Rollup merge of rust-lang#72045 - RalfJung:incomplete-unsound, r=petrochenkov

Incomplete features can also be unsound

Some incomplete features do not just ICE, they are also currently unsound (e.g. rust-lang#72029, and also `specialization` -- which is not yet marked incomplete but [should be](rust-lang#71420)). This makes the message reflect that.

While at it I also added a link to the tracking issue, which hopefully should explain what is incomplete/unsound about the feature.

Author: RalfJung

src/librustc_lint/builtin.rs

@@ -28,8 +28,8 @@ use rustc_ast::visit::{FnCtxt, FnKind};
 use rustc_ast_pretty::pprust::{self, expr_to_string};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{Applicability, DiagnosticBuilder};
-use rustc_feature::Stability;
 use rustc_feature::{deprecated_attributes, AttributeGate, AttributeTemplate, AttributeType};
+use rustc_feature::{GateIssue, Stability};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -1817,13 +1817,21 @@ impl EarlyLintPass for IncompleteFeatures {
             .map(|(name, span, _)| (name, span))
             .chain(features.declared_lib_features.iter().map(|(name, span)| (name, span)))
             .filter(|(name, _)| rustc_feature::INCOMPLETE_FEATURES.iter().any(|f| name == &f))
-            .for_each(|(name, &span)| {
+            .for_each(|(&name, &span)| {
                 cx.struct_span_lint(INCOMPLETE_FEATURES, span, |lint| {
-                    lint.build(&format!(
-                        "the feature `{}` is incomplete and may cause the compiler to crash",
+                    let mut builder = lint.build(&format!(
+                        "the feature `{}` is incomplete and may not be safe to use \
+                         and/or cause compiler crashes",
                         name,
-                    ))
-                    .emit()
+                    ));
+                    if let Some(n) = rustc_feature::find_feature_issue(name, GateIssue::Language) {
+                        builder.note(&format!(
+                            "see issue #{} <https://github.com/rust-lang/rust/issues/{}> \
+                             for more information",
+                            n, n,
+                        ));
+                    }
+                    builder.emit();
                 })
             });
     }

src/test/incremental/const-generics/issue-62536.rs

@@ -1,6 +1,6 @@
 // revisions:cfail1
 #![feature(const_generics)]
-//[cfail1]~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+//[cfail1]~^ WARN the feature `const_generics` is incomplete
 
 struct S<T, const N: usize>([T; N]);
 

src/test/incremental/const-generics/issue-64087.rs

@@ -1,6 +1,6 @@
 // revisions:cfail1
 #![feature(const_generics)]
-//[cfail1]~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+//[cfail1]~^ WARN the feature `const_generics` is incomplete
 
 fn combinator<T, const S: usize>() -> [T; S] {}
 //[cfail1]~^ ERROR mismatched types

src/test/ui/array-slice-vec/match_arr_unknown_len.rs

@@ -1,5 +1,5 @@
 #![feature(const_generics)]
-//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+//~^ WARN the feature `const_generics` is incomplete
 
 fn is_123<const N: usize>(x: [u32; N]) -> bool {
     match x {

src/test/ui/array-slice-vec/match_arr_unknown_len.stderr

@@ -1,10 +1,11 @@
-warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/match_arr_unknown_len.rs:1:12
    |
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
 
 error[E0308]: mismatched types
   --> $DIR/match_arr_unknown_len.rs:6:9

src/test/ui/associated-type-bounds/duplicate.rs

@@ -2,7 +2,7 @@
 
 #![feature(associated_type_bounds)]
 #![feature(type_alias_impl_trait)]
-#![feature(impl_trait_in_bindings)] //~ WARN the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash [incomplete_features]
+#![feature(impl_trait_in_bindings)] //~ WARN the feature `impl_trait_in_bindings` is incomplete
 #![feature(untagged_unions)]
 
 use std::iter;

src/test/ui/associated-type-bounds/duplicate.stderr

@@ -1,10 +1,11 @@
-warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
+warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/duplicate.rs:5:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
 
 error[E0719]: the value of the associated type `Item` (from trait `std::iter::Iterator`) is already specified
   --> $DIR/duplicate.rs:10:36

src/test/ui/associated-type-bounds/dyn-lcsit.stderr

@@ -1,10 +1,11 @@
-warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
+warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/dyn-lcsit.rs:4:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
 
 warning: 1 warning emitted
 

src/test/ui/associated-type-bounds/lcsit.stderr

@@ -1,10 +1,11 @@
-warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
+warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/lcsit.rs:4:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
 
 warning: 1 warning emitted
 

src/test/ui/binding/const-param.stderr

@@ -1,10 +1,11 @@
-warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/const-param.rs:3:12
    |
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
 
 error[E0158]: const parameters cannot be referenced in patterns
   --> $DIR/const-param.rs:7:9