diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
index d935fc83b7212..1464784ae288c 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
@@ -2372,7 +2372,7 @@ fn compute_type_parameters(cx: &CodegenCx<'ll, 'tcx>, ty: Ty<'tcx>) -> &'ll DIAr
     fn get_parameter_names(cx: &CodegenCx<'_, '_>, generics: &ty::Generics) -> Vec<Symbol> {
         let mut names = generics
             .parent
-            .map_or(vec![], |def_id| get_parameter_names(cx, cx.tcx.generics_of(def_id)));
+            .map_or_else(Vec::new, |def_id| get_parameter_names(cx, cx.tcx.generics_of(def_id)));
         names.extend(generics.params.iter().map(|param| param.name));
         names
     }
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
index 955e739b2c126..c28b0d64651e3 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
@@ -481,9 +481,9 @@ impl DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         }
 
         fn get_parameter_names(cx: &CodegenCx<'_, '_>, generics: &ty::Generics) -> Vec<Symbol> {
-            let mut names = generics
-                .parent
-                .map_or(vec![], |def_id| get_parameter_names(cx, cx.tcx.generics_of(def_id)));
+            let mut names = generics.parent.map_or_else(Vec::new, |def_id| {
+                get_parameter_names(cx, cx.tcx.generics_of(def_id))
+            });
             names.extend(generics.params.iter().map(|param| param.name));
             names
         }
diff --git a/compiler/rustc_codegen_llvm/src/metadata.rs b/compiler/rustc_codegen_llvm/src/metadata.rs
index 3912d6a3a48b6..b007df5730621 100644
--- a/compiler/rustc_codegen_llvm/src/metadata.rs
+++ b/compiler/rustc_codegen_llvm/src/metadata.rs
@@ -65,8 +65,8 @@ fn search_meta_section<'a>(
         while llvm::LLVMIsSectionIteratorAtEnd(of.llof, si.llsi) == False {
             let mut name_buf = None;
             let name_len = llvm::LLVMRustGetSectionName(si.llsi, &mut name_buf);
-            let name = name_buf.map_or(
-                String::new(), // We got a NULL ptr, ignore `name_len`.
+            let name = name_buf.map_or_else(
+                String::new, // We got a NULL ptr, ignore `name_len`.
                 |buf| {
                     String::from_utf8(
                         slice::from_raw_parts(buf.as_ptr() as *const u8, name_len as usize)
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 08932e26bf1c7..caa6a6a8e3a26 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -2082,7 +2082,7 @@ fn add_upstream_rust_crates<'a, B: ArchiveBuilder<'a>>(
         let filestem = cratepath.file_stem().unwrap().to_str().unwrap();
         cmd.link_rust_dylib(
             Symbol::intern(&unlib(&sess.target, filestem)),
-            parent.unwrap_or(Path::new("")),
+            parent.unwrap_or_else(|| Path::new("")),
         );
     }
 }
diff --git a/compiler/rustc_lint/src/non_fmt_panic.rs b/compiler/rustc_lint/src/non_fmt_panic.rs
index bfe37ce6959e7..5a27135581747 100644
--- a/compiler/rustc_lint/src/non_fmt_panic.rs
+++ b/compiler/rustc_lint/src/non_fmt_panic.rs
@@ -201,7 +201,7 @@ fn check_panic_str<'tcx>(
             Some(v) if v.len() == 1 => "panic message contains a brace",
             _ => "panic message contains braces",
         };
-        cx.struct_span_lint(NON_FMT_PANIC, brace_spans.unwrap_or(vec![span]), |lint| {
+        cx.struct_span_lint(NON_FMT_PANIC, brace_spans.unwrap_or_else(|| vec![span]), |lint| {
             let mut l = lint.build(msg);
             l.note("this message is not used as a format string, but will be in Rust 2021");
             if span.contains(arg.span) {
diff --git a/compiler/rustc_macros/src/query.rs b/compiler/rustc_macros/src/query.rs
index 3e67525567f97..291e7ef045e4f 100644
--- a/compiler/rustc_macros/src/query.rs
+++ b/compiler/rustc_macros/src/query.rs
@@ -378,14 +378,14 @@ fn add_query_description_impl(
                 let t = &(t.0).0;
                 quote! { #t }
             })
-            .unwrap_or(quote! { _ });
+            .unwrap_or_else(|| quote! { _ });
         let value = args
             .as_ref()
             .map(|t| {
                 let t = &(t.1).0;
                 quote! { #t }
             })
-            .unwrap_or(quote! { _ });
+            .unwrap_or_else(|| quote! { _ });
         // expr is a `Block`, meaning that `{ #expr }` gets expanded
         // to `{ { stmts... } }`, which triggers the `unused_braces` lint.
         quote! {
@@ -409,7 +409,7 @@ fn add_query_description_impl(
     };
 
     let (tcx, desc) = modifiers.desc;
-    let tcx = tcx.as_ref().map_or(quote! { _ }, |t| quote! { #t });
+    let tcx = tcx.as_ref().map_or_else(|| quote! { _ }, |t| quote! { #t });
 
     let desc = quote! {
         #[allow(unused_variables)]
diff --git a/compiler/rustc_macros/src/session_diagnostic.rs b/compiler/rustc_macros/src/session_diagnostic.rs
index 5c061a9d3c794..8a0fce209b7fb 100644
--- a/compiler/rustc_macros/src/session_diagnostic.rs
+++ b/compiler/rustc_macros/src/session_diagnostic.rs
@@ -473,9 +473,9 @@ impl<'a> SessionDiagnosticDeriveBuilder<'a> {
                                         .map(
                                             |applicability_idx| quote!(#binding.#applicability_idx),
                                         )
-                                        .unwrap_or(quote!(
-                                            rustc_errors::Applicability::Unspecified
-                                        ));
+                                        .unwrap_or_else(|| {
+                                            quote!(rustc_errors::Applicability::Unspecified)
+                                        });
                                     return Ok((span, applicability));
                                 }
                                 throw_span_err!(
diff --git a/compiler/rustc_mir/src/const_eval/eval_queries.rs b/compiler/rustc_mir/src/const_eval/eval_queries.rs
index de898460368b8..fa234ff5feb64 100644
--- a/compiler/rustc_mir/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_mir/src/const_eval/eval_queries.rs
@@ -50,7 +50,7 @@ fn eval_body_using_ecx<'mir, 'tcx>(
 
     let name =
         with_no_trimmed_paths(|| ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id())));
-    let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
+    let prom = cid.promoted.map_or_else(String::new, |p| format!("::promoted[{:?}]", p));
     trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
 
     ecx.push_stack_frame(
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 0f49386dec07b..5b4939b740726 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -223,7 +223,7 @@ impl<'a> Parser<'a> {
         fn tokens_to_string(tokens: &[TokenType]) -> String {
             let mut i = tokens.iter();
             // This might be a sign we need a connect method on `Iterator`.
-            let b = i.next().map_or(String::new(), |t| t.to_string());
+            let b = i.next().map_or_else(String::new, |t| t.to_string());
             i.enumerate().fold(b, |mut b, (i, a)| {
                 if tokens.len() > 2 && i == tokens.len() - 2 {
                     b.push_str(", or ");
diff --git a/compiler/rustc_resolve/src/late/lifetimes.rs b/compiler/rustc_resolve/src/late/lifetimes.rs
index c0e4a1bdf07a4..493f25f4992d1 100644
--- a/compiler/rustc_resolve/src/late/lifetimes.rs
+++ b/compiler/rustc_resolve/src/late/lifetimes.rs
@@ -1971,7 +1971,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         // Therefore, we would compute `object_lifetime_defaults` to a
         // vector like `['x, 'static]`. Note that the vector only
         // includes type parameters.
-        let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
+        let object_lifetime_defaults = type_def_id.map_or_else(Vec::new, |def_id| {
             let in_body = {
                 let mut scope = self.scope;
                 loop {
diff --git a/compiler/rustc_session/src/filesearch.rs b/compiler/rustc_session/src/filesearch.rs
index 47f14fa6b7a74..2df326628e787 100644
--- a/compiler/rustc_session/src/filesearch.rs
+++ b/compiler/rustc_session/src/filesearch.rs
@@ -169,7 +169,7 @@ pub fn get_or_default_sysroot() -> PathBuf {
 
     // Check if sysroot is found using env::args().next(), and if is not found,
     // use env::current_exe() to imply sysroot.
-    from_env_args_next().unwrap_or(from_current_exe())
+    from_env_args_next().unwrap_or_else(from_current_exe)
 }
 
 // The name of the directory rustc expects libraries to be located.
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index 0133a961c11a7..264cc4f248cec 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -349,7 +349,7 @@ fn report_negative_positive_conflict(
         E0751,
         "found both positive and negative implementation of trait `{}`{}:",
         overlap.trait_desc,
-        overlap.self_desc.clone().map_or(String::new(), |ty| format!(" for type `{}`", ty))
+        overlap.self_desc.clone().map_or_else(String::new, |ty| format!(" for type `{}`", ty))
     );
 
     match tcx.span_of_impl(negative_impl_def_id) {
@@ -397,7 +397,10 @@ fn report_conflicting_impls(
         let msg = format!(
             "conflicting implementations of trait `{}`{}:{}",
             overlap.trait_desc,
-            overlap.self_desc.clone().map_or(String::new(), |ty| { format!(" for type `{}`", ty) }),
+            overlap
+                .self_desc
+                .clone()
+                .map_or_else(String::new, |ty| { format!(" for type `{}`", ty) }),
             match used_to_be_allowed {
                 Some(FutureCompatOverlapErrorKind::Issue33140) => " (E0119)",
                 _ => "",
@@ -415,7 +418,7 @@ fn report_conflicting_impls(
                     impl_span,
                     format!(
                         "conflicting implementation{}",
-                        overlap.self_desc.map_or(String::new(), |ty| format!(" for `{}`", ty))
+                        overlap.self_desc.map_or_else(String::new, |ty| format!(" for `{}`", ty))
                     ),
                 );
             }
diff --git a/compiler/rustc_typeck/src/check/method/probe.rs b/compiler/rustc_typeck/src/check/method/probe.rs
index c25aec5b2b7e9..38ff88553e8d1 100644
--- a/compiler/rustc_typeck/src/check/method/probe.rs
+++ b/compiler/rustc_typeck/src/check/method/probe.rs
@@ -1716,7 +1716,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             } else {
                 self.fcx
                     .associated_item(def_id, name, Namespace::ValueNS)
-                    .map_or(Vec::new(), |x| vec![x])
+                    .map_or_else(Vec::new, |x| vec![x])
             }
         } else {
             self.tcx.associated_items(def_id).in_definition_order().copied().collect()
diff --git a/compiler/rustc_typeck/src/check/mod.rs b/compiler/rustc_typeck/src/check/mod.rs
index fce7ae8119e17..bb85336d7fb2d 100644
--- a/compiler/rustc_typeck/src/check/mod.rs
+++ b/compiler/rustc_typeck/src/check/mod.rs
@@ -1062,7 +1062,10 @@ fn report_unexpected_variant_res(tcx: TyCtxt<'_>, res: Res, span: Span) {
         E0533,
         "expected unit struct, unit variant or constant, found {}{}",
         res.descr(),
-        tcx.sess.source_map().span_to_snippet(span).map_or(String::new(), |s| format!(" `{}`", s)),
+        tcx.sess
+            .source_map()
+            .span_to_snippet(span)
+            .map_or_else(|_| String::new(), |s| format!(" `{}`", s)),
     )
     .emit();
 }
diff --git a/compiler/rustc_typeck/src/check/pat.rs b/compiler/rustc_typeck/src/check/pat.rs
index 897b1f01569f4..f8ca916caf127 100644
--- a/compiler/rustc_typeck/src/check/pat.rs
+++ b/compiler/rustc_typeck/src/check/pat.rs
@@ -879,7 +879,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let sm = tcx.sess.source_map();
             let path_str = sm
                 .span_to_snippet(sm.span_until_char(pat.span, '('))
-                .map_or(String::new(), |s| format!(" `{}`", s.trim_end()));
+                .map_or_else(|_| String::new(), |s| format!(" `{}`", s.trim_end()));
             let msg = format!(
                 "expected tuple struct or tuple variant, found {}{}",
                 res.descr(),
diff --git a/compiler/rustc_typeck/src/check/writeback.rs b/compiler/rustc_typeck/src/check/writeback.rs
index 4f78598215077..af82a3bb4f59a 100644
--- a/compiler/rustc_typeck/src/check/writeback.rs
+++ b/compiler/rustc_typeck/src/check/writeback.rs
@@ -348,9 +348,9 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
                 let min_list_wb = min_list
                     .iter()
                     .map(|captured_place| {
-                        let locatable = captured_place.info.path_expr_id.unwrap_or(
-                            self.tcx().hir().local_def_id_to_hir_id(closure_def_id.expect_local()),
-                        );
+                        let locatable = captured_place.info.path_expr_id.unwrap_or_else(|| {
+                            self.tcx().hir().local_def_id_to_hir_id(closure_def_id.expect_local())
+                        });
 
                         self.resolve(captured_place.clone(), &locatable)
                     })
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index fc8e50b4b65ba..9fbc56f051beb 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -2387,7 +2387,7 @@ fn compute_sig_of_foreign_fn_decl<'tcx>(
                     .sess
                     .source_map()
                     .span_to_snippet(ast_ty.span)
-                    .map_or(String::new(), |s| format!(" `{}`", s));
+                    .map_or_else(|_| String::new(), |s| format!(" `{}`", s));
                 tcx.sess
                     .struct_span_err(
                         ast_ty.span,