impl Add<Output = String> for &str

Inspired by https://fasterthanli.me/articles/some-mistakes-rust-doesnt-catch.

Author: jyn514

compiler/rustc_hir/src/lang_items.rs

@@ -171,6 +171,7 @@ language_item_table! {
     Slice,                   sym::slice,               slice_impl,                 Target::Impl,           GenericRequirement::None;
     SliceU8,                 sym::slice_u8,            slice_u8_impl,              Target::Impl,           GenericRequirement::None;
     StrAlloc,                sym::str_alloc,           str_alloc_impl,             Target::Impl,           GenericRequirement::None;
+    StrAllocAdd,             sym::str_alloc_add,       str_alloc_add_impl,         Target::Impl,           GenericRequirement::None;
     SliceAlloc,              sym::slice_alloc,         slice_alloc_impl,           Target::Impl,           GenericRequirement::None;
     SliceU8Alloc,            sym::slice_u8_alloc,      slice_u8_alloc_impl,        Target::Impl,           GenericRequirement::None;
     ConstPtr,                sym::const_ptr,           const_ptr_impl,             Target::Impl,           GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -1330,6 +1330,7 @@ symbols! {
         store,
         str,
         str_alloc,
+        str_alloc_add,
         stringify,
         stringify_macro,
         struct_field_attributes,

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -439,6 +439,11 @@ pub enum OrphanCheckErr<'tcx> {
 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
     debug!("orphan_check({:?})", impl_def_id);
 
+    // `alloc` breaks coherence so it can define `&str + &str = String`
+    if Some(impl_def_id) == tcx.lang_items().str_alloc_add_impl() {
+        return Ok(());
+    }
+
     // We only except this routine to be invoked on implementations
     // of a trait, not inherent implementations.
     let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();

library/alloc/src/string.rs

@@ -2215,6 +2215,23 @@ impl AddAssign<&str> for String {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[cfg(not(bootstrap))]
+#[lang = "str_alloc_add"]
+#[stable(feature = "stradd", since = "1.60.0")]
+/// # Examples
+/// ```
+/// let hello: String = "hello, " + "world!";
+/// println!("{}", hello);
+/// ```
+impl core::ops::Add<&str> for &str {
+    type Output = String;
+
+    fn add(self, other: &str) -> String {
+        String::from(self) + other
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ops::Index<ops::Range<usize>> for String {
     type Output = str;