Auto merge of #53793 - toidiu:ak-stabalize, r=nikomatsakis

stabilize outlives requirements

#44493

r? @nikomatsakis

Author: bors

src/doc/unstable-book/src/language-features/infer-outlives-requirements.md

@@ -77,7 +77,6 @@
 #![cfg_attr(not(test), feature(fn_traits))]
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![cfg_attr(test, feature(test))]
 
 #![feature(allocator_api)]

src/liballoc/lib.rs

@@ -17,7 +17,6 @@
 #![feature(libc)]
 #![feature(linkage)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(staged_api)]
 #![feature(rustc_attrs)]
 #![cfg_attr(dummy_jemalloc, allow(dead_code, unused_extern_crates))]

src/liballoc_jemalloc/lib.rs

@@ -18,7 +18,6 @@
 #![feature(allocator_api)]
 #![feature(core_intrinsics)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(staged_api)]
 #![feature(rustc_attrs)]
 #![cfg_attr(any(unix, target_os = "cloudabi", target_os = "redox"), feature(libc))]

src/liballoc_system/lib.rs

@@ -27,7 +27,6 @@
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(raw_vec_internals)]
 #![cfg_attr(test, feature(test))]
 

src/libarena/lib.rs

@@ -92,7 +92,6 @@
 #![feature(link_llvm_intrinsics)]
 #![feature(never_type)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(exhaustive_patterns)]
 #![feature(macro_at_most_once_rep)]
 #![feature(no_core)]

src/libcore/lib.rs

@@ -21,7 +21,6 @@
        test(attr(deny(warnings))))]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 
 pub use self::Piece::*;
 pub use self::Position::*;

src/libfmt_macros/lib.rs

@@ -289,7 +289,6 @@
        test(attr(allow(unused_variables), deny(warnings))))]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(str_escape)]
 
 use self::LabelText::*;

src/libgraphviz/lib.rs

@@ -25,7 +25,6 @@
 #![feature(core_intrinsics)]
 #![feature(libc)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(panic_runtime)]
 #![feature(staged_api)]
 #![feature(rustc_attrs)]

src/libpanic_abort/lib.rs

@@ -35,7 +35,6 @@
 #![feature(lang_items)]
 #![feature(libc)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(panic_unwind)]
 #![feature(raw)]
 #![feature(staged_api)]

src/libpanic_unwind/lib.rs

@@ -28,7 +28,6 @@
        test(attr(allow(dead_code, deprecated, unused_variables, unused_mut))))]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(rustc_private)]
 #![feature(staged_api)]
 #![feature(lang_items)]

src/libproc_macro/lib.rs

@@ -16,5 +16,4 @@
             issue = "0")]
 #![allow(unused_features)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(staged_api)]

src/libprofiler_builtins/lib.rs

@@ -1232,41 +1232,54 @@ let x: i32 = "I am not a number!";
 "##,
 
 E0309: r##"
-Types in type definitions have lifetimes associated with them that represent
-how long the data stored within them is guaranteed to be live. This lifetime
-must be as long as the data needs to be alive, and missing the constraint that
-denotes this will cause this error.
+The type definition contains some field whose type
+requires an outlives annotation. Outlives annotations
+(e.g., `T: 'a`) are used to guarantee that all the data in T is valid
+for at least the lifetime `'a`. This scenario most commonly
+arises when the type contains an associated type reference
+like `<T as SomeTrait<'a>>::Output`, as shown in this example:
 
 ```compile_fail,E0309
-// This won't compile because T is not constrained, meaning the data
-// stored in it is not guaranteed to last as long as the reference
+// This won't compile because the applicable impl of
+// `SomeTrait` (below) requires that `T: 'a`, but the struct does
+// not have a matching where-clause.
 struct Foo<'a, T> {
-    foo: &'a T
+    foo: <T as SomeTrait<'a>>::Output,
 }
-```
 
-This will compile, because it has the constraint on the type parameter:
+trait SomeTrait<'a> {
+    type Output;
+}
 
-```
-struct Foo<'a, T: 'a> {
-    foo: &'a T
+impl<'a, T> SomeTrait<'a> for T
+where
+    T: 'a,
+{
+    type Output = u32;
 }
 ```
 
-To see why this is important, consider the case where `T` is itself a reference
-(e.g., `T = &str`). If we don't include the restriction that `T: 'a`, the
-following code would be perfectly legal:
+Here, the where clause `T: 'a` that appears on the impl is not known to be
+satisfied on the struct. To make this example compile, you have to add
+a where-clause like `T: 'a` to the struct definition:
 
-```compile_fail,E0309
-struct Foo<'a, T> {
-    foo: &'a T
+```
+struct Foo<'a, T>
+where
+    T: 'a,
+{
+    foo: <T as SomeTrait<'a>>::Output
 }
 
-fn main() {
-    let v = "42".to_string();
-    let f = Foo{foo: &v};
-    drop(v);
-    println!("{}", f.foo); // but we've already dropped v!
+trait SomeTrait<'a> {
+    type Output;
+}
+
+impl<'a, T> SomeTrait<'a> for T
+where
+    T: 'a,
+{
+    type Output = u32;
 }
 ```
 "##,
@@ -1465,30 +1478,31 @@ A reference has a longer lifetime than the data it references.
 Erroneous code example:
 
 ```compile_fail,E0491
-// struct containing a reference requires a lifetime parameter,
-// because the data the reference points to must outlive the struct (see E0106)
-struct Struct<'a> {
-    ref_i32: &'a i32,
+trait SomeTrait<'a> {
+    type Output;
 }
 
-// However, a nested struct like this, the signature itself does not tell
-// whether 'a outlives 'b or the other way around.
-// So it could be possible that 'b of reference outlives 'a of the data.
-struct Nested<'a, 'b> {
-    ref_struct: &'b Struct<'a>, // compile error E0491
+impl<'a, T> SomeTrait<'a> for T {
+    type Output = &'a T; // compile error E0491
 }
 ```
 
-To fix this issue, you can specify a bound to the lifetime like below:
+Here, the problem is that a reference type like `&'a T` is only valid
+if all the data in T outlives the lifetime `'a`. But this impl as written
+is applicable to any lifetime `'a` and any type `T` -- we have no guarantee
+that `T` outlives `'a`. To fix this, you can add a where clause like
+`where T: 'a`.
 
 ```
-struct Struct<'a> {
-    ref_i32: &'a i32,
+trait SomeTrait<'a> {
+    type Output;
 }
 
-// 'a: 'b means 'a outlives 'b
-struct Nested<'a: 'b, 'b> {
-    ref_struct: &'b Struct<'a>,
+impl<'a, T> SomeTrait<'a> for T
+where
+    T: 'a,
+{
+    type Output = &'a T; // compile error E0491
 }
 ```
 "##,

src/librustc/diagnostics.rs

@@ -52,7 +52,6 @@
 #![feature(exhaustive_patterns)]
 #![feature(extern_types)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(non_exhaustive)]
 #![feature(proc_macro_internals)]
 #![feature(quote)]

src/librustc/lib.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(rustc_private)]
 
 #[macro_use] extern crate log;

src/librustc_allocator/lib.rs

@@ -46,7 +46,6 @@
 #![forbid(unsafe_code)]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(try_from)]
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
 #[allow(unused_extern_crates)]

src/librustc_apfloat/lib.rs

@@ -11,7 +11,6 @@
 #![sanitizer_runtime]
 #![feature(alloc_system)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(sanitizer_runtime)]
 #![feature(staged_api)]
 #![no_std]

src/librustc_asan/lib.rs

@@ -15,7 +15,6 @@
 #![allow(non_camel_case_types)]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(quote)]
 
 #![recursion_limit="256"]

src/librustc_borrowck/lib.rs

@@ -27,7 +27,6 @@
 #![allow(unused_attributes)]
 #![feature(libc)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(quote)]
 #![feature(range_contains)]
 #![feature(rustc_diagnostic_macros)]

src/librustc_codegen_llvm/lib.rs

@@ -20,7 +20,6 @@
 #![feature(box_syntax)]
 #![feature(custom_attribute)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![allow(unused_attributes)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]

src/librustc_codegen_utils/lib.rs

@@ -12,7 +12,6 @@
 #![allow(unused_extern_crates)]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 
 extern crate bitflags;
 extern crate log;

src/librustc_cratesio_shim/src/lib.rs

@@ -29,7 +29,6 @@
 #![feature(optin_builtin_traits)]
 #![cfg_attr(stage0, feature(macro_vis_matcher))]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(allow_internal_unstable)]
 #![feature(vec_resize_with)]
 

src/librustc_data_structures/lib.rs

@@ -21,7 +21,6 @@
 #![feature(box_syntax)]
 #![cfg_attr(unix, feature(libc))]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(option_replace)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]

src/librustc_driver/lib.rs

@@ -17,7 +17,6 @@
 #![feature(range_contains)]
 #![cfg_attr(unix, feature(libc))]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(optin_builtin_traits)]
 
 extern crate atty;

src/librustc_errors/lib.rs

@@ -15,7 +15,6 @@
       html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(specialization)]
 
 #![recursion_limit="256"]

src/librustc_incremental/lib.rs

@@ -28,7 +28,6 @@
 #![feature(box_syntax)]
 #![cfg_attr(stage0, feature(macro_vis_matcher))]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
 #![feature(macro_at_most_once_rep)]

src/librustc_lint/lib.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(static_nobundle)]
 
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",

src/librustc_llvm/lib.rs

@@ -11,7 +11,6 @@
 #![sanitizer_runtime]
 #![feature(alloc_system)]
 #![cfg_attr(not(stage0), feature(nll))]
-#![cfg_attr(not(stage0), feature(infer_outlives_requirements))]
 #![feature(sanitizer_runtime)]
 #![feature(staged_api)]
 #![no_std]