Rollup of 3 pull requests

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -409,9 +409,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
     pub(super) fn report_ambiguous_assoc_item(
         &self,
-        bound1: ty::PolyTraitRef<'tcx>,
-        bound2: ty::PolyTraitRef<'tcx>,
-        matching_candidates: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
+        matching_candidates: &[ty::PolyTraitRef<'tcx>],
         qself: AssocItemQSelf,
         assoc_tag: ty::AssocTag,
         assoc_ident: Ident,
@@ -443,7 +441,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // predicates!).
         // FIXME: Turn this into a structured, translatable & more actionable suggestion.
         let mut where_bounds = vec![];
-        for bound in [bound1, bound2].into_iter().chain(matching_candidates) {
+        for &bound in matching_candidates {
             let bound_id = bound.def_id();
             let assoc_item = tcx.associated_items(bound_id).find_by_ident_and_kind(
                 tcx,

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -24,6 +24,7 @@ use std::{assert_matches, slice};
 use rustc_abi::FIRST_VARIANT;
 use rustc_ast::LitKind;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::sso::SsoHashSet;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed, FatalError, StashKey,
@@ -1262,6 +1263,74 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         )
     }
 
+    /// When there are multiple traits which contain an identically named
+    /// associated item, this function eliminates any traits which are a
+    /// supertrait of another candidate trait.
+    ///
+    /// This is the type-level analogue of
+    /// `rustc_hir_typeck::method::probe::ProbeContext::collapse_candidates_to_subtrait_pick`;
+    /// keep both implementations in sync.
+    ///
+    /// This implements RFC #3624.
+    fn collapse_candidates_to_subtrait_pick(
+        &self,
+        matching_candidates: &[ty::PolyTraitRef<'tcx>],
+    ) -> Option<ty::PolyTraitRef<'tcx>> {
+        if !self.tcx().features().supertrait_item_shadowing() {
+            return None;
+        }
+
+        let mut child_trait = matching_candidates[0];
+        let mut supertraits: SsoHashSet<_> =
+            traits::supertrait_def_ids(self.tcx(), child_trait.def_id()).collect();
+
+        let mut remaining_candidates: Vec<_> = matching_candidates[1..].iter().copied().collect();
+        while !remaining_candidates.is_empty() {
+            let mut made_progress = false;
+            let mut next_round = vec![];
+
+            for remaining_trait in remaining_candidates {
+                if supertraits.contains(&remaining_trait.def_id()) {
+                    made_progress = true;
+                    continue;
+                }
+
+                // This candidate is not a supertrait of the `child_trait`.
+                // Check if it's a subtrait of the `child_trait`, instead.
+                // If it is, then it must have been a subtrait of every
+                // other pick we've eliminated at this point. It will
+                // take over at this point.
+                let remaining_trait_supertraits: SsoHashSet<_> =
+                    traits::supertrait_def_ids(self.tcx(), remaining_trait.def_id()).collect();
+                if remaining_trait_supertraits.contains(&child_trait.def_id()) {
+                    child_trait = remaining_trait;
+                    supertraits = remaining_trait_supertraits;
+                    made_progress = true;
+                    continue;
+                }
+
+                // Neither `child_trait` or the current candidate are
+                // supertraits of each other.
+                // Don't bail here, since we may be comparing two supertraits
+                // of a common subtrait. These two supertraits won't be related
+                // at all, but we will pick them up next round when we find their
+                // child as we continue iterating in this round.
+                next_round.push(remaining_trait);
+            }
+
+            if made_progress {
+                // If we've made progress, iterate again.
+                remaining_candidates = next_round;
+            } else {
+                // Otherwise, we must have at least two candidates which
+                // are not related to each other at all.
+                return None;
+            }
+        }
+
+        Some(child_trait)
+    }
+
     /// Search for a single trait bound whose trait defines the associated item given by
     /// `assoc_ident`.
     ///
@@ -1296,10 +1365,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         };
 
         if let Some(bound2) = matching_candidates.next() {
+            let all_matching_candidates: Vec<_> =
+                [bound1, bound2].into_iter().chain(matching_candidates).collect();
+            if let Some(bound) = self.collapse_candidates_to_subtrait_pick(&all_matching_candidates)
+            {
+                return Ok(bound);
+            }
+
             return Err(self.report_ambiguous_assoc_item(
-                bound1,
-                bound2,
-                matching_candidates,
+                &all_matching_candidates,
                 qself,
                 assoc_tag,
                 assoc_ident,

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -2359,6 +2359,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// multiple conflicting picks if there is one pick whose trait container is a subtrait
     /// of the trait containers of all of the other picks.
     ///
+    /// This is the method-probe analogue of
+    /// `rustc_hir_analysis::hir_ty_lowering::HirTyLowerer::collapse_candidates_to_subtrait_pick`;
+    /// keep both implementations in sync.
+    ///
     /// This implements RFC #3624.
     fn collapse_candidates_to_subtrait_pick(
         &self,

library/std/src/attribute_docs.rs

@@ -85,3 +85,253 @@
 /// [`unused_must_use`]: ../rustc/lints/listing/warn-by-default.html#unused-must-use
 /// [the `must_use` attribute]: ../reference/attributes/diagnostics.html#the-must_use-attribute
 mod must_use_attribute {}
+
+#[doc(attribute = "allow")]
+//
+/// The `allow` attribute suppresses lint diagnostics that would otherwise produce
+/// warnings or errors. It can be used on any lint or lint group (except those
+/// set to `forbid`).
+///
+/// ```rust
+/// #[allow(dead_code)]
+/// fn unused_function() {
+///     // ...
+/// }
+///
+/// fn main() {
+///   // `unused_function` does not generate a compiler warning.
+/// }
+/// ```
+///
+/// Without `#[allow(dead_code)]`, the example above would emit:
+///
+/// ```text
+/// warning: function `unused_function` is never used
+///  --> main.rs:1:4
+///   |
+/// 1 | fn unused_function() {
+///   |    ^^^^^^^^^^^^^^^
+///   |
+///   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
+///
+///   warning: 1 warning emitted
+/// ```
+///
+/// Multiple lints can be set to `allow` at once with commas:
+///
+/// ```rust
+/// #[allow(unused_variables, unused_mut)]
+/// fn main() {
+///     let mut x: u32 = 42;
+/// }
+/// ```
+///
+/// This is mostly used to prevent lint warnings or errors while still under development.
+///
+/// It cannot override a lint that has been set to `forbid`.
+///
+/// It's also important to consider that overusing `allow` could make code harder to maintain
+/// and possibly hide issues. To mitigate this issue, using the `expect` attribute is preferred.
+///
+/// `allow` can be overridden by `warn`, `deny`, and `forbid`.
+///
+/// The lint checks supported by rustc can be found via `rustc -W help`,
+/// along with their default settings and are documented in [the `rustc` book].
+///
+/// [the `rustc` book]: ../rustc/lints/listing/index.html
+///
+/// For more information, see the Reference on [the `allow` attribute].
+///
+/// [the `allow` attribute]: ../reference/attributes/diagnostics.html#lint-check-attributes
+mod allow_attribute {}
+
+#[doc(attribute = "cfg")]
+//
+/// Used for conditional compilation.
+///
+/// The `cfg` attribute allows compiling an item under specific conditions, otherwise it
+/// will be ignored.
+///
+/// ```rust
+/// // Only compiles this function for Linux.
+/// #[cfg(target_os = "linux")]
+/// fn platform_specific() {
+///     println!("Running on Linux");
+/// }
+///
+/// // Only compiles this function if not for Linux.
+/// #[cfg(not(target_os = "linux"))]
+/// fn platform_specific() {
+///     println!("Running on something else");
+/// }
+/// ```
+///
+/// Depending on the platform you're targeting, only one of these two functions will be considered
+/// during the compilation.
+///
+/// Conditions can also be combined with `all(...)`, `any(...)`, and `not(...)`.
+///
+/// * `all`: True if all given predicates are true.
+/// * `any`: True if at least one of the given predicates is true.
+/// * `not`: True if the predicate is false and false if the predicate is true.
+///
+/// ```rust
+/// #[cfg(all(unix, target_pointer_width = "64"))]
+/// fn unix_64bit() {
+/// }
+/// ```
+///
+/// If you want to use this mechanism in an `if` condition in your code, you
+/// can use the [`cfg!`] macro. To conditionally apply an attribute,
+/// see [`cfg_attr`].
+///
+/// For more information, see the Reference on [the `cfg` attribute].
+///
+/// [`cfg_attr`]: ../reference/conditional-compilation.html#the-cfg_attr-attribute
+/// [the `cfg` attribute]: ../reference/conditional-compilation.html#the-cfg-attribute
+mod cfg_attribute {}
+
+#[doc(attribute = "deny")]
+//
+/// Emits an error, preventing the compilation from finishing, when a lint check has failed.
+/// This is useful for enforcing rules or preventing certain patterns:
+///
+/// ```rust,compile_fail
+/// #[deny(unused)]
+/// fn foo() {
+///     let x = 42; // Emits an error because x is unused.
+/// }
+/// ```
+///
+/// `deny` can be overridden by `allow`, `warn`, and `forbid`:
+///
+/// ```rust
+/// #![deny(unused)]
+///
+/// #[allow(unused)] // We override the `deny` for this function.
+/// fn foo() {
+///     let x = 42; // No lint emitted even though `x` is unused.
+/// }
+/// ```
+///
+/// Multiple lints can also be set to `deny` at once:
+///
+/// ```rust,compile_fail
+/// #![deny(unused_imports, unused_variables)]
+/// use std::collections::*;
+///
+/// fn main() {
+///     let mut x = 10;
+/// }
+/// ```
+///
+/// The lint checks supported by rustc can be found via `rustc -W help`,
+/// along with their default settings and are documented in [the `rustc` book].
+///
+/// [the `rustc` book]: ../rustc/lints/listing/index.html
+///
+/// For more information, see the Reference on [the `deny` attribute].
+///
+/// [the `deny` attribute]: ../reference/attributes/diagnostics.html#lint-check-attributes
+mod deny_attribute {}
+
+#[doc(attribute = "forbid")]
+//
+/// Emits an error, preventing the compilation from finishing, when a lint check has failed.
+///
+/// A lint set to `forbid` cannot be overridden by `allow` or `warn`.
+/// Attempting either will result in a compilation error. Writing `#[deny(...)]` on the same lint inside a
+/// `forbid` scope is permitted, but has no effect; the lint remains at the `forbid` level.
+///
+/// This is useful for enforcing strict policies that should not be relaxed
+/// anywhere in the codebase. Example:
+///
+/// ```rust
+/// #![forbid(unsafe_code)]
+///
+/// // This would cause a compilation error if uncommented:
+/// // #[allow(unsafe_code)] // error: cannot override `forbid`
+/// ```
+///
+/// Multiple lints can be set to `forbid` at once:
+///
+/// ```rust
+/// #![forbid(unsafe_code, unused)]
+/// ```
+///
+/// The lint checks supported by rustc can be found via `rustc -W help`,
+/// along with their default settings and are documented in [the `rustc` book].
+///
+/// [the `rustc` book]: ../rustc/lints/listing/index.html
+///
+/// For more information, see the Reference on [the `forbid` attribute].
+///
+/// [the `forbid` attribute]: ../reference/attributes/diagnostics.html#lint-check-attributes
+mod forbid_attribute {}
+
+#[doc(attribute = "deprecated")]
+//
+/// Emits a warning during compilation when an item with this attribute is used.
+/// `since` and `note` are optional fields giving more detail about why the item is deprecated.
+///
+/// * `since`: the version since when the item is deprecated.
+/// * `note`: the reason why an item is deprecated.
+///
+/// Example:
+///
+/// ```rust
+/// #[deprecated(since = "1.0.0", note = "Use bar instead")]
+/// struct Foo;
+/// struct Bar;
+/// ```
+///
+/// `deprecated` attribute helps developers transition away from old code by providing warnings when
+/// deprecated items are used. Note that during `Cargo` builds, warnings on dependencies get silenced
+/// by default, so you may not see a deprecation warning unless you build that dependency directly.
+///
+/// For more information, see the Reference on [the `deprecated` attribute].
+///
+/// [the `deprecated` attribute]: ../reference/attributes/diagnostics.html#the-deprecated-attribute
+mod deprecated_attribute {}
+
+#[doc(attribute = "warn")]
+//
+/// Emits a warning during compilation when a lint check failed.
+///
+/// Unlike `deny` or `forbid`, `warn` does not produce a hard error: the compilation continues, but
+/// the compiler emits a warning message. `warn` can be overridden by `allow`, `deny`, and `forbid`.
+///
+/// Example:
+///
+/// ```rust,compile_fail
+/// #![allow(unused)]
+///
+/// #[warn(unused)] // We override the allowed `unused` lint.
+/// fn foo() {
+///     // This lint warns by default even without #[warn(unused)] being explicitly set
+///     let x = 42; // warning: unused variable `x`
+/// }
+/// ```
+///
+///
+/// Many lints, including `unused`, are already set to `warn` by default so this attribute is
+/// mainly useful for lints that are normally `allow` by default.
+///
+/// Multiple lints can be set to `warn` at once:
+///
+/// ```rust,compile_fail
+/// #[warn(unused_mut, unused_variables)]
+/// fn main() {
+///     let mut x = 42;
+/// }
+/// ```
+///
+/// The lint checks supported by rustc can be found via `rustc -W help`,
+/// along with their default settings and are documented in [the `rustc` book].
+///
+/// [the `rustc` book]: ../rustc/lints/listing/index.html
+///
+/// For more information, see the Reference on [the `warn` attribute].
+///
+/// [the `warn` attribute]: ../reference/attributes/diagnostics.html#lint-check-attributes
+mod warn_attribute {}