Add #[inline] to small functions in core

compiler/rustc_mir_transform/messages.ftl

@@ -42,6 +42,9 @@ mir_transform_requires_unsafe = {$details} is unsafe and requires unsafe {$op_in
     }
     .not_inherited = items do not inherit unsafety from separate enclosing items
 
+mir_transform_small_fn_without_inline = this function looks small ({$statements}) but doesn't have #[inline], consider adding it
+    .suggestion = add the inline attribute
+
 mir_transform_target_feature_call_label = call to function with `#[target_feature]`
 mir_transform_target_feature_call_note = can only be called if the required target features are available
 

compiler/rustc_mir_transform/src/cross_crate_inline.rs

@@ -62,13 +62,36 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     };
 
     let mir = tcx.optimized_mir(def_id);
-    let mut checker =
-        CostChecker { tcx, callee_body: mir, calls: 0, statements: 0, landing_pads: 0, resumes: 0 };
+    let mut checker = CostChecker {
+        tcx,
+        callee_body: mir,
+        calls: 0,
+        statements: 0,
+        landing_pads: 0,
+        resumes: 0,
+        branches: 0,
+        asserts: 0,
+    };
     checker.visit_body(mir);
-    checker.calls == 0
+    let is_leaf = checker.calls == 0
         && checker.resumes == 0
         && checker.landing_pads == 0
-        && checker.statements <= threshold
+        && checker.statements <= threshold;
+
+    let is_trivial_wrapper = checker.calls == 1
+        && checker.resumes == 0
+        && checker.landing_pads == 0
+        && mir.basic_blocks.len() == 2;
+
+    if is_trivial_wrapper {
+        let span = tcx.def_span(def_id);
+        tcx.sess.emit_warning(crate::errors::SuggestAddingInline {
+            place: span,
+            suggest_inline: span.with_hi(span.lo()),
+            statements: checker.statements,
+        });
+    }
+    is_leaf
 }
 
 struct CostChecker<'b, 'tcx> {
@@ -78,6 +101,8 @@ struct CostChecker<'b, 'tcx> {
     statements: usize,
     landing_pads: usize,
     resumes: usize,
+    branches: usize,
+    asserts: usize,
 }
 
 impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
@@ -111,7 +136,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                 }
             }
             TerminatorKind::Assert { unwind, .. } => {
-                self.calls += 1;
+                self.asserts += 1;
                 if let UnwindAction::Cleanup(_) = unwind {
                     self.landing_pads += 1;
                 }
@@ -123,6 +148,10 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                     self.landing_pads += 1;
                 }
             }
+            TerminatorKind::SwitchInt { .. } => {
+                self.statements += 1;
+                self.branches += 1;
+            }
             TerminatorKind::Return => {}
             _ => self.statements += 1,
         }

compiler/rustc_mir_transform/src/errors.rs

@@ -278,3 +278,13 @@ pub(crate) struct MustNotSuspendReason {
     pub span: Span,
     pub reason: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(mir_transform_small_fn_without_inline)]
+pub struct SuggestAddingInline {
+    #[primary_span]
+    pub place: Span,
+    #[suggestion(code = "#[inline]\n", applicability = "machine-applicable")]
+    pub suggest_inline: Span,
+    pub statements: usize,
+}

library/core/src/alloc/layout.rs

@@ -482,6 +482,7 @@ impl Error for LayoutError {}
 // (we need this for downstream impl of trait Error)
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 impl fmt::Display for LayoutError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("invalid parameters to Layout::from_size_align")
     }

library/core/src/alloc/mod.rs

@@ -43,6 +43,7 @@ impl Error for AllocError {}
 // (we need this for downstream impl of trait Error)
 #[unstable(feature = "allocator_api", issue = "32838")]
 impl fmt::Display for AllocError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("memory allocation failed")
     }

library/core/src/any.rs

@@ -630,6 +630,7 @@ impl TypeId {
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_type_id", issue = "77125")]
+    #[inline]
     pub const fn of<T: ?Sized + 'static>() -> TypeId {
         let t: u128 = intrinsics::type_id::<T>();
         TypeId { t }

library/core/src/array/mod.rs

@@ -321,6 +321,7 @@ impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
     type Item = &'a T;
     type IntoIter = Iter<'a, T>;
 
+    #[inline]
     fn into_iter(self) -> Iter<'a, T> {
         self.iter()
     }

library/core/src/ascii.rs

@@ -90,6 +90,7 @@ pub struct EscapeDefault(escape::EscapeIterInner<4>);
 /// assert_eq!(b'd', escaped.next().unwrap());
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
+#[inline]
 pub fn escape_default(c: u8) -> EscapeDefault {
     let mut data = [Char::Null; 4];
     let range = escape::escape_ascii_into(&mut data, c);

library/core/src/ascii/ascii_char.rs

@@ -559,6 +559,7 @@ impl [AsciiChar] {
 
 #[unstable(feature = "ascii_char", issue = "110998")]
 impl fmt::Display for AsciiChar {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         <str as fmt::Display>::fmt(self.as_str(), f)
     }

library/core/src/cell.rs

@@ -722,6 +722,7 @@ impl Debug for BorrowError {
 
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Display for BorrowError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         Display::fmt("already mutably borrowed", f)
     }
@@ -749,6 +750,7 @@ impl Debug for BorrowMutError {
 
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Display for BorrowMutError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         Display::fmt("already borrowed", f)
     }

library/core/src/char/convert.rs

@@ -264,6 +264,7 @@ pub struct CharTryFromError(());
 
 #[stable(feature = "try_from", since = "1.34.0")]
 impl fmt::Display for CharTryFromError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         "converted integer out of range for `char`".fmt(f)
     }

library/core/src/char/mod.rs

@@ -152,6 +152,7 @@ pub const fn from_digit(num: u32, radix: u32) -> Option<char> {
 pub struct EscapeUnicode(escape::EscapeIterInner<10>);
 
 impl EscapeUnicode {
+    #[inline]
     fn new(chr: char) -> Self {
         let mut data = [ascii::Char::Null; 10];
         let range = escape::escape_unicode_into(&mut data, chr);
@@ -219,11 +220,13 @@ impl fmt::Display for EscapeUnicode {
 pub struct EscapeDefault(escape::EscapeIterInner<10>);
 
 impl EscapeDefault {
+    #[inline]
     fn printable(chr: ascii::Char) -> Self {
         let data = [chr];
         Self(escape::EscapeIterInner::from_array(data))
     }
 
+    #[inline]
     fn backslash(chr: ascii::Char) -> Self {
         let data = [ascii::Char::ReverseSolidus, chr];
         Self(escape::EscapeIterInner::from_array(data))
@@ -308,6 +311,7 @@ impl EscapeDebug {
         Self(EscapeDebugInner::Char(chr))
     }
 
+    #[inline]
     fn backslash(chr: ascii::Char) -> Self {
         let data = [ascii::Char::ReverseSolidus, chr];
         let iter = escape::EscapeIterInner::from_array(data);
@@ -318,6 +322,7 @@ impl EscapeDebug {
         Self(EscapeDebugInner::Bytes(esc.0))
     }
 
+    #[inline]
     fn clear(&mut self) {
         let bytes = escape::EscapeIterInner::from_array([]);
         self.0 = EscapeDebugInner::Bytes(bytes);
@@ -386,6 +391,7 @@ pub struct ToLowercase(CaseMappingIter);
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Iterator for ToLowercase {
     type Item = char;
+    #[inline]
     fn next(&mut self) -> Option<char> {
         self.0.next()
     }
@@ -396,6 +402,7 @@ impl Iterator for ToLowercase {
 
 #[stable(feature = "case_mapping_double_ended", since = "1.59.0")]
 impl DoubleEndedIterator for ToLowercase {
+    #[inline]
     fn next_back(&mut self) -> Option<char> {
         self.0.next_back()
     }
@@ -420,6 +427,7 @@ pub struct ToUppercase(CaseMappingIter);
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Iterator for ToUppercase {
     type Item = char;
+    #[inline]
     fn next(&mut self) -> Option<char> {
         self.0.next()
     }
@@ -430,6 +438,7 @@ impl Iterator for ToUppercase {
 
 #[stable(feature = "case_mapping_double_ended", since = "1.59.0")]
 impl DoubleEndedIterator for ToUppercase {
+    #[inline]
     fn next_back(&mut self) -> Option<char> {
         self.0.next_back()
     }
@@ -534,13 +543,15 @@ impl fmt::Display for CaseMappingIter {
 
 #[stable(feature = "char_struct_display", since = "1.16.0")]
 impl fmt::Display for ToLowercase {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)
     }
 }
 
 #[stable(feature = "char_struct_display", since = "1.16.0")]
 impl fmt::Display for ToUppercase {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)
     }
@@ -553,6 +564,7 @@ pub struct TryFromCharError(pub(crate) ());
 
 #[stable(feature = "u8_from_char", since = "1.59.0")]
 impl fmt::Display for TryFromCharError {
+    #[inline]
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         "unicode code point out of range".fmt(fmt)
     }

library/core/src/escape.rs

@@ -82,10 +82,12 @@ impl<const N: usize> EscapeIterInner<N> {
         Self::new(data, 0..M as u8)
     }
 
+    #[inline]
     pub fn as_ascii(&self) -> &[ascii::Char] {
         &self.data[usize::from(self.alive.start)..usize::from(self.alive.end)]
     }
 
+    #[inline]
     pub fn as_str(&self) -> &str {
         self.as_ascii().as_str()
     }

library/core/src/ffi/c_str.rs

@@ -158,6 +158,7 @@ pub struct FromBytesUntilNulError(());
 
 #[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
 impl fmt::Display for FromBytesUntilNulError {
+    #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "data provided does not contain a nul")
     }
@@ -623,6 +624,7 @@ impl CStr {
     /// ```
     #[stable(feature = "cstr_to_str", since = "1.4.0")]
     #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
+    #[inline]
     pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {
         // N.B., when `CStr` is changed to perform the length check in `.to_bytes()`
         // instead of in `from_ptr()`, it may be worth considering if this should

library/core/src/fmt/builders.rs

@@ -18,6 +18,7 @@ impl Default for PadAdapterState {
 }
 
 impl<'buf, 'state> PadAdapter<'buf, 'state> {
+    #[inline]
     fn wrap<'slot, 'fmt: 'buf + 'slot>(
         fmt: &'fmt mut fmt::Formatter<'_>,
         slot: &'slot mut Option<Self>,
@@ -91,6 +92,7 @@ pub struct DebugStruct<'a, 'b: 'a> {
     has_fields: bool,
 }
 
+#[inline]
 pub(super) fn debug_struct_new<'a, 'b>(
     fmt: &'a mut fmt::Formatter<'b>,
     name: &str,
@@ -138,6 +140,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
     /// This method is equivalent to [`DebugStruct::field`], but formats the
     /// value using a provided closure rather than by calling [`Debug::fmt`].
     #[unstable(feature = "debug_closure_helpers", issue = "117729")]
+    #[inline]
     pub fn field_with<F>(&mut self, name: &str, value_fmt: F) -> &mut Self
     where
         F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
@@ -293,6 +296,7 @@ pub struct DebugTuple<'a, 'b: 'a> {
     empty_name: bool,
 }
 
+#[inline]
 pub(super) fn debug_tuple_new<'a, 'b>(
     fmt: &'a mut fmt::Formatter<'b>,
     name: &str,
@@ -326,6 +330,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     /// );
     /// ```
     #[stable(feature = "debug_builders", since = "1.2.0")]
+    #[inline]
     pub fn field(&mut self, value: &dyn fmt::Debug) -> &mut Self {
         self.field_with(|f| value.fmt(f))
     }
@@ -335,6 +340,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
     /// This method is equivalent to [`DebugTuple::field`], but formats the
     /// value using a provided closure rather than by calling [`Debug::fmt`].
     #[unstable(feature = "debug_closure_helpers", issue = "117729")]
+    #[inline]
     pub fn field_with<F>(&mut self, value_fmt: F) -> &mut Self
     where
         F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
@@ -409,6 +415,7 @@ struct DebugInner<'a, 'b: 'a> {
 }
 
 impl<'a, 'b: 'a> DebugInner<'a, 'b> {
+    #[inline]
     fn entry_with<F>(&mut self, entry_fmt: F)
     where
         F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
@@ -471,6 +478,7 @@ pub struct DebugSet<'a, 'b: 'a> {
     inner: DebugInner<'a, 'b>,
 }
 
+#[inline]
 pub(super) fn debug_set_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugSet<'a, 'b> {
     let result = fmt.write_str("{");
     DebugSet { inner: DebugInner { fmt, result, has_fields: false } }
@@ -614,6 +622,7 @@ pub struct DebugList<'a, 'b: 'a> {
     inner: DebugInner<'a, 'b>,
 }
 
+#[inline]
 pub(super) fn debug_list_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugList<'a, 'b> {
     let result = fmt.write_str("[");
     DebugList { inner: DebugInner { fmt, result, has_fields: false } }
@@ -762,6 +771,7 @@ pub struct DebugMap<'a, 'b: 'a> {
     state: PadAdapterState,
 }
 
+#[inline]
 pub(super) fn debug_map_new<'a, 'b>(fmt: &'a mut fmt::Formatter<'b>) -> DebugMap<'a, 'b> {
     let result = fmt.write_str("{");
     DebugMap { fmt, result, has_fields: false, has_key: false, state: Default::default() }
@@ -836,6 +846,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     /// This method is equivalent to [`DebugMap::key`], but formats the
     /// key using a provided closure rather than by calling [`Debug::fmt`].
     #[unstable(feature = "debug_closure_helpers", issue = "117729")]
+    #[inline]
     pub fn key_with<F>(&mut self, key_fmt: F) -> &mut Self
     where
         F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
@@ -912,6 +923,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
     /// This method is equivalent to [`DebugMap::value`], but formats the
     /// value using a provided closure rather than by calling [`Debug::fmt`].
     #[unstable(feature = "debug_closure_helpers", issue = "117729")]
+    #[inline]
     pub fn value_with<F>(&mut self, value_fmt: F) -> &mut Self
     where
         F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,