Auto merge of #52761 - toidiu:ak-static-infer-fg, r=nikomatsakis

static infer feature gate

https://github.com/rust-lang/rust/issues/44493

r? @nikomatsakis

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

@@ -0,0 +1,45 @@
+# `infer_static_outlives_requirements`
+
+The tracking issue for this feature is: [#44493]
+
+[#44493]: https://github.com/rust-lang/rust/issues/44493
+
+------------------------
+The `infer_static_outlives_requirements` feature indicates that certain
+`'static` outlives requirements can be infered by the compiler rather than
+stating them explicitly.
+
+Note: It is an accompanying feature to `infer_outlives_requirements`,
+which must be enabled to infer outlives requirements.
+
+For example, currently generic struct definitions that contain
+references, require where-clauses of the form T: 'static. By using
+this feature the outlives predicates will be infered, although
+they may still be written explicitly.
+
+```rust,ignore (pseudo-Rust)
+struct Foo<U> where U: 'static { // <-- currently required
+    bar: Bar<U>
+}
+struct Bar<T: 'static> {
+    x: T,
+}
+```
+
+
+## Examples:
+
+```rust,ignore (pseudo-Rust)
+#![feature(infer_outlives_requirements)]
+#![feature(infer_static_outlives_requirements)]
+
+#[rustc_outlives]
+// Implicitly infer U: 'static
+struct Foo<U> {
+    bar: Bar<U>
+}
+struct Bar<T: 'static> {
+    x: T,
+}
+```
+

src/librustc_typeck/outlives/utils.rs

@@ -27,7 +27,7 @@ pub fn insert_outlives_predicate<'tcx>(
 ) {
     // If the `'a` region is bound within the field type itself, we
     // don't want to propagate this constraint to the header.
-    if !is_free_region(outlived_region) {
+    if !is_free_region(tcx, outlived_region) {
         return;
     }
 
@@ -120,7 +120,7 @@ pub fn insert_outlives_predicate<'tcx>(
         }
 
         UnpackedKind::Lifetime(r) => {
-            if !is_free_region(r) {
+            if !is_free_region(tcx, r) {
                 return;
             }
             required_predicates.insert(ty::OutlivesPredicate(kind, outlived_region));
@@ -128,19 +128,36 @@ pub fn insert_outlives_predicate<'tcx>(
     }
 }
 
-fn is_free_region(region: Region<'_>) -> bool {
+fn is_free_region<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, region: Region<'_>) -> bool {
     // First, screen for regions that might appear in a type header.
     match region {
-        // *These* correspond to `T: 'a` relationships where `'a` is
-        // either declared on the type or `'static`:
+        // These correspond to `T: 'a` relationships:
         //
         //     struct Foo<'a, T> {
         //         field: &'a T, // this would generate a ReEarlyBound referencing `'a`
-        //         field2: &'static T, // this would generate a ReStatic
         //     }
         //
         // We care about these, so fall through.
-        RegionKind::ReStatic | RegionKind::ReEarlyBound(_) => true,
+        RegionKind::ReEarlyBound(_) => true,
+
+        // These correspond to `T: 'static` relationships which can be
+        // rather surprising. We are therefore putting this behind a
+        // feature flag:
+        //
+        //     struct Foo<'a, T> {
+        //         field: &'static T, // this would generate a ReStatic
+        //     }
+        RegionKind::ReStatic => {
+            if tcx
+                .sess
+                .features_untracked()
+                .infer_static_outlives_requirements
+            {
+                true
+            } else {
+                false
+            }
+        }
 
         // Late-bound regions can appear in `fn` types:
         //

src/libsyntax/feature_gate.rs

@@ -396,6 +396,9 @@ declare_features! (
     // Infer outlives requirements; RFC 2093
     (active, infer_outlives_requirements, "1.26.0", Some(44493), None),
 
+    // Infer outlives requirements; RFC 2093
+    (active, infer_static_outlives_requirements, "1.26.0", Some(44493), None),
+
     // Multiple patterns with `|` in `if let` and `while let`
     (active, if_while_or_patterns, "1.26.0", Some(48215), None),
 
@@ -1057,6 +1060,12 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                    "infer outlives requirements is an experimental feature",
                                    cfg_fn!(infer_outlives_requirements))),
 
+    // RFC #2093
+    ("infer_static_outlives_requirements", Normal, Gated(Stability::Unstable,
+                                   "infer_static_outlives_requirements",
+                                   "infer 'static lifetime requirements",
+                                   cfg_fn!(infer_static_outlives_requirements))),
+
     // RFC 2070
     ("panic_implementation", Normal, Gated(Stability::Unstable,
                            "panic_implementation",

src/test/ui/feature-gate-infer_static_outlives_requirements.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Needs an explicit where clause stating outlives condition. (RFC 2093)
+
+// Type T needs to outlive lifetime 'static.
+struct Foo<U> {
+    bar: Bar<U> //~ ERROR 15:5: 15:16: the parameter type `U` may not live long enough [E0310]
+}
+struct Bar<T: 'static> {
+    x: T,
+}
+
+
+fn main() { }

src/test/ui/feature-gate-infer_static_outlives_requirements.stderr

@@ -0,0 +1,17 @@
+error[E0310]: the parameter type `U` may not live long enough
+  --> $DIR/feature-gate-infer_static_outlives_requirements.rs:15:5
+   |
+LL | struct Foo<U> {
+   |            - help: consider adding an explicit lifetime bound `U: 'static`...
+LL |     bar: Bar<U> //~ ERROR 15:5: 15:16: the parameter type `U` may not live long enough [E0310]
+   |     ^^^^^^^^^^^
+   |
+note: ...so that the type `U` will meet its required lifetime bounds
+  --> $DIR/feature-gate-infer_static_outlives_requirements.rs:15:5
+   |
+LL |     bar: Bar<U> //~ ERROR 15:5: 15:16: the parameter type `U` may not live long enough [E0310]
+   |     ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.

src/test/ui/rfc-2093-infer-outlives/dont-infer-static.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-tidy-linelength
+
+#![feature(infer_outlives_requirements)]
+
+/*
+ * We don't infer `T: 'static` outlives relationships by default.
+ * Instead an additional feature gate `infer_static_outlives_requirements`
+ * is required.
+ */
+
+struct Foo<U> {
+    bar: Bar<U> //~ ERROR 22:5: 22:16: the parameter type `U` may not live long enough [E0310]
+}
+struct Bar<T: 'static> {
+    x: T,
+}
+
+fn main() {}
+

src/test/ui/rfc-2093-infer-outlives/dont-infer-static.stderr

@@ -0,0 +1,17 @@
+error[E0310]: the parameter type `U` may not live long enough
+  --> $DIR/dont-infer-static.rs:22:5
+   |
+LL | struct Foo<U> {
+   |            - help: consider adding an explicit lifetime bound `U: 'static`...
+LL |     bar: Bar<U> //~ ERROR 22:5: 22:16: the parameter type `U` may not live long enough [E0310]
+   |     ^^^^^^^^^^^
+   |
+note: ...so that the type `U` will meet its required lifetime bounds
+  --> $DIR/dont-infer-static.rs:22:5
+   |
+LL |     bar: Bar<U> //~ ERROR 22:5: 22:16: the parameter type `U` may not live long enough [E0310]
+   |     ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.

src/test/ui/rfc-2093-infer-outlives/infer-static.rs

@@ -0,0 +1,24 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+#![feature(infer_outlives_requirements)]
+#![feature(infer_static_outlives_requirements)]
+
+#[rustc_outlives]
+struct Foo<U> { //~ ERROR 16:1: 18:2: rustc_outlives
+    bar: Bar<U>
+}
+struct Bar<T: 'static> {
+    x: T,
+}
+
+fn main() {}
+

src/test/ui/rfc-2093-infer-outlives/infer-static.stderr

@@ -0,0 +1,12 @@
+error: rustc_outlives
+  --> $DIR/infer-static.rs:16:1
+   |
+LL | / struct Foo<U> { //~ ERROR 16:1: 18:2: rustc_outlives
+LL | |     bar: Bar<U>
+LL | | }
+   | |_^
+   |
+   = note: U : 'static
+
+error: aborting due to previous error
+