Honor #[mutants::skip] on const and static items

[sandersaares]

Fixes #508.

Problem

#[mutants::skip] (and #[cfg_attr(..., mutants::skip)]) placed on a const or static item was silently ignored. The DiscoveryVisitor in src/visit.rs had no visit_item_const / visit_item_static / visit_impl_item_const / visit_trait_item_const, so syn's default visitor walked straight into the initializer expression and produced the usual operator mutants without consulting the item's attributes.

Reproduced before the fix:

src/lib.rs:1:31: replace | with &     # plain const
src/lib.rs:1:31: replace | with ^
src/lib.rs:4:39: replace | with &     # const with #[mutants::skip] - still mutated
src/lib.rs:4:39: replace | with ^

Fix

Added four visitor methods to DiscoveryVisitor that bail out via attrs_excluded(&i.attrs) when the item carries a skip attribute, and otherwise delegate to the default syn::visit::visit_* recursion. Behavior for unskipped consts/statics is unchanged.

The attribute is now honoured on:

• Top-level const FOO: T = ...;
• Top-level static FOO: T = ...;
• Associated const inside impl blocks
• Associated const defaults inside trait blocks

Both #[mutants::skip] and #[cfg_attr(..., mutants::skip)] work, via the existing attrs_excluded helper.

Tests

Five new tests under src/visit/test/ following the existing skip_attr_impl.rs pattern:

• skip_attr_item_const.rs — covers #[mutants::skip] and #[cfg_attr(test, mutants::skip)] on a top-level const
• skip_attr_item_static.rs — covers static
• skip_attr_impl_item_const.rs — covers associated const in impl
• skip_attr_trait_item_const.rs — covers associated const default in trait

Each test puts a distinct operator on the skipped item (^) and on the unskipped sibling (|), then asserts on Mutant::original_text() rather than line numbers, so reformatting the indoc block cannot turn the assertions into silent passes.

Documentation

• book/src/attrs.md: added a Scope bullet for const / static items.
• NEWS.md: added an Unreleased entry describing the fix.

Out of scope

Generating fresh "replace value" mutants for const initializers (per @kpreid's follow-up on the issue) would need a new mutation genre and is a separate, larger feature.

Validation

• cargo fmt --check passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo nextest run --all-features --no-fail-fast: 401/402 pass (the one failure, symlink_in_source_tree_is_copied, is a pre-existing Windows symlink-privilege issue unrelated to this change)
• End-to-end repro confirms each item kind now honors both #[mutants::skip] and #[cfg_attr(..., mutants::skip)] while unskipped items continue to produce mutants as before.

[Copilot]

Pull request overview

This pull request fixes an attribute-handling gap in the mutation discovery visitor so that #[mutants::skip] (including #[cfg_attr(..., mutants::skip)]) is honored on const and static items, preventing mutants from being generated inside their initializer expressions.

Changes:

• Added Visit overrides in DiscoveryVisitor for ItemConst, ItemStatic, ImplItemConst, and TraitItemConst to early-return when attrs_excluded(&i.attrs) matches.
• Added targeted tests ensuring skipped const/static initializers don’t produce operator mutants while unskipped siblings still do.
• Updated documentation and changelog to reflect the newly supported skip scope.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/visit.rs	Adds visitor hooks for const/static items (including associated consts) to honor #[mutants::skip] before descending into initializer/default expressions.
src/visit/test/skip_attr_item_const.rs	Tests skipping top-level const initializers, including cfg_attr(..., mutants::skip).
src/visit/test/skip_attr_item_static.rs	Tests skipping top-level static initializers.
src/visit/test/skip_attr_impl_item_const.rs	Tests skipping associated const initializers inside impl blocks.
src/visit/test/skip_attr_trait_item_const.rs	Tests skipping associated const defaults inside trait blocks.
book/src/attrs.md	Documents that skip applies to const/static initializer expressions (including associated consts).
NEWS.md	Adds an Unreleased changelog entry describing the fix and linking to #508.

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