Auto merge of rust-lang#79078 - petrochenkov:derattr, r=Aaron1011

expand/resolve: Turn `#[derive]` into a regular macro attribute

This PR turns `#[derive]` into a regular attribute macro declared in libcore and defined in `rustc_builtin_macros`, like it was previously done with other "active" attributes in rust-lang#62086, rust-lang#62735 and other PRs.
This PR is also a continuation of rust-lang#65252, rust-lang#69870 and other PRs linked from them, which layed the ground for converting `#[derive]` specifically.

`#[derive]` still asks `rustc_resolve` to resolve paths inside `derive(...)`, and `rustc_expand` gets those resolution results through some backdoor (which I'll try to address later), but otherwise `#[derive]` is treated as any other macro attributes, which simplifies the resolution-expansion infra pretty significantly.

The change has several observable effects on language and library.
Some of the language changes are **feature-gated** by [`feature(macro_attributes_in_derive_output)`](rust-lang#81119).

#### Library

- `derive` is now available through standard library as `{core,std}::prelude::v1::derive`.

#### Language

- `derive` now goes through name resolution, so it can now be renamed - `use derive as my_derive; #[my_derive(Debug)] struct S;`.
- `derive` now goes through name resolution, so this resolution can fail in corner cases. Crater found one such regression, where import `use foo as derive` goes into a cycle with `#[derive(Something)]`.
- **[feature-gated]** `#[derive]` is now expanded as any other attributes in left-to-right order. This allows to remove the restriction on other macro attributes following `#[derive]` (rust-lang/reference#566). The following macro attributes become a part of the derive's input (this is not a change, non-macro attributes following `#[derive]` were treated in the same way previously).
- `#[derive]` is now expanded as any other attributes in left-to-right order. This means two derive attributes `#[derive(Foo)] #[derive(Bar)]` are now expanded separately rather than together. It doesn't generally make difference, except for esoteric cases. For example `#[derive(Foo)]` can now produce an import bringing `Bar` into scope, but previously both `Foo` and `Bar` were required to be resolved before expanding any of them.
- **[feature-gated]** `#[derive()]` (with empty list in parentheses) actually becomes useful. For historical reasons `#[derive]` *fully configures* its input, eagerly evaluating `cfg` everywhere in its target, for example on fields.
Expansion infra doesn't do that for other attributes, but now when macro attributes attributes are allowed to be written after `#[derive]`, it means that derive can *fully configure* items for them.
    ```rust
	#[derive()]
	#[my_attr]
	struct S {
		#[cfg(FALSE)] // this field in removed by `#[derive()]` and not observed by `#[my_attr]`
		field: u8
	}
    ```
- `#[derive]` on some non-item targets is now prohibited. This was accidentally allowed as noop in the past, but was warned about since early 2018 (rust-lang#50092), despite that crater found a few such cases in unmaintained crates.
- Derive helper attributes used before their introduction are now reported with a deprecation lint. This change is long overdue (since macro modularization, rust-lang#52226 (comment)), but it was hard to do without fixing expansion order for derives. The deprecation is tracked by rust-lang#79202.
```rust
    #[trait_helper] // warning: derive helper attribute is used before it is introduced
    #[derive(Trait)]
    struct S {}
```

Crater analysis: rust-lang#79078 (comment)

Author: bors

Diff for: Cargo.lock

@@ -3582,6 +3582,7 @@ dependencies = [
  "rustc_errors",
  "rustc_expand",
  "rustc_feature",
+ "rustc_lexer",
  "rustc_parse",
  "rustc_parse_format",
  "rustc_session",

Diff for: compiler/rustc_builtin_macros/Cargo.toml

@@ -15,6 +15,7 @@ rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
+rustc_lexer = { path = "../rustc_lexer" }
 rustc_parse = { path = "../rustc_parse" }
 rustc_target = { path = "../rustc_target" }
 rustc_session = { path = "../rustc_session" }

Diff for: compiler/rustc_builtin_macros/src/derive.rs

@@ -0,0 +1,132 @@
+use rustc_ast::{self as ast, token, ItemKind, MetaItemKind, NestedMetaItem, StmtKind};
+use rustc_errors::{struct_span_err, Applicability};
+use rustc_expand::base::{Annotatable, ExpandResult, ExtCtxt, Indeterminate, MultiItemModifier};
+use rustc_expand::config::StripUnconfigured;
+use rustc_feature::AttributeTemplate;
+use rustc_parse::validate_attr;
+use rustc_session::Session;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
+
+crate struct Expander;
+
+impl MultiItemModifier for Expander {
+    fn expand(
+        &self,
+        ecx: &mut ExtCtxt<'_>,
+        span: Span,
+        meta_item: &ast::MetaItem,
+        item: Annotatable,
+    ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
+        let sess = ecx.sess;
+        if report_bad_target(sess, &item, span) {
+            // We don't want to pass inappropriate targets to derive macros to avoid
+            // follow up errors, all other errors below are recoverable.
+            return ExpandResult::Ready(vec![item]);
+        }
+
+        let template =
+            AttributeTemplate { list: Some("Trait1, Trait2, ..."), ..Default::default() };
+        let attr = ecx.attribute(meta_item.clone());
+        validate_attr::check_builtin_attribute(&sess.parse_sess, &attr, sym::derive, template);
+
+        let derives: Vec<_> = attr
+            .meta_item_list()
+            .unwrap_or_default()
+            .into_iter()
+            .filter_map(|nested_meta| match nested_meta {
+                NestedMetaItem::MetaItem(meta) => Some(meta),
+                NestedMetaItem::Literal(lit) => {
+                    // Reject `#[derive("Debug")]`.
+                    report_unexpected_literal(sess, &lit);
+                    None
+                }
+            })
+            .map(|meta| {
+                // Reject `#[derive(Debug = "value", Debug(abc))]`, but recover the paths.
+                report_path_args(sess, &meta);
+                meta.path
+            })
+            .collect();
+
+        // FIXME: Try to cache intermediate results to avoid collecting same paths multiple times.
+        match ecx.resolver.resolve_derives(ecx.current_expansion.id, derives, ecx.force_mode) {
+            Ok(()) => {
+                let mut visitor =
+                    StripUnconfigured { sess, features: ecx.ecfg.features, modified: false };
+                let mut item = visitor.fully_configure(item);
+                if visitor.modified {
+                    // Erase the tokens if cfg-stripping modified the item
+                    // This will cause us to synthesize fake tokens
+                    // when `nt_to_tokenstream` is called on this item.
+                    match &mut item {
+                        Annotatable::Item(item) => item,
+                        Annotatable::Stmt(stmt) => match &mut stmt.kind {
+                            StmtKind::Item(item) => item,
+                            _ => unreachable!(),
+                        },
+                        _ => unreachable!(),
+                    }
+                    .tokens = None;
+                }
+                ExpandResult::Ready(vec![item])
+            }
+            Err(Indeterminate) => ExpandResult::Retry(item),
+        }
+    }
+}
+
+fn report_bad_target(sess: &Session, item: &Annotatable, span: Span) -> bool {
+    let item_kind = match item {
+        Annotatable::Item(item) => Some(&item.kind),
+        Annotatable::Stmt(stmt) => match &stmt.kind {
+            StmtKind::Item(item) => Some(&item.kind),
+            _ => None,
+        },
+        _ => None,
+    };
+
+    let bad_target =
+        !matches!(item_kind, Some(ItemKind::Struct(..) | ItemKind::Enum(..) | ItemKind::Union(..)));
+    if bad_target {
+        struct_span_err!(
+            sess,
+            span,
+            E0774,
+            "`derive` may only be applied to structs, enums and unions",
+        )
+        .emit();
+    }
+    bad_target
+}
+
+fn report_unexpected_literal(sess: &Session, lit: &ast::Lit) {
+    let help_msg = match lit.token.kind {
+        token::Str if rustc_lexer::is_ident(&lit.token.symbol.as_str()) => {
+            format!("try using `#[derive({})]`", lit.token.symbol)
+        }
+        _ => "for example, write `#[derive(Debug)]` for `Debug`".to_string(),
+    };
+    struct_span_err!(sess, lit.span, E0777, "expected path to a trait, found literal",)
+        .help(&help_msg)
+        .emit();
+}
+
+fn report_path_args(sess: &Session, meta: &ast::MetaItem) {
+    let report_error = |title, action| {
+        let span = meta.span.with_lo(meta.path.span.hi());
+        sess.struct_span_err(span, title)
+            .span_suggestion(span, action, String::new(), Applicability::MachineApplicable)
+            .emit();
+    };
+    match meta.kind {
+        MetaItemKind::Word => {}
+        MetaItemKind::List(..) => report_error(
+            "traits in `#[derive(...)]` don't accept arguments",
+            "remove the arguments",
+        ),
+        MetaItemKind::NameValue(..) => {
+            report_error("traits in `#[derive(...)]` don't accept values", "remove the value")
+        }
+    }
+}

Diff for: compiler/rustc_builtin_macros/src/lib.rs

@@ -27,6 +27,7 @@ mod cfg_accessible;
 mod compile_error;
 mod concat;
 mod concat_idents;
+mod derive;
 mod deriving;
 mod env;
 mod format;
@@ -88,6 +89,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
     register_attr! {
         bench: test::expand_bench,
         cfg_accessible: cfg_accessible::Expander,
+        derive: derive::Expander,
         global_allocator: global_allocator::expand,
         test: test::expand_test,
         test_case: test::expand_test_case,

Diff for: compiler/rustc_expand/src/base.rs

@@ -141,7 +141,10 @@ impl Annotatable {
     }
 
     crate fn into_tokens(self, sess: &ParseSess) -> TokenStream {
-        nt_to_tokenstream(&self.into_nonterminal(), sess, CanSynthesizeMissingTokens::No)
+        // Tokens of an attribute target may be invalidated by some outer `#[derive]` performing
+        // "full configuration" (attributes following derives on the same item should be the most
+        // common case), that's why synthesizing tokens is allowed.
+        nt_to_tokenstream(&self.into_nonterminal(), sess, CanSynthesizeMissingTokens::Yes)
     }
 
     pub fn expect_item(self) -> P<ast::Item> {
@@ -234,25 +237,6 @@ impl Annotatable {
             _ => panic!("expected variant"),
         }
     }
-
-    pub fn derive_allowed(&self) -> bool {
-        match *self {
-            Annotatable::Stmt(ref stmt) => match stmt.kind {
-                ast::StmtKind::Item(ref item) => matches!(
-                    item.kind,
-                    ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..)
-                ),
-                _ => false,
-            },
-            Annotatable::Item(ref item) => match item.kind {
-                ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..) => {
-                    true
-                }
-                _ => false,
-            },
-            _ => false,
-        }
-    }
 }
 
 /// Result of an expansion that may need to be retried.
@@ -854,12 +838,6 @@ impl SyntaxExtension {
     }
 }
 
-/// Result of resolving a macro invocation.
-pub enum InvocationRes {
-    Single(Lrc<SyntaxExtension>),
-    DeriveContainer(Vec<Lrc<SyntaxExtension>>),
-}
-
 /// Error type that denotes indeterminacy.
 pub struct Indeterminate;
 
@@ -885,16 +863,29 @@ pub trait ResolverExpand {
         invoc: &Invocation,
         eager_expansion_root: ExpnId,
         force: bool,
-    ) -> Result<InvocationRes, Indeterminate>;
+    ) -> Result<Lrc<SyntaxExtension>, Indeterminate>;
 
     fn check_unused_macros(&mut self);
 
     /// Some parent node that is close enough to the given macro call.
-    fn lint_node_id(&mut self, expn_id: ExpnId) -> NodeId;
+    fn lint_node_id(&self, expn_id: ExpnId) -> NodeId;
 
     // Resolver interfaces for specific built-in macros.
     /// Does `#[derive(...)]` attribute with the given `ExpnId` have built-in `Copy` inside it?
     fn has_derive_copy(&self, expn_id: ExpnId) -> bool;
+    /// Resolve paths inside the `#[derive(...)]` attribute with the given `ExpnId`.
+    fn resolve_derives(
+        &mut self,
+        expn_id: ExpnId,
+        derives: Vec<ast::Path>,
+        force: bool,
+    ) -> Result<(), Indeterminate>;
+    /// Take resolutions for paths inside the `#[derive(...)]` attribute with the given `ExpnId`
+    /// back from resolver.
+    fn take_derive_resolutions(
+        &mut self,
+        expn_id: ExpnId,
+    ) -> Option<Vec<(Lrc<SyntaxExtension>, ast::Path)>>;
     /// Path resolution logic for `#[cfg_accessible(path)]`.
     fn cfg_accessible(&mut self, expn_id: ExpnId, path: &ast::Path) -> Result<bool, Indeterminate>;
 }