diff --git a/compiler/rustc_builtin_macros/src/format_foreign.rs b/compiler/rustc_builtin_macros/src/format_foreign.rs
index 85cf4c42e94..b39423b86e7 100644
--- a/compiler/rustc_builtin_macros/src/format_foreign.rs
+++ b/compiler/rustc_builtin_macros/src/format_foreign.rs
@@ -166,14 +166,14 @@ pub mod printf {
             let cap = self.span.len() + if has_options { 2 } else { 0 };
             let mut s = String::with_capacity(cap);
 
-            s.push_str("{");
+            s.push('{');
 
             if let Some(arg) = self.parameter {
                 write!(s, "{}", arg.checked_sub(1)?).ok()?;
             }
 
             if has_options {
-                s.push_str(":");
+                s.push(':');
 
                 let align = if let Some(fill) = fill {
                     s.push_str(fill);
@@ -191,11 +191,11 @@ pub mod printf {
                 }
 
                 if alt {
-                    s.push_str("#");
+                    s.push('#');
                 }
 
                 if zero_fill {
-                    s.push_str("0");
+                    s.push('0');
                 }
 
                 if let Some(width) = width {
@@ -203,7 +203,7 @@ pub mod printf {
                 }
 
                 if let Some(precision) = precision {
-                    s.push_str(".");
+                    s.push('.');
                     precision.translate(&mut s).ok()?;
                 }
 
@@ -212,7 +212,7 @@ pub mod printf {
                 }
             }
 
-            s.push_str("}");
+            s.push('}');
             Some(s)
         }
     }
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index c044020d930..faeb727202c 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1076,7 +1076,7 @@ fn exec_linker(
             }
             .to_string(),
         );
-        args.push_str("\n");
+        args.push('\n');
     }
     let file = tmpdir.join("linker-arguments");
     let bytes = if sess.target.target.options.is_like_msvc {
diff --git a/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs b/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
index 54e5d4d00f6..0c0f1bc681c 100644
--- a/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
+++ b/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
@@ -37,7 +37,7 @@ pub fn push_debuginfo_type_name<'tcx>(
         ty::Bool => output.push_str("bool"),
         ty::Char => output.push_str("char"),
         ty::Str => output.push_str("str"),
-        ty::Never => output.push_str("!"),
+        ty::Never => output.push('!'),
         ty::Int(int_ty) => output.push_str(int_ty.name_str()),
         ty::Uint(uint_ty) => output.push_str(uint_ty.name_str()),
         ty::Float(float_ty) => output.push_str(float_ty.name_str()),
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index b53605b0796..1225776db45 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -2093,7 +2093,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 _ => String::new(),
             };
             if !s.is_empty() {
-                s.push_str(" ");
+                s.push(' ');
             }
             s
         };
diff --git a/compiler/rustc_infer/src/infer/lub.rs b/compiler/rustc_infer/src/infer/lub.rs
index 3e2ea3d0f8f..9f43fac0916 100644
--- a/compiler/rustc_infer/src/infer/lub.rs
+++ b/compiler/rustc_infer/src/infer/lub.rs
@@ -50,7 +50,7 @@ impl TypeRelation<'tcx> for Lub<'combine, 'infcx, 'tcx> {
             ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
             ty::Covariant => self.relate(a, b),
             // FIXME(#41044) -- not correct, need test
-            ty::Bivariant => Ok(a.clone()),
+            ty::Bivariant => Ok(a),
             ty::Contravariant => self.fields.glb(self.a_is_expected).relate(a, b),
         }
     }
diff --git a/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs b/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
index f51bf7730ea..3cee32834be 100644
--- a/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
+++ b/compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs
@@ -150,8 +150,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             Some(mut descr) => {
                 // Surround descr with `backticks`.
                 descr.reserve(2);
-                descr.insert_str(0, "`");
-                descr.push_str("`");
+                descr.insert(0, '`');
+                descr.push('`');
                 descr
             }
             None => "value".to_string(),
@@ -222,7 +222,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             if self.upvars[var_index].by_ref {
                                 buf.push_str(&name);
                             } else {
-                                buf.push_str(&format!("*{}", &name));
+                                buf.push('*');
+                                buf.push_str(&name);
                             }
                         } else {
                             if autoderef {
@@ -234,7 +235,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                     &including_downcast,
                                 )?;
                             } else {
-                                buf.push_str(&"*");
+                                buf.push('*');
                                 self.append_place_to_string(
                                     PlaceRef { local, projection: proj_base },
                                     buf,
@@ -272,7 +273,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                 autoderef,
                                 &including_downcast,
                             )?;
-                            buf.push_str(&format!(".{}", field_name));
+                            buf.push('.');
+                            buf.push_str(&field_name);
                         }
                     }
                     ProjectionElem::Index(index) => {
@@ -284,11 +286,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             autoderef,
                             &including_downcast,
                         )?;
-                        buf.push_str("[");
+                        buf.push('[');
                         if self.append_local_to_string(*index, buf).is_err() {
-                            buf.push_str("_");
+                            buf.push('_');
                         }
-                        buf.push_str("]");
+                        buf.push(']');
                     }
                     ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {
                         autoderef = true;
@@ -301,7 +303,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             autoderef,
                             &including_downcast,
                         )?;
-                        buf.push_str(&"[..]");
+                        buf.push_str("[..]");
                     }
                 };
             }
@@ -648,7 +650,7 @@ impl UseSpans {
                     " in closure".to_string()
                 }
             }
-            _ => "".to_string(),
+            _ => String::new(),
         }
     }
 
diff --git a/compiler/rustc_mir/src/borrow_check/region_infer/values.rs b/compiler/rustc_mir/src/borrow_check/region_infer/values.rs
index 8a5a600cfdd..f247d07e1f0 100644
--- a/compiler/rustc_mir/src/borrow_check/region_infer/values.rs
+++ b/compiler/rustc_mir/src/borrow_check/region_infer/values.rs
@@ -417,7 +417,7 @@ crate fn location_set_str(
 
 fn region_value_str(elements: impl IntoIterator<Item = RegionElement>) -> String {
     let mut result = String::new();
-    result.push_str("{");
+    result.push('{');
 
     // Set to Some(l1, l2) when we have observed all the locations
     // from l1..=l2 (inclusive) but not yet printed them. This
@@ -478,7 +478,7 @@ fn region_value_str(elements: impl IntoIterator<Item = RegionElement>) -> String
         push_location_range(&mut result, location1, location2);
     }
 
-    result.push_str("}");
+    result.push('}');
 
     return result;
 
diff --git a/compiler/rustc_mir/src/monomorphize/partitioning/mod.rs b/compiler/rustc_mir/src/monomorphize/partitioning/mod.rs
index e96af77bbb8..0f6f078d968 100644
--- a/compiler/rustc_mir/src/monomorphize/partitioning/mod.rs
+++ b/compiler/rustc_mir/src/monomorphize/partitioning/mod.rs
@@ -382,7 +382,7 @@ fn collect_and_partition_mono_items<'tcx>(
                 cgus.sort_by_key(|(name, _)| *name);
                 cgus.dedup();
                 for &(ref cgu_name, (linkage, _)) in cgus.iter() {
-                    output.push_str(" ");
+                    output.push(' ');
                     output.push_str(&cgu_name.as_str());
 
                     let linkage_abbrev = match linkage {
@@ -399,9 +399,9 @@ fn collect_and_partition_mono_items<'tcx>(
                         Linkage::Common => "Common",
                     };
 
-                    output.push_str("[");
+                    output.push('[');
                     output.push_str(linkage_abbrev);
-                    output.push_str("]");
+                    output.push(']');
                 }
                 output
             })
diff --git a/compiler/rustc_mir/src/transform/instrument_coverage.rs b/compiler/rustc_mir/src/transform/instrument_coverage.rs
index 8f43df8a6cb..a5b30a25a9b 100644
--- a/compiler/rustc_mir/src/transform/instrument_coverage.rs
+++ b/compiler/rustc_mir/src/transform/instrument_coverage.rs
@@ -353,7 +353,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
                             if !INCLUDE_COVERAGE_STATEMENTS {
                                 continue;
                             }
-                            format!("unreachable")
+                            String::from("unreachable")
                         }
                     },
                     _ => format!("{:?}", statement),
diff --git a/compiler/rustc_mir/src/util/pretty.rs b/compiler/rustc_mir/src/util/pretty.rs
index 54bc248bc5b..75567181b69 100644
--- a/compiler/rustc_mir/src/util/pretty.rs
+++ b/compiler/rustc_mir/src/util/pretty.rs
@@ -514,7 +514,7 @@ fn write_scope_tree(
                 write!(indented_decl, " as {:?}", user_ty).unwrap();
             }
         }
-        indented_decl.push_str(";");
+        indented_decl.push(';');
 
         let local_name =
             if local == RETURN_PLACE { " return place".to_string() } else { String::new() };
diff --git a/compiler/rustc_save_analysis/src/lib.rs b/compiler/rustc_save_analysis/src/lib.rs
index c95e7e193be..032d7cb3ed6 100644
--- a/compiler/rustc_save_analysis/src/lib.rs
+++ b/compiler/rustc_save_analysis/src/lib.rs
@@ -438,7 +438,7 @@ impl<'tcx> SaveContext<'tcx> {
                                     .next()
                                     .map(|item| item.def_id);
                             }
-                            qualname.push_str(">");
+                            qualname.push('>');
 
                             (qualname, trait_id, decl_id, docs, attrs)
                         }
diff --git a/compiler/rustc_save_analysis/src/sig.rs b/compiler/rustc_save_analysis/src/sig.rs
index 6dd7f89d594..747e198cd93 100644
--- a/compiler/rustc_save_analysis/src/sig.rs
+++ b/compiler/rustc_save_analysis/src/sig.rs
@@ -497,7 +497,7 @@ impl<'hir> Sig for hir::Item<'hir> {
                     sig.text.push_str(&bounds_to_string(bounds));
                 }
                 // FIXME where clause
-                sig.text.push_str(";");
+                sig.text.push(';');
 
                 Ok(sig)
             }
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 19cd2385992..4aecb35294a 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -581,9 +581,9 @@ impl OutputFilenames {
 
         if !ext.is_empty() {
             if !extension.is_empty() {
-                extension.push_str(".");
+                extension.push('.');
                 extension.push_str(RUST_CGU_EXT);
-                extension.push_str(".");
+                extension.push('.');
             }
 
             extension.push_str(ext);