ b/src/librustc_typeck/outlives/mod.rs
@@ -54,9 +54,9 @@ fn inferred_outlives_of<'a, 'tcx>(
                     let mut pred: Vec<String> = predicates
                         .iter()
                         .map(|out_pred| match out_pred {
-                            ty::Predicate::RegionOutlives(p) => format!("{}", &p),
+                            ty::Predicate::RegionOutlives(p) => p.to_string(),
 
-                            ty::Predicate::TypeOutlives(p) => format!("{}", &p),
+                            ty::Predicate::TypeOutlives(p) => p.to_string(),
 
                             err => bug!("unexpected predicate {:?}", err),
                         })
diff --git a/src/librustdoc/html/format.rs b/src/librustdoc/html/format.rs
index feec513b975..58034d1df5a 100644
--- a/src/librustdoc/html/format.rs
+++ b/src/librustdoc/html/format.rs
@@ -206,7 +206,7 @@ impl<'a> fmt::Display for WhereClause<'a> {
                             clause.push_str(" + ");
                         }
 
-                        clause.push_str(&format!("{}", lifetime));
+                        clause.push_str(&lifetime.to_string());
                     }
                 }
                 &clean::WherePredicate::EqPredicate { ref lhs, ref rhs } => {
@@ -458,10 +458,10 @@ fn resolved_path(w: &mut fmt::Formatter, did: DefId, path: &clean::Path,
                             fqp[..fqp.len() - 1].join("::"),
                             HRef::new(did, fqp.last().unwrap()))
                 }
-                None => format!("{}", HRef::new(did, &last.name)),
+                None => HRef::new(did, &last.name).to_string(),
             }
         } else {
-            format!("{}", HRef::new(did, &last.name))
+            HRef::new(did, &last.name).to_string()
         };
         write!(w, "{}{}", path, last.args)?;
     }
@@ -883,7 +883,7 @@ impl<'a> fmt::Display for Method<'a> {
                 if f.alternate() {
                     args.push_str(&format!("{:#}", input.type_));
                 } else {
-                    args.push_str(&format!("{}", input.type_));
+                    args.push_str(&input.type_.to_string());
                 }
                 args_plain.push_str(&format!("{:#}", input.type_));
             }
@@ -902,7 +902,7 @@ impl<'a> fmt::Display for Method<'a> {
         let arrow = if f.alternate() {
             format!("{:#}", decl.output)
         } else {
-            format!("{}", decl.output)
+            decl.output.to_string()
         };
 
         let pad = " ".repeat(name_len);
diff --git a/src/librustdoc/html/markdown.rs b/src/librustdoc/html/markdown.rs
index 8040548ce6b..8d85adfb3d0 100644
--- a/src/librustdoc/html/markdown.rs
+++ b/src/librustdoc/html/markdown.rs
@@ -881,14 +881,14 @@ mod tests {
     #[test]
     fn issue_17736() {
         let markdown = "# title";
-        format!("{}", Markdown(markdown, &[]));
+        Markdown(markdown, &[]).to_string();
         reset_ids(true);
     }
 
     #[test]
     fn test_header() {
         fn t(input: &str, expect: &str) {
-            let output = format!("{}", Markdown(input, &[]));
+            let output = Markdown(input, &[]).to_string();
             assert_eq!(output, expect, "original: {}", input);
             reset_ids(true);
         }
@@ -910,7 +910,7 @@ mod tests {
     #[test]
     fn test_header_ids_multiple_blocks() {
         fn t(input: &str, expect: &str) {
-            let output = format!("{}", Markdown(input, &[]));
+            let output = Markdown(input, &[]).to_string();
             assert_eq!(output, expect, "original: {}", input);
         }
 
@@ -951,7 +951,7 @@ mod tests {
     #[test]
     fn test_markdown_html_escape() {
         fn t(input: &str, expect: &str) {
-            let output = format!("{}", MarkdownHtml(input));
+            let output = MarkdownHtml(input).to_string();
             assert_eq!(output, expect, "original: {}", input);
         }
 
diff --git a/src/librustdoc/html/render.rs b/src/librustdoc/html/render.rs
index 9f4f20e3539..a1ab66ea81c 100644
--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -2058,7 +2058,7 @@ impl<'a> Item<'a> {
         };
 
         let lines = if self.item.source.loline == self.item.source.hiline {
-            format!("{}", self.item.source.loline)
+            self.item.source.loline.to_string()
         } else {
             format!("{}-{}", self.item.source.loline, self.item.source.hiline)
         };
@@ -2233,7 +2233,7 @@ fn document_short(w: &mut fmt::Formatter, item: &clean::Item, link: AssocItemLin
             format!("{} [Read more]({})",
                     &plain_summary_line(Some(s)), naive_assoc_href(item, link))
         } else {
-            format!("{}", &plain_summary_line(Some(s)))
+            plain_summary_line(Some(s)).to_string()
         };
         render_markdown(w, &markdown, item.links(), prefix)?;
     } else if !prefix.is_empty() {
@@ -2730,7 +2730,7 @@ fn bounds(t_bounds: &[clean::GenericBound]) -> String {
                 bounds.push_str(" + ");
                 bounds_plain.push_str(" + ");
             }
-            bounds.push_str(&format!("{}", *p));
+            bounds.push_str(&(*p).to_string());
             bounds_plain.push_str(&format!("{:#}", *p));
         }
     }
@@ -3391,7 +3391,7 @@ fn render_attribute(attr: &ast::MetaItem) -> Option<String> {
     let name = attr.name();
 
     if attr.is_word() {
-        Some(format!("{}", name))
+        Some(name.to_string())
     } else if let Some(v) = attr.value_str() {
         Some(format!("{} = {:?}", name, v.as_str()))
     } else if let Some(values) = attr.meta_item_list() {
@@ -3639,7 +3639,7 @@ fn render_assoc_items(w: &mut fmt::Formatter,
             }
         }
 
-        let impls = format!("{}", RendererStruct(cx, concrete, containing_item));
+        let impls = RendererStruct(cx, concrete, containing_item).to_string();
         if !impls.is_empty() {
             write!(w, "\
                 <h2 id='implementations' class='small-section-header'>\
@@ -3755,7 +3755,7 @@ fn spotlight_decl(decl: &clean::FnDecl) -> Result<String, fmt::Error> {
                             &format!("<h3 class=\"important\">Important traits for {}</h3>\
                                       <code class=\"content\">",
                                      impl_.for_));
-                        trait_.push_str(&format!("{}", impl_.for_));
+                        trait_.push_str(&impl_.for_.to_string());
                     }
 
                     //use the "where" class here to make it small
diff --git a/src/librustdoc/html/toc.rs b/src/librustdoc/html/toc.rs
index f3379b33155..88ada5c7f7f 100644
--- a/src/librustdoc/html/toc.rs
+++ b/src/librustdoc/html/toc.rs
@@ -157,7 +157,7 @@ impl TocBuilder {
                 sec_number.push_str("0.");
             }
             let number = toc.count_entries_with_level(level);
-            sec_number.push_str(&format!("{}", number + 1))
+            sec_number.push_str(&(number + 1).to_string())
         }
 
         self.chain.push(TocEntry {
diff --git a/src/librustdoc/markdown.rs b/src/librustdoc/markdown.rs
index bf7b025884d..36a8fc94dba 100644
--- a/src/librustdoc/markdown.rs
+++ b/src/librustdoc/markdown.rs
@@ -89,9 +89,9 @@ pub fn render(input: &Path, mut output: PathBuf, matches: &getopts::Matches,
     reset_ids(false);
 
     let text = if include_toc {
-        format!("{}", MarkdownWithToc(text))
+        MarkdownWithToc(text).to_string()
     } else {
-        format!("{}", Markdown(text, &[]))
+        Markdown(text, &[]).to_string()
     };
 
     let err = write!(
diff --git a/src/libserialize/json.rs b/src/libserialize/json.rs
index 88cc9373113..b15321f4ba7 100644
--- a/src/libserialize/json.rs
+++ b/src/libserialize/json.rs
@@ -2094,7 +2094,7 @@ macro_rules! expect {
         match $e {
             Json::Null => Ok(()),
             other => Err(ExpectedError("Null".to_owned(),
-                                       format!("{}", other)))
+                                       other.to_string()))
         }
     });
     ($e:expr, $t:ident) => ({
@@ -2102,7 +2102,7 @@ macro_rules! expect {
             Json::$t(v) => Ok(v),
             other => {
                 Err(ExpectedError(stringify!($t).to_owned(),
-                                  format!("{}", other)))
+                                  other.to_string()))
             }
         }
     })
@@ -2114,14 +2114,14 @@ macro_rules! read_primitive {
             match self.pop() {
                 Json::I64(f) => Ok(f as $ty),
                 Json::U64(f) => Ok(f as $ty),
-                Json::F64(f) => Err(ExpectedError("Integer".to_owned(), format!("{}", f))),
+                Json::F64(f) => Err(ExpectedError("Integer".to_owned(), f.to_string())),
                 // re: #12967.. a type w/ numeric keys (ie HashMap<usize, V> etc)
                 // is going to have a string here, as per JSON spec.
                 Json::String(s) => match s.parse().ok() {
                     Some(f) => Ok(f),
                     None => Err(ExpectedError("Number".to_owned(), s)),
                 },
-                value => Err(ExpectedError("Number".to_owned(), format!("{}", value))),
+                value => Err(ExpectedError("Number".to_owned(), value.to_string())),
             }
         }
     }
@@ -2163,7 +2163,7 @@ impl ::Decoder for Decoder {
                 }
             },
             Json::Null => Ok(f64::NAN),
-            value => Err(ExpectedError("Number".to_owned(), format!("{}", value)))
+            value => Err(ExpectedError("Number".to_owned(), value.to_string()))
         }
     }
 
@@ -2181,7 +2181,7 @@ impl ::Decoder for Decoder {
                 _ => ()
             }
         }
-        Err(ExpectedError("single character string".to_owned(), format!("{}", s)))
+        Err(ExpectedError("single character string".to_owned(), s.to_string()))
     }
 
     fn read_str(&mut self) -> DecodeResult<Cow<str>> {
@@ -2204,7 +2204,7 @@ impl ::Decoder for Decoder {
                 let n = match o.remove(&"variant".to_owned()) {
                     Some(Json::String(s)) => s,
                     Some(val) => {
-                        return Err(ExpectedError("String".to_owned(), format!("{}", val)))
+                        return Err(ExpectedError("String".to_owned(), val.to_string()))
                     }
                     None => {
                         return Err(MissingFieldError("variant".to_owned()))
@@ -2217,7 +2217,7 @@ impl ::Decoder for Decoder {
                         }
                     },
                     Some(val) => {
-                        return Err(ExpectedError("Array".to_owned(), format!("{}", val)))
+                        return Err(ExpectedError("Array".to_owned(), val.to_string()))
                     }
                     None => {
                         return Err(MissingFieldError("fields".to_owned()))
@@ -2226,7 +2226,7 @@ impl ::Decoder for Decoder {
                 n
             }
             json => {
-                return Err(ExpectedError("String or Object".to_owned(), format!("{}", json)))
+                return Err(ExpectedError("String or Object".to_owned(), json.to_string()))
             }
         };
         let idx = match names.iter().position(|n| *n == &name[..]) {
@@ -2845,21 +2845,21 @@ mod tests {
     fn test_write_enum() {
         let animal = Dog;
         assert_eq!(
-            format!("{}", super::as_json(&animal)),
+            super::as_json(&animal).to_string(),
             "\"Dog\""
         );
         assert_eq!(
-            format!("{}", super::as_pretty_json(&animal)),
+            super::as_pretty_json(&animal).to_string(),
             "\"Dog\""
         );
 
         let animal = Frog("Henry".to_string(), 349);
         assert_eq!(
-            format!("{}", super::as_json(&animal)),
+            super::as_json(&animal).to_string(),
             "{\"variant\":\"Frog\",\"fields\":[\"Henry\",349]}"
         );
         assert_eq!(
-            format!("{}", super::as_pretty_json(&animal)),
+            super::as_pretty_json(&animal).to_string(),
             "{\n  \
                \"variant\": \"Frog\",\n  \
                \"fields\": [\n    \
@@ -2872,10 +2872,10 @@ mod tests {
 
     macro_rules! check_encoder_for_simple {
         ($value:expr, $expected:expr) => ({
-            let s = format!("{}", super::as_json(&$value));
+            let s = super::as_json(&$value).to_string();
             assert_eq!(s, $expected);
 
-            let s = format!("{}", super::as_pretty_json(&$value));
+            let s = super::as_pretty_json(&$value).to_string();
             assert_eq!(s, $expected);
         })
     }
diff --git a/src/libstd/sys/redox/net/udp.rs b/src/libstd/sys/redox/net/udp.rs
index 2ed67bd2836..22af02079e7 100644
--- a/src/libstd/sys/redox/net/udp.rs
+++ b/src/libstd/sys/redox/net/udp.rs
@@ -58,7 +58,7 @@ impl UdpSocket {
 
     pub fn recv(&self, buf: &mut [u8]) -> Result<usize> {
         if let Some(addr) = *self.get_conn() {
-            let from = self.0.dup(format!("{}", addr).as_bytes())?;
+            let from = self.0.dup(addr.to_string().as_bytes())?;
             from.read(buf)
         } else {
             Err(Error::new(ErrorKind::Other, "UdpSocket::recv not connected"))
diff --git a/src/libstd/sys_common/wtf8.rs b/src/libstd/sys_common/wtf8.rs
index bbb5980bd9d..45204b56ead 100644
--- a/src/libstd/sys_common/wtf8.rs
+++ b/src/libstd/sys_common/wtf8.rs
@@ -1245,7 +1245,7 @@ mod tests {
     #[test]
     fn wtf8_display() {
         fn d(b: &[u8]) -> String {
-            format!("{}", &unsafe { Wtf8::from_bytes_unchecked(b) })
+            (&unsafe { Wtf8::from_bytes_unchecked(b) }).to_string()
         }
 
         assert_eq!("", d("".as_bytes()));
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index b84046d1050..9f8909e1626 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -380,7 +380,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                                                             structs, enums and unions");
                         if let ast::AttrStyle::Inner = attr.style {
                             let trait_list = traits.iter()
-                                .map(|t| format!("{}", t)).collect::<Vec<_>>();
+                                .map(|t| t.to_string()).collect::<Vec<_>>();
                             let suggestion = format!("#[derive({})]", trait_list.join(", "));
                             err.span_suggestion_with_applicability(
                                 span, "try an outer attribute", suggestion,
@@ -558,7 +558,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         invoc.expansion_data.mark.set_expn_info(ExpnInfo {
             call_site: attr.span,
             def_site: None,
-            format: MacroAttribute(Symbol::intern(&format!("{}", attr.path))),
+            format: MacroAttribute(Symbol::intern(&attr.path.to_string())),
             allow_internal_unstable: false,
             allow_internal_unsafe: false,
             local_inner_macros: false,
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 56760546c50..9011b6e48b9 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -6320,7 +6320,7 @@ impl<'a> Parser<'a> {
                 mod_name: mod_name.clone(),
                 default_path: default_path_str,
                 secondary_path: secondary_path_str,
-                dir_path: format!("{}", dir_path.display()),
+                dir_path: dir_path.display().to_string(),
             }),
             (true, true) => Err(Error::DuplicatePaths {
                 mod_name: mod_name.clone(),
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 08c9ec4c989..54ce06f61ef 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -631,7 +631,7 @@ pub trait PrintState<'a> {
                         self.writer().word(&ut.val_to_string(i))
                     }
                     ast::LitIntType::Unsuffixed => {
-                        self.writer().word(&format!("{}", i))
+                        self.writer().word(&i.to_string())
                     }
                 }
             }
diff --git a/src/libsyntax_ext/concat.rs b/src/libsyntax_ext/concat.rs
index 99dba8af754..dcdd2c590e0 100644
--- a/src/libsyntax_ext/concat.rs
+++ b/src/libsyntax_ext/concat.rs
@@ -42,10 +42,10 @@ pub fn expand_syntax_ext(
                 ast::LitKind::Int(i, ast::LitIntType::Unsigned(_))
                 | ast::LitKind::Int(i, ast::LitIntType::Signed(_))
                 | ast::LitKind::Int(i, ast::LitIntType::Unsuffixed) => {
-                    accumulator.push_str(&format!("{}", i));
+                    accumulator.push_str(&i.to_string());
                 }
                 ast::LitKind::Bool(b) => {
-                    accumulator.push_str(&format!("{}", b));
+                    accumulator.push_str(&b.to_string());
                 }
                 ast::LitKind::Byte(..) | ast::LitKind::ByteStr(..) => {
                     cx.span_err(e.span, "cannot concatenate a byte string literal");
diff --git a/src/libterm/terminfo/parser/compiled.rs b/src/libterm/terminfo/parser/compiled.rs
index d5e5df54733..57dcfb0d906 100644
--- a/src/libterm/terminfo/parser/compiled.rs
+++ b/src/libterm/terminfo/parser/compiled.rs
@@ -189,7 +189,7 @@ pub fn parse(file: &mut dyn io::Read, longnames: bool) -> Result<TermInfo, Strin
     macro_rules! t( ($e:expr) => (
         match $e {
             Ok(e) => e,
-            Err(e) => return Err(format!("{}", e))
+            Err(e) => return Err(e.to_string())
         }
     ) );