diff --git a/src/items/use-declarations.md b/src/items/use-declarations.md index 94638b6a1..284f2be3c 100644 --- a/src/items/use-declarations.md +++ b/src/items/use-declarations.md @@ -116,6 +116,7 @@ They may create bindings for: * [Built-in types] * [Attributes] * [Derive macros] +* [Macros by example] r[items.use.path.disallowed] They cannot import [associated items], [generic parameters], [local variables], paths with [`Self`], or [tool attributes]. More restrictions are described below. @@ -389,58 +390,9 @@ r[items.use.restrictions.variant] use TypeAlias::MyVariant; //~ ERROR ``` -r[items.use.ambiguities] -## Ambiguities - -> [!NOTE] -> This section is incomplete. - -r[items.use.ambiguities.intro] -Some situations are an error when there is an ambiguity as to which name a `use` declaration refers. This happens when there are two name candidates that do not resolve to the same entity. - -r[items.use.ambiguities.glob] -Glob imports are allowed to import conflicting names in the same namespace as long as the name is not used. -For example: - -```rust -mod foo { - pub struct Qux; -} - -mod bar { - pub struct Qux; -} - -use foo::*; -use bar::*; //~ OK, no name conflict. - -fn main() { - // This would be an error, due to the ambiguity. - //let x = Qux; -} -``` - -Multiple glob imports are allowed to import the same name, and that name is allowed to be used, if the imports are of the same item (following re-exports). The visibility of the name is the maximum visibility of the imports. For example: - -```rust -mod foo { - pub struct Qux; -} - -mod bar { - pub use super::foo::Qux; -} - -// These both import the same `Qux`. The visibility of `Qux` -// is `pub` because that is the maximum visibility between -// these two `use` declarations. -pub use bar::*; -use foo::*; - -fn main() { - let _: Qux = Qux; -} -``` +TODO mention ambiguities and link to name-res. Moved to name-res because +ambiguities are fundamentally a product of the place of use, not the use +declaration. [`extern crate`]: extern-crates.md [`macro_rules`]: ../macros-by-example.md diff --git a/src/macros-by-example.md b/src/macros-by-example.md index 2fa104ed4..f582fee93 100644 --- a/src/macros-by-example.md +++ b/src/macros-by-example.md @@ -326,6 +326,28 @@ fn foo() { // m!(); // Error: m is not in scope. ``` +* textual scope name bindings for macros may shadow path-based scope bindings + to macros + +```rust +macro_rules! m { + () => { + println!("m"); + }; +} + +#[macro_export] +macro_rules! m2 { + () => { + println!("m2"); + }; +} + +use crate::m2 as m; + +m!(); // prints "m\n" +``` + r[macro.decl.scope.macro_use] ### The `macro_use` attribute @@ -480,6 +502,25 @@ By default, macros only have [textual scope][macro.decl.scope.textual] and canno > # fn main() {} > ``` +r[macro.decl.scope.path.reexport] + +* macros can be re-exported to give them path-based scope from a module other than the crate root. + * there's some visibility stuff here that may already be mentioned + elsewhere. I'm pretty sure that w/o a #[macro_export] the macro being + re-exported is implicitly pub(crate) and with one it is implicitly pub. + The later is mentioned below, don't remember where I saw the former. + +``` +mac::m!(); // OK: Path-based lookup finds m in the mac module. + +mod mac { + macro_rules! m { + () => {}; + } + pub(crate) use m; +} +``` + r[macro.decl.scope.macro_export.export] The `macro_export` attribute causes a macro to be exported from the crate root so that it can be referred to in other crates by path. diff --git a/src/names/name-resolution.md b/src/names/name-resolution.md index da60d7eb3..47e570ab7 100644 --- a/src/names/name-resolution.md +++ b/src/names/name-resolution.md @@ -1,4 +1,423 @@ +r[names.resolution] # Name resolution +r[names.resolution.intro] + +_Name resolution_ is the process of tying paths and other identifiers to the +declarations of those entities. Names are segregated into different +[namespaces], allowing entities in different namespaces to share the same name +without conflict. Each name is valid within a [scope], or a region of source +text where that name may be referenced. Access to certain names may be +restricted based on their [visibility]. + +Name resolution is split into three stages throughout the compilation process. +The first stage, Expansion-time resolution, resolves all [use declarations] and +[macro invocations]. The second stage, Primary resolution, resolves all names +that have not yet been resolved that do not depend on type information to +resolve. The last stage, Type-relative resolution, resolves the remaining names +once type information is available. + +> Note +> +> * Expansion-time resolution is also known as "Early Resolution" +> * Primary resolution is also known as "Late Resolution" + +r[names.resolution.expansion] +## Expansion-time name resolution + +r[names.resolution.expansion.intro] + +Expansion-time name resolution is the stage of name resolution necessary to +complete macro expansion and fully generate a crate's AST. This stage requires +the resolution of macro invocations and use declarations. Resolving use +declarations is required to resolve [path-based scope] macro invocations. +Resolving macro invocations is required in order to expand them. + +The expansion process is iterative, alternately resolving imports, resolving +and expanding macro invocations, then repeating until there are no further +macros invocations to resolve. Once this process is completed all the imports +are resolved again to ensure that the macro expansion process did not introduce +any new ambiguious imports. + +TODO: do we want to talk about this? feels like an implementation detail but +also really helps to understand certain kinds of ambiguity errors that users +can run into. + +> Note +> +> This causes so called time traveling ambiguities, such as when a glob import introduces an item that is ambiguous with its own base path. +> +```rust +macro_rules! m { + () => { mod bar {} } +} + +mod bar { + pub(crate) use m; +} + +fn f() { + // * initially speculatively resolve bar to the module in the crate root + // * expansion of m introduces a second bar module inside the body of f + // * expansion-time resolution finalizes resolutions by re-resolving all + // imports and macro invocations, sees the introduced ambiguity + // and reports it as an error + bar::m!(); // ERROR `bar` is ambiguous +} +``` + +TODO I would like to be able to link to a path-based scope section that + discusses the various kinds of macros that can be invoked via path-based scope. + Right now the section I know of off of the top of my head lives in the macros + by example chapter. + +r[names.resolution.expansion.imports] + +All use declarations are fully resolved during this stage of resolution. +Type-relative paths cannot be resolved at this stage of compilation and will +produce an error. + +* `Type::assoc_item`, `::assoc_item`, `::Variant` and `EnumTyAlias::Variant` are resolved during type checking + * `Trait::assoc_item`, `::assoc_item` and `Enum::Variant` are resolved during late resolution + +```rust +mod my_mod { + pub const Const: () = (); + + pub enum MyEnum { + MyVariant + } + + impl MyEnum { + pub const Const: () = (); + } + + pub type TypeAlias = MyEnum; +} + +fn foo() { + use my_mod::MyEnum; // OK + use my_mod::MyEnum::MyVariant; // OK + use my_mod::TypeAlias; // OK + use my_mod::TypeAlias::MyVariant; // Doesn't work + use my_mod::MyEnum::Const; // Doesn't work + use my_mod::Const; // OK + let _ = my_mod::TypeAlias::MyVariant; // OK + let _ = my_mod::MyEnum::Const; // OK +} +``` + +r[names.resolution.expansion.imports.shadowing] + +The following is a list of situations where shadowing of use declarations is permitted: + +* [use glob shadowing] +* [macro textual scope shadowing] + +r[names.resolution.expansion.imports.errors] +r[names.resolution.expansion.imports.errors.ambiguity] + +TODO shadowing and ambiguity may or may not represent the same section or one may be a subsection of the other + +The following is a list of situations where shadowing of use declarations is +_NOT_ permitted, otherwise known as ambiguity errors: + +* Builtin Attributes +* Derive Helpers +* Textual Vs Path-based Scope +* Glob vs Outer +* Glob vs Glob +* ~~Glob vs Expanded~~ pretty certain we don't want to mention this one +* More Expanded vs Outer + +r[names.resolution.expansion.imports.errors.ambiguity] +## Ambiguities + +r[items.use.ambiguities.intro] +Some situations are an error when there is an ambiguity as to which name a +`use` declaration refers. This happens when there are two name candidates that +do not resolve to the same entity where neither import is +[permitted](names.resolution.expansion.imports.shadowing) to shadow the other. + +r[names.resolution.early.imports.errors.ambiguity.globvsglob] +* it is an error to name an item through ambiguous use declarations + * two globs imports which both have an item matching that name where the items are different + * this is not an error even if is a third non glob binding resolution to an item with the same name +* it is not an error to have two glob imports which include items which would be ambiguous so long as you do not name one of those items through the ambiguous glob imports + * Should this live alongside use decls item page or in the name resolution page? + +r[items.use.ambiguities.glob] +Glob imports are allowed to import conflicting names in the same namespace as +long as the name is not used. Names may not be resolved through ambiguous glob +statements. Conflicting names from ambiguous glob statements may still be +shadowed and used without producing an error. + +For example: + +```rust +mod foo { + pub struct Qux; +} + +mod bar { + pub struct Qux; +} + +use foo::*; +use bar::*; //~ OK, no name conflict. + +fn ambiguous_use() { + // This would be an error, due to the ambiguity. + //let x = Qux; +} + +fn ambiguous_shadow() { + // This is permitted, since resolution is not through the ambiguous globs + struct Qux; + let x = Qux; +} +``` + + +Multiple glob imports are allowed to import the same name, and that name is allowed to be used, if the imports are of the same item (following re-exports). The visibility of the name is the maximum visibility of the imports. For example: + +```rust +mod foo { + pub struct Qux; +} + +mod bar { + pub use super::foo::Qux; +} + +// These both import the same `Qux`. The visibility of `Qux` +// is `pub` because that is the maximum visibility between +// these two `use` declarations. +pub use bar::*; +use foo::*; + +fn main() { + let _: Qux = Qux; +} +``` + +r[names.resolution.early.imports.errors.ambiguity.builtin-attr] +It is an error to use a user defined attribute or derive macro with the same +name as a builtin attribute (e.g. inline) +* I think we may special case this one and allow certain kinds of ambiguities + where the builtin-attr is shadowed by a user attribute (not sure if this + actually exists or is just proposed, TODO investigate) + + +```rust,ignore +// myinline/src/lib.rs +use proc_macro::TokenStream; + +#[proc_macro_attribute] +pub fn inline(_attr: TokenStream, item: TokenStream) -> TokenStream { + item +} +``` + + +```rust,ignore +// src/lib.rs +use myinline::inline; +use myinline::inline as myinline; + +#[myinline::inline] +pub fn foo() {} + +#[crate::inline] +pub fn bar() {} + +#[myinline] +pub fn baz() {} + +#[inline] // ERROR `inline` is ambiguous +pub fn qux() {} +``` + +r[names.resolution.early.imports.errors.ambiguity.derivehelper] +* derive helpers used before their associated derive may not shadow other attributes or other derive helpers that are otherwise in scope after their derive + * TODO example? This ones harder to do concisely afaik + +Helper attributes may not be used before the macro that introduces them. + +* What happens if two macros introduce the same helper, will the second one not + be able to see the attribute of the first anymore, assuming their order is + "firstmacro" "helper" "secondmacro"? + > [!NOTE] -> This is a placeholder for future expansion. +> rustc currently allows derive helpers to be used before their attribute macro +> introduces them into scope so long as they do not shadow any other attributes +> or derive helpers that are otherwise correctly in scope. This behavior +> deprecated and slated for removal. +> +> ```rust,ignore +> #[helper] // deprecated, hard error in the future +> #[derive(WithHelperAttr)] +> struct Struct { +> field: (), +> } +> ``` +> +> For more details, see [Rust issue #79202](https://github.com/rust-lang/rust/issues/79202). + + +r[names.resolution.early.imports.errors.ambiguity.pathvstextualmacro] +Path-based scope bindings for macros may not shadow textual scope bindings to macros. + +```rust +#[macro_export] +macro_rules! m2 { + () => {} +} +macro_rules! m { + () => {} +} +pub fn foo() { + m!(); // ERROR `m` is ambiguous + use crate::m2 as m; // in scope for entire function body +} +``` + +r[names.resolution.early.imports.errors.ambiguity.globvsouter] +it is an error to shadow an outer name binding with a glob import. + +```rust +mod bar { + pub mod foo { + pub struct Name; + } +} + +mod baz { + pub mod foo { + pub struct Name; + } +} + +use baz::foo; + +pub fn qux() { + use bar::*; + use foo::Name; // `foo` is ambiguous +} +``` + +```rust +mod bar { + macro_rules! name { + () => {} + } + pub(crate) use name; +} + +mod baz { + macro_rules! name { + () => {} + } + pub(crate) use name; +} + +use baz::name; + +pub fn foo() { + use bar::*; + name!(); // ERROR `name` is ambiguous +} +``` + +> **NOTE** These ambiguity errors are specific to imports, even though they are +> only observed when those imports are used, having multiple candidates +> available for a given name during later stages of resolution is not +> considered an error, so long as none of the imports themselves are ambiguous, +> there will always be a single unambiguous closest resolution during later +> stages. +> +> ```rust +> mod bar { +> pub struct Name; +> } +> +> mod baz { +> pub struct Name; +> } +> +> use baz::Name; +> +> pub fn foo() { +> use bar::*; +> Name; // resolves to bar::Name +> } +> ``` + +r[macro.decl.scope.textual.ambiguity.moreexpandedvsouter] +* it is an error for name bindings from macro expansions to shadow name bindings from outside of those expansions + +```rust +macro_rules! name { + () => {} +} + +macro_rules! define_name { + () => { + macro_rules! name { + () => {} + } + } +} + +fn foo() { + define_name!(); + name!(); // ERROR `name` is ambiguous +} +``` + + +r[names.resolution.early.imports.errors.ambiguity.globvsexpanded] +* Grey Area + +r[names.resolution.expansion.macros] + +* .visitation-order + * derive helpers + * not visited when resolving derive macros in the parent scope (starting scope) + * derive helpers compat + * always visited + * macro rules bindings (textual scope macros) + * always visited + * modules (path-based scope macros) + * always visited + * macrouseprelude + * not visited in 2018 and later when `#[no_implicit_prelude]` is present + * stdlibprelude + * always visited for macro resolutions + * is it? what about no-std + no-core? + * builtinattrs + * always visited +* .subnamespaces + * macros are split into two subnamespaces, one for bang macros, and the other for attributes and derives. Resolution candidates from the incorrect subnamespace are ignored + * https://doc.rust-lang.org/nightly/reference/names/namespaces.html#r-names.namespaces.sub-namespaces + +r[names.resolution.expansion.macros.errors.reserved-names] + +the names cfg and cfg_attr are reserved in the macro attribute sub-namespace + +* https://doc.rust-lang.org/nightly/reference/names/namespaces.html#r-names.namespaces.sub-namespaces + + +r[names.resolution.late] + +r[names.resolution.type-dependent] + +[use glob shadowing]: ../items/use-declarations.md#r-items.use.glob.shadowing +[Macros]: ../macros.md +[use declarations]: ../items/use-declarations.md +[macro textual scope shadowing]: ../macros-by-example.md#r-macro.decl.scope.textual.shadow +[`let` bindings]: ../statements.md#let-statements +[item definitions]: ../items.md +[namespaces]: ../names/namespaces.md +[scope]: ../names/scopes.md +[visibility]: ../visibility-and-privacy.md diff --git a/src/names/namespaces.md b/src/names/namespaces.md index b3560d2c1..8050cfd1f 100644 --- a/src/names/namespaces.md +++ b/src/names/namespaces.md @@ -119,6 +119,7 @@ For example, the [`cfg` attribute] and the [`cfg` macro] are two different entit r[names.namespaces.sub-namespaces.use-shadow] It is still an error for a [`use` import] to shadow another macro, regardless of their sub-namespaces. +* TODO revisit [`cfg` attribute]: ../conditional-compilation.md#the-cfg-attribute [`cfg` macro]: ../conditional-compilation.md#the-cfg-macro