Auto merge of #54877 - arielb1:destabilize-outlives, r=nikomatsakis

[beta] back out #53793 - stabilize outlives requirements

Fixes #54467 for beta, looks like a less risky fix than #54701

Author: bors

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

@@ -0,0 +1,67 @@
+# `infer_outlives_requirements`
+
+The tracking issue for this feature is: [#44493]
+
+[#44493]: https://github.com/rust-lang/rust/issues/44493
+
+------------------------
+The `infer_outlives_requirements` feature indicates that certain
+outlives requirements can be inferred by the compiler rather than
+stating them explicitly.
+
+For example, currently generic struct definitions that contain
+references, require where-clauses of the form T: 'a. By using
+this feature the outlives predicates will be inferred, although
+they may still be written explicitly.
+
+```rust,ignore (pseudo-Rust)
+struct Foo<'a, T>
+  where T: 'a // <-- currently required
+  {
+      bar: &'a T,
+  }
+```
+
+
+## Examples:
+
+
+```rust,ignore (pseudo-Rust)
+#![feature(infer_outlives_requirements)]
+
+// Implicitly infer T: 'a
+struct Foo<'a, T> {
+    bar: &'a T,
+}
+```
+
+```rust,ignore (pseudo-Rust)
+#![feature(infer_outlives_requirements)]
+
+// Implicitly infer `U: 'b`
+struct Foo<'b, U> {
+    bar: Bar<'b, U>
+}
+
+struct Bar<'a, T> where T: 'a {
+    x: &'a (),
+    y: T,
+}
+```
+
+```rust,ignore (pseudo-Rust)
+#![feature(infer_outlives_requirements)]
+
+// Implicitly infer `b': 'a`
+struct Foo<'a, 'b, T> {
+    x: &'a &'b T
+}
+```
+
+```rust,ignore (pseudo-Rust)
+#![feature(infer_outlives_requirements)]
+
+// Implicitly infer `<T as std::iter::Iterator>::Item : 'a`
+struct Foo<'a, T: Iterator> {
+    bar: &'a T::Item
+```

src/liballoc/lib.rs

@@ -77,6 +77,7 @@
 #![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_jemalloc/lib.rs

@@ -17,6 +17,7 @@
 #![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_system/lib.rs

@@ -18,6 +18,7 @@
 #![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/libarena/lib.rs

@@ -27,6 +27,7 @@
 #![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/libcore/lib.rs

@@ -92,6 +92,7 @@
 #![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/libfmt_macros/lib.rs

@@ -21,6 +21,7 @@
        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/libgraphviz/lib.rs

@@ -289,6 +289,7 @@
        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/libpanic_abort/lib.rs

@@ -25,6 +25,7 @@
 #![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_unwind/lib.rs

@@ -35,6 +35,7 @@
 #![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/libproc_macro/lib.rs

@@ -28,6 +28,7 @@
        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/libprofiler_builtins/lib.rs

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

src/librustc/diagnostics.rs

@@ -1232,54 +1232,41 @@ let x: i32 = "I am not a number!";
 "##,
 
 E0309: r##"
-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:
+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.
 
 ```compile_fail,E0309
-// 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.
+// 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
 struct Foo<'a, T> {
-    foo: <T as SomeTrait<'a>>::Output,
-}
-
-trait SomeTrait<'a> {
-    type Output;
-}
-
-impl<'a, T> SomeTrait<'a> for T
-where
-    T: 'a,
-{
-    type Output = u32;
+    foo: &'a T
 }
 ```
 
-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:
+This will compile, because it has the constraint on the type parameter:
 
 ```
-struct Foo<'a, T>
-where
-    T: 'a,
-{
-    foo: <T as SomeTrait<'a>>::Output
+struct Foo<'a, T: 'a> {
+    foo: &'a T
 }
+```
 
-trait SomeTrait<'a> {
-    type Output;
+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:
+
+```compile_fail,E0309
+struct Foo<'a, T> {
+    foo: &'a T
 }
 
-impl<'a, T> SomeTrait<'a> for T
-where
-    T: 'a,
-{
-    type Output = u32;
+fn main() {
+    let v = "42".to_string();
+    let f = Foo{foo: &v};
+    drop(v);
+    println!("{}", f.foo); // but we've already dropped v!
 }
 ```
 "##,
@@ -1478,31 +1465,30 @@ A reference has a longer lifetime than the data it references.
 Erroneous code example:
 
 ```compile_fail,E0491
-trait SomeTrait<'a> {
-    type Output;
+// 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,
 }
 
-impl<'a, T> SomeTrait<'a> for T {
-    type Output = &'a T; // compile error E0491
+// 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
 }
 ```
 
-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`.
+To fix this issue, you can specify a bound to the lifetime like below:
 
 ```
-trait SomeTrait<'a> {
-    type Output;
+struct Struct<'a> {
+    ref_i32: &'a i32,
 }
 
-impl<'a, T> SomeTrait<'a> for T
-where
-    T: 'a,
-{
-    type Output = &'a T; // compile error E0491
+// 'a: 'b means 'a outlives 'b
+struct Nested<'a: 'b, 'b> {
+    ref_struct: &'b Struct<'a>,
 }
 ```
 "##,

src/librustc/lib.rs

@@ -52,6 +52,7 @@
 #![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_allocator/lib.rs

@@ -9,6 +9,7 @@
 // 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_apfloat/lib.rs

@@ -46,6 +46,7 @@
 #![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_asan/lib.rs

@@ -11,6 +11,7 @@
 #![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_borrowck/lib.rs

@@ -15,6 +15,7 @@
 #![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_codegen_llvm/lib.rs

@@ -27,6 +27,7 @@
 #![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_utils/lib.rs

@@ -20,6 +20,7 @@
 #![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_cratesio_shim/src/lib.rs

@@ -12,6 +12,7 @@
 #![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_data_structures/lib.rs

@@ -29,6 +29,7 @@
 #![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_driver/lib.rs

@@ -21,6 +21,7 @@
 #![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_errors/lib.rs

@@ -17,6 +17,7 @@
 #![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_incremental/lib.rs

@@ -15,6 +15,7 @@
       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_lint/lib.rs

@@ -28,6 +28,7 @@
 #![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_llvm/lib.rs

@@ -9,6 +9,7 @@
 // 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_lsan/lib.rs

@@ -11,6 +11,7 @@
 #![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]