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alloc: add ToString specialization for &&str #128759

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Aug 14, 2024
Merged

alloc: add ToString specialization for &&str #128759

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1b587a6
alloc: add ToString specialization for `&&str`
notriddle Aug 6, 2024
20c833c
diagnostics: `Box<dyn Trait>` suggestion with multiple matching impl
notriddle Aug 7, 2024
3312f5d
alloc: make `to_string_str!` a bit less complex
notriddle Aug 7, 2024
c6fb0f3
diagnostics: use `DeepRejectCtxt` for check
notriddle Aug 13, 2024
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43 changes: 35 additions & 8 deletions library/alloc/src/string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2643,14 +2643,41 @@ impl ToString for i8 {
}
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
impl ToString for str {
#[inline]
fn to_string(&self) -> String {
String::from(self)
}
// Generic/generated code can sometimes have multiple, nested references
// for strings, including `&&&str`s that would never be written
// by hand. This macro generates twelve layers of nested `&`-impl
// for primitive strings.
macro_rules! to_string_str {
{type ; x $($x:ident)*} => {
&to_string_str! { type ; $($x)* }
};
{type ;} => { str };
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{impl ; x $($x:ident)*} => {
to_string_str! { $($x)* }
};
{impl ;} => { };
{$self:expr ; x $($x:ident)*} => {
*(to_string_str! { $self ; $($x)* })
};
{$self:expr ;} => { $self };
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{$($x:ident)*} => {
#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
impl ToString for to_string_str!(type ; $($x)*) {
#[inline]
fn to_string(&self) -> String {
String::from(to_string_str!(self ; $($x)*))
}
}
to_string_str! { impl ; $($x)* }
};
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}
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to_string_str! {
x x x x
x x x x
x x x x
}

#[doc(hidden)]
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36 changes: 36 additions & 0 deletions tests/codegen/issues/str-to-string-128690.rs
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@@ -0,0 +1,36 @@
//@ compile-flags: -C opt-level=3 -Z merge-functions=disabled
#![crate_type = "lib"]

//! Make sure str::to_string is specialized not to use fmt machinery.

// CHECK-LABEL: define {{(dso_local )?}}void @one_ref
#[no_mangle]
pub fn one_ref(input: &str) -> String {
// CHECK-NOT: {{(call|invoke).*}}fmt
input.to_string()
}

// CHECK-LABEL: define {{(dso_local )?}}void @two_ref
#[no_mangle]
pub fn two_ref(input: &&str) -> String {
// CHECK-NOT: {{(call|invoke).*}}fmt
input.to_string()
}

// CHECK-LABEL: define {{(dso_local )?}}void @thirteen_ref
#[no_mangle]
pub fn thirteen_ref(input: &&&&&&&&&&&&&str) -> String {
// CHECK-NOT: {{(call|invoke).*}}fmt
input.to_string()
}

// This is a known performance cliff because of the macro-generated
// specialized impl. If this test suddenly starts failing,
// consider removing the `to_string_str!` macro in `alloc/str/string.rs`.
//
// CHECK-LABEL: define {{(dso_local )?}}void @fourteen_ref
#[no_mangle]
pub fn fourteen_ref(input: &&&&&&&&&&&&&&str) -> String {
// CHECK: {{(call|invoke).*}}fmt
input.to_string()
}
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