Start using #[expect] instead of #[allow] (#9696)

* Start using `#[expect]` instead of `#[allow]`

In Rust 1.81, our new MSRV, a new feature was added to Rust to use
`#[expect]` to control lint levels. This new lint annotation will
silence a lint but will itself cause a lint if it doesn't actually
silence anything. This is quite useful to ensure that annotations don't
get stale over time.

Another feature is the ability to use a `reason` directive on the
attribute with a string explaining why the attribute is there. This
string is then rendered in compiler messages if a warning or error
happens.

This commit migrates applies a few changes across the workspace:

* Some `#[allow]` are changed to `#[expect]` with a `reason`.
* Some `#[allow]` have a `reason` added if the lint conditionally fires
  (mostly related to macros).
* Some `#[allow]` are removed since the lint doesn't actually fire.
* The workspace configures `clippy::allow_attributes_without_reason = 'warn'`
  as a "ratchet" to prevent future regressions.
* Many crates are annotated to allow `allow_attributes_without_reason`
  during this transitionary period.

The end-state is that all crates should use
`#[expect(..., reason = "...")]` for any lint that unconditionally fires
but is expected. The `#[allow(..., reason = "...")]` lint should be used
for conditionally firing lints, primarily in macro-related code.
The `allow_attributes_without_reason = 'warn'` level is intended to be
permanent but the transitionary
`#[expect(clippy::allow_attributes_without_reason)]` crate annotations
to go away over time.

* Fix adapter build

prtest:full

* Fix one-core build of icache coherence

* Use `allow` for missing_docs

Work around rust-lang/rust#130021 which was fixed in Rust 1.83 and isn't
fixed for our MSRV at this time.

* More MSRV compat

Cargo.toml

@@ -193,6 +193,7 @@ manual_strip = 'warn'
 unnecessary_mut_passed = 'warn'
 unnecessary_fallible_conversions = 'warn'
 unnecessary_cast = 'warn'
+allow_attributes_without_reason = 'warn'
 
 [workspace.dependencies]
 arbitrary = { version = "1.3.1" }

benches/call.rs

@@ -1,3 +1,5 @@
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
+
 use criterion::measurement::WallTime;
 use criterion::{criterion_group, criterion_main, BenchmarkGroup, Criterion};
 use std::fmt::Debug;

cranelift/codegen/meta/src/shared/instructions.rs

@@ -1,4 +1,4 @@
-#![allow(non_snake_case)]
+#![expect(non_snake_case, reason = "DSL style here")]
 
 use crate::cdsl::instructions::{
     AllInstructions, InstructionBuilder as Inst, InstructionGroupBuilder,

cranelift/codegen/src/lib.rs

@@ -10,6 +10,7 @@
 // built for one platform we don't have to worry too much about trimming
 // everything down.
 #![cfg_attr(not(feature = "all-arch"), allow(dead_code))]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 #[allow(unused_imports)] // #[macro_use] is required for no_std
 #[macro_use]

cranelift/entity/src/lib.rs

@@ -145,29 +145,29 @@ macro_rules! entity_impl {
 
         impl $entity {
             /// Create a new instance from a `u32`.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn from_u32(x: u32) -> Self {
                 debug_assert!(x < $crate::__core::u32::MAX);
                 $entity(x)
             }
 
             /// Return the underlying index value as a `u32`.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn as_u32(self) -> u32 {
                 self.0
             }
 
             /// Return the raw bit encoding for this instance.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn as_bits(self) -> u32 {
                 self.0
             }
 
             /// Create a new instance from the raw bit encoding.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn from_bits(x: u32) -> Self {
                 $entity(x)
@@ -225,7 +225,7 @@ macro_rules! entity_impl {
 
         impl $entity {
             /// Create a new instance from a `u32`.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn from_u32(x: u32) -> Self {
                 debug_assert!(x < $crate::__core::u32::MAX);
@@ -234,7 +234,7 @@ macro_rules! entity_impl {
             }
 
             /// Return the underlying index value as a `u32`.
-            #[allow(dead_code)]
+            #[allow(dead_code, reason = "macro-generated code")]
             #[inline]
             pub fn as_u32(self) -> u32 {
                 let $arg = self;

cranelift/filetests/src/lib.rs

@@ -4,6 +4,7 @@
 //! available filetest commands.
 
 #![deny(missing_docs)]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 pub use crate::function_runner::TestFileCompiler;
 use crate::runner::TestRunner;

cranelift/frontend/src/lib.rs

@@ -157,6 +157,7 @@
 
 #![deny(missing_docs)]
 #![no_std]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 #[allow(unused_imports)] // #[macro_use] is required for no_std
 #[macro_use]

cranelift/interpreter/src/lib.rs

@@ -2,6 +2,8 @@
 //!
 //! This module is a project for interpreting Cranelift IR.
 
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
+
 pub mod address;
 pub mod environment;
 pub mod frame;

cranelift/isle/isle/src/lib.rs

@@ -1,5 +1,6 @@
 #![doc = include_str!("../README.md")]
 #![deny(missing_docs)]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 macro_rules! declare_id {
     (

cranelift/module/src/lib.rs

@@ -2,6 +2,7 @@
 
 #![deny(missing_docs)]
 #![no_std]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 #[cfg(not(feature = "std"))]
 #[macro_use]

cranelift/reader/src/lib.rs

@@ -4,6 +4,7 @@
 //! testing Cranelift, but is not essential for a JIT compiler.
 
 #![deny(missing_docs)]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 pub use crate::error::{Location, ParseError, ParseResult};
 pub use crate::isaspec::{parse_option, parse_options, IsaSpec, ParseOptionError};

cranelift/src/bugpoint.rs

@@ -987,7 +987,7 @@ impl<'a> CrashCheckContext<'a> {
         }
     }
 
-    #[cfg_attr(test, allow(unreachable_code))]
+    #[cfg_attr(test, expect(unreachable_code, reason = "test-specific code"))]
     fn check_for_crash(&mut self, func: &Function) -> CheckResult {
         self.context.clear();
 

crates/c-api/src/lib.rs

@@ -12,7 +12,7 @@
 //! but otherwise an accompanying `wasmtime.h` API is provided which is more
 //! specific to Wasmtime and has fewer gymnastics to implement.
 
-#![allow(non_snake_case, non_camel_case_types, non_upper_case_globals)]
+#![expect(non_camel_case_types, reason = "matching C style, not Rust")]
 
 pub use wasmtime;
 

crates/cli-flags/src/opt.rs

@@ -47,7 +47,7 @@ macro_rules! wasmtime_option_group {
         }
 
         #[derive(Clone, Debug,PartialEq)]
-        #[allow(non_camel_case_types)]
+        #[expect(non_camel_case_types, reason = "macro-generated code")]
         enum $option {
             $(
                 $opt($payload),

crates/component-macro/tests/codegen.rs

@@ -1,4 +1,5 @@
 #![allow(dead_code)]
+#![expect(clippy::allow_attributes_without_reason, reason = "crate not migrated")]
 
 macro_rules! gentest {
     ($id:ident $name:tt $path:tt) => {

crates/cranelift/src/debug.rs

@@ -3,7 +3,11 @@
 // FIXME: this whole crate opts-in to these two noisier-than-default lints, but
 // this module has lots of hits on this warning which aren't the easiest to
 // resolve. Ideally all warnings would be resolved here though.
-#![allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
+#![expect(
+    clippy::cast_possible_truncation,
+    clippy::cast_sign_loss,
+    reason = "haven't had a chance to fix these yet"
+)]
 
 use crate::CompiledFunctionMetadata;
 use core::fmt;

crates/cranelift/src/debug/transform/expression.rs

@@ -1,5 +1,3 @@
-#![allow(trivial_numeric_casts)]
-
 use super::address_transform::AddressTransform;
 use crate::debug::ModuleMemoryOffset;
 use crate::translate::get_vmctx_value_label;
@@ -796,6 +794,7 @@ impl std::fmt::Debug for JumpTargetMarker {
 }
 
 #[cfg(test)]
+#[expect(trivial_numeric_casts, reason = "macro-generated code")]
 mod tests {
     use super::{
         compile_expression, AddressTransform, CompiledExpression, CompiledExpressionPart,

crates/cranelift/src/debug/transform/unit.rs

@@ -120,7 +120,7 @@ fn replace_pointer_type(
     macro_rules! add_tag {
         ($parent_id:ident, $tag:expr => $die:ident as $die_id:ident { $($a:path = $v:expr),* }) => {
             let $die_id = comp_unit.add($parent_id, $tag);
-            #[allow(unused_variables)]
+            #[allow(unused_variables, reason = "sometimes not used below")]
             let $die = comp_unit.get_mut($die_id);
             $( $die.set($a, $v); )*
         };

crates/cranelift/src/debug/write_debuginfo.rs

@@ -8,14 +8,12 @@ use cranelift_codegen::isa::{
 use gimli::write::{Address, Dwarf, EndianVec, FrameTable, Result, Sections, Writer};
 use gimli::{RunTimeEndian, SectionId};
 
-#[allow(missing_docs)]
 pub struct DwarfSection {
     pub name: &'static str,
     pub body: Vec<u8>,
     pub relocs: Vec<DwarfSectionReloc>,
 }
 
-#[allow(missing_docs)]
 #[derive(Clone)]
 pub struct DwarfSectionReloc {
     pub target: DwarfSectionRelocTarget,
@@ -24,7 +22,6 @@ pub struct DwarfSectionReloc {
     pub size: u8,
 }
 
-#[allow(missing_docs)]
 #[derive(Clone)]
 pub enum DwarfSectionRelocTarget {
     Func(usize),

crates/cranelift/src/obj.rs

@@ -287,7 +287,7 @@ struct UnwindInfoBuilder<'a> {
 // platforms. Note that all of these specifiers here are relative to a "base
 // address" which we define as the base of where the text section is eventually
 // loaded.
-#[allow(non_camel_case_types)]
+#[expect(non_camel_case_types, reason = "matching Windows style, not Rust")]
 struct RUNTIME_FUNCTION {
     begin: u32,
     end: u32,

crates/environ/src/builtin.rs

@@ -274,7 +274,7 @@ macro_rules! declare_indexes {
          )*
     ) => {
         $( #[$this_attr] )*
-        #[allow(missing_docs)]
+        #[allow(missing_docs, reason = "macro-generated")]
         pub const fn $this_name() -> Self {
             Self($index)
         }

crates/environ/src/compile/module_environ.rs

@@ -118,7 +118,7 @@ pub struct FunctionBodyData<'a> {
 }
 
 #[derive(Debug, Default)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct DebugInfoData<'a> {
     pub dwarf: Dwarf<'a>,
     pub name_section: NameSection<'a>,
@@ -131,21 +131,21 @@ pub struct DebugInfoData<'a> {
     pub debug_tu_index: gimli::DebugTuIndex<Reader<'a>>,
 }
 
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing")]
 pub type Dwarf<'input> = gimli::Dwarf<Reader<'input>>;
 
 type Reader<'input> = gimli::EndianSlice<'input, gimli::LittleEndian>;
 
 #[derive(Debug, Default)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct NameSection<'a> {
     pub module_name: Option<&'a str>,
     pub func_names: HashMap<FuncIndex, &'a str>,
     pub locals_names: HashMap<FuncIndex, HashMap<u32, &'a str>>,
 }
 
 #[derive(Debug, Default)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct WasmFileInfo {
     pub path: Option<PathBuf>,
     pub code_section_offset: u64,
@@ -154,7 +154,7 @@ pub struct WasmFileInfo {
 }
 
 #[derive(Debug)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct FunctionMetadata {
     pub params: Box<[WasmValType]>,
     pub locals: Box<[(u32, WasmValType)]>,

crates/environ/src/component/dfg.rs

@@ -37,8 +37,8 @@ use std::hash::Hash;
 use std::ops::Index;
 use wasmparser::component_types::ComponentCoreModuleTypeId;
 
+/// High-level representation of a component as a "data-flow graph".
 #[derive(Default)]
-#[allow(missing_docs)]
 pub struct ComponentDfg {
     /// Same as `Component::import_types`
     pub import_types: PrimaryMap<ImportIndex, (String, TypeDef)>,
@@ -153,7 +153,7 @@ pub enum SideEffect {
 macro_rules! id {
     ($(pub struct $name:ident(u32);)*) => ($(
         #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]
-        #[allow(missing_docs)]
+        #[allow(missing_docs, reason = "tedious to document")]
         pub struct $name(u32);
         cranelift_entity::entity_impl!($name);
     )*)
@@ -169,7 +169,7 @@ id! {
 }
 
 /// Same as `info::InstantiateModule`
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "tedious to document variants")]
 pub enum Instance {
     Static(StaticModuleIndex, Box<[CoreDef]>),
     Import(
@@ -179,7 +179,7 @@ pub enum Instance {
 }
 
 /// Same as `info::Export`
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "tedious to document variants")]
 pub enum Export {
     LiftedFunction {
         ty: TypeFuncIndex,
@@ -203,7 +203,7 @@ pub enum Export {
 
 /// Same as `info::CoreDef`, except has an extra `Adapter` variant.
 #[derive(Debug, Clone, Hash, Eq, PartialEq)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "tedious to document variants")]
 pub enum CoreDef {
     Export(CoreExport<EntityIndex>),
     InstanceFlags(RuntimeComponentInstanceIndex),
@@ -230,14 +230,14 @@ where
 
 /// Same as `info::CoreExport`
 #[derive(Debug, Clone, Hash, Eq, PartialEq)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct CoreExport<T> {
     pub instance: InstanceId,
     pub item: ExportItem<T>,
 }
 
 impl<T> CoreExport<T> {
-    #[allow(missing_docs)]
+    #[allow(missing_docs, reason = "self-describing function")]
     pub fn map_index<U>(self, f: impl FnOnce(T) -> U) -> CoreExport<U> {
         CoreExport {
             instance: self.instance,
@@ -251,7 +251,7 @@ impl<T> CoreExport<T> {
 
 /// Same as `info::Trampoline`
 #[derive(Clone, PartialEq, Eq, Hash)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub enum Trampoline {
     LowerImport {
         import: RuntimeImportIndex,
@@ -277,7 +277,7 @@ pub enum Trampoline {
 
 /// Same as `info::CanonicalOptions`
 #[derive(Clone, Hash, Eq, PartialEq)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct CanonicalOptions {
     pub instance: RuntimeComponentInstanceIndex,
     pub string_encoding: StringEncoding,
@@ -287,7 +287,7 @@ pub struct CanonicalOptions {
 }
 
 /// Same as `info::Resource`
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing fields")]
 pub struct Resource {
     pub rep: WasmValType,
     pub dtor: Option<CoreDef>,

crates/environ/src/component/info.rs

@@ -457,7 +457,7 @@ pub struct CanonicalOptions {
 // `extern "C" fn()` function argument which is called from cranelift-compiled
 // code so we must know the representation of this.
 #[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq, Hash)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing variants")]
 #[repr(u8)]
 pub enum StringEncoding {
     Utf8,
@@ -469,7 +469,7 @@ pub enum StringEncoding {
 ///
 /// Note that each transcoding operation may have a unique signature depending
 /// on the precise operation.
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing variants")]
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]
 pub enum Transcode {
     Copy(FixedEncoding),
@@ -525,7 +525,7 @@ impl Transcode {
 }
 
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq, Serialize, Deserialize)]
-#[allow(missing_docs)]
+#[allow(missing_docs, reason = "self-describing variants")]
 pub enum FixedEncoding {
     Utf8,
     Utf16,