Add assert_hierarchy lint

[mqqz]

This PR was split out from #13

Add a new assert_hierarchy lint that suggests to use stronger build-time assertion forms when possible.

The lint implements the assertion ordering:

• static_assert! > const_assert! > build_assert!

and warns when a weaker assertion can be replaced with a stronger one.

e.g. cases where

• build_assert! can be replaced with static_assert!
• build_assert! can be replaced with const_assert!
• const_assert! can be replaced with static_assert!

Implementation Notes

This change adds a dedicated late HIR lint in src/assert_hierarchy.rs.

The lint:

• recognizes build_assert! and const_assert! via diagnostic-item support and macro backtraces
• recovers both the assertion invocation span and the asserted condition span
• classifies conditions into Build, Const or Static

The condition classifier handles:

• literals, const items, assoc consts, and const params
• local expression structure such as blocks, lets, if, match, tuples, arrays, and structs
• const fn and const-usable method calls
• generic-argument dependence via typeck substitutions

This lint does not use any of the mono-graph, interprocedural summary and propagation machinery from the build_assert_not_inlined work in #13. It is only local and source-oriented; it inspects the asserted condition itself, along with local expression structure, resolution, typeck information, const-function legality, and generic dependence.

Other Changes/Refactors

• Add diagnostic-item support for assert macros in src/diagnostic_items/out_of_band.rs.
• Extract out ClosureDiag from infalliable_allocation.rs to be reused for this lint (reverted in favour of using a derive diagnostic).
• Allow klint:assert_hierarchy in build_error.rs, so the output isn't polluted by this lint.

Tests

tests/ui/assert_hierarchy.rs

UI test coverage includes:

• literals
• const generics
• wrapper macros
• macro-generated assertions
• local const fn helpers
• comments around macro commas
• outer local bindings
• local bindings inside the asserted condition
• const_assert! -> static_assert!

Docs

Add documentation in doc/assert_hierarchy.md and the corresponding entry in README "Implemented Lints" section.

---

Lint output on Linux kernel tree (Rust-for-Linux/linux@3a2486c)

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/dma.rs:404:9
    |
404 | /         build_assert!(
405 | |             core::mem::size_of::<T>() > 0,
406 | |             "It doesn't make sense for the allocated type to be a ZST"
407 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/dma.rs:405:13
    |
405 |             core::mem::size_of::<T>() > 0,
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: `-D klint::assert-hierarchy` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(klint::assert_hierarchy)]`

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/i2c.rs:113:9
    |
113 | /         build_assert!(
114 | |             T::ACPI_ID_TABLE.is_some() || T::OF_ID_TABLE.is_some() || T::I2C_ID_TABLE.is_some(),
115 | |             "At least one of ACPI/OF/Legacy tables must be present when registering an i2c driver"
116 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/i2c.rs:114:13
    |
114 |             T::ACPI_ID_TABLE.is_some() || T::OF_ID_TABLE.is_some() || T::I2C_ID_TABLE.is_some(),
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/list/arc.rs:255:9
    |
255 |         build_assert!(ID != ID2);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/list/arc.rs:255:23
    |
255 |         build_assert!(ID != ID2);
    |                       ^^^^^^^^^

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/sync/locked_by.rs:103:9
    |
103 | /         build_assert!(
104 | |             size_of::<Lock<U, B>>() > 0,
105 | |             "The lock type cannot be a ZST because it may be impossible to distinguish instances"
106 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/sync/locked_by.rs:104:13
    |
104 |             size_of::<Lock<U, B>>() > 0,
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/sync/locked_by.rs:129:9
    |
129 | /         build_assert!(
130 | |             size_of::<U>() > 0,
131 | |             "`U` cannot be a ZST because `owner` wouldn't be unique"
132 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/sync/locked_by.rs:130:13
    |
130 |             size_of::<U>() > 0,
    |             ^^^^^^^^^^^^^^^^^^

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/sync/locked_by.rs:158:9
    |
158 | /         build_assert!(
159 | |             size_of::<U>() > 0,
160 | |             "`U` cannot be a ZST because `owner` wouldn't be unique"
161 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/sync/locked_by.rs:159:13
    |
159 |             size_of::<U>() > 0,
    |             ^^^^^^^^^^^^^^^^^^

error: this `build_assert!` can be written as `const_assert!` instead
   --> rust/kernel/xarray.rs:234:9
    |
234 | /         build_assert!(
235 | |             T::FOREIGN_ALIGN >= 4,
236 | |             "pointers stored in XArray must be 4-byte aligned"
237 | |         );
    | |_________^
    |
note: this asserted condition is valid in a generic-aware const context, so `build_assert!` is unnecessary here
   --> rust/kernel/xarray.rs:235:13
    |
235 |             T::FOREIGN_ALIGN >= 4,
    |             ^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 7 previous errors

(same cases as #13 btw)