Add ClosestAstOrMacro to allow detecting macro expansions

Following PR rust-lang#72389, we create many more spans with
`ExpnKind::Desugaring`. This exposed a latent bug in Clippy - only the
top-most `ExpnData` is considered when checking if code is the result of
a macro expansion. If code emitted by a macro expansion gets annotated
with an `ExpnKind::Desugaring` (e.g. an operator or a for loop), Clippy
will incorrectly act as though this code is not the result of a macro
expansion.

This PR introduces the `ClosestAstOrMacro` enum, which allows linting
code to quickly determine if a given `Span` is the result of a macro
expansion. For any `ExpnId`, we keep track of closest `ExpnKind::Macro`
or `ExpnKind::AstPass` in the `call_site` chain. This is determined when
the `ExpnData` is set for an `ExpnId`, which allows us to avoid walking
the entire chain in Clippy.

src/librustc_middle/lint.rs

@@ -7,8 +7,8 @@ use rustc_errors::{DiagnosticBuilder, DiagnosticId};
 use rustc_hir::HirId;
 use rustc_session::lint::{builtin, Level, Lint, LintId};
 use rustc_session::{DiagnosticMessageId, Session};
-use rustc_span::hygiene::MacroKind;
-use rustc_span::source_map::{DesugaringKind, ExpnKind, MultiSpan};
+use rustc_span::hygiene::{ClosestAstOrMacro, MacroKind};
+use rustc_span::source_map::{ExpnKind, MultiSpan};
 use rustc_span::{Span, Symbol};
 
 /// How a lint level was set.
@@ -337,16 +337,19 @@ pub fn struct_lint_level<'s, 'd>(
 /// This is used to test whether a lint should not even begin to figure out whether it should
 /// be reported on the current node.
 pub fn in_external_macro(sess: &Session, span: Span) -> bool {
-    let expn_data = span.ctxt().outer_expn_data();
-    match expn_data.kind {
-        ExpnKind::Root
-        | ExpnKind::Desugaring(DesugaringKind::ForLoop(_))
-        | ExpnKind::Desugaring(DesugaringKind::Operator) => false,
-        ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
-        ExpnKind::Macro(MacroKind::Bang, _) => {
-            // Dummy span for the `def_site` means it's an external macro.
-            expn_data.def_site.is_dummy() || sess.source_map().is_imported(expn_data.def_site)
+    match span.ctxt().outer_expn().closest_ast_or_macro() {
+        ClosestAstOrMacro::Expn(expn_id) => {
+            let data = expn_id.expn_data();
+            match data.kind {
+                ExpnKind::Macro(MacroKind::Bang, _) => {
+                    // Dummy span for the `def_site` means it's an external macro.
+                    data.def_site.is_dummy() || sess.source_map().is_imported(data.def_site)
+                }
+                ExpnKind::Macro(_, _) => true, // definitely a plugin
+                ExpnKind::AstPass(_) => true,  // well, it's "External"
+                _ => unreachable!("unexpected ExpnData {:?}", data),
+            }
         }
-        ExpnKind::Macro(..) => true, // definitely a plugin
+        ClosestAstOrMacro::None => false,
     }
 }

src/librustc_span/hygiene.rs

@@ -104,9 +104,19 @@ impl ExpnId {
         HygieneData::with(|data| data.expn_data(self).clone())
     }
 
+    #[inline]
+    pub fn closest_ast_or_macro(self) -> ClosestAstOrMacro {
+        HygieneData::with(|data| data.closest_ast_or_macro(self))
+    }
+
     #[inline]
     pub fn set_expn_data(self, expn_data: ExpnData) {
         HygieneData::with(|data| {
+            let closest = data.determine_closest_ast_or_macro(self, &expn_data);
+            let old_closest = &mut data.closest_ast_or_macro[self.0 as usize];
+            assert!(old_closest.is_none(), "closest ast/macro data reset for an expansion ID");
+            *old_closest = Some(closest);
+
             let old_expn_data = &mut data.expn_data[self.0 as usize];
             assert!(old_expn_data.is_none(), "expansion data is reset for an expansion ID");
             *old_expn_data = Some(expn_data);
@@ -149,6 +159,10 @@ crate struct HygieneData {
     /// between creation of an expansion ID and obtaining its data (e.g. macros are collected
     /// first and then resolved later), so we use an `Option` here.
     expn_data: Vec<Option<ExpnData>>,
+    /// Stores the computed `ClosestAstOrMacro` for each `ExpnId`. This is updated
+    /// at the same time as `expn_data`, and its contents it determined entirely
+    /// by the `ExpnData` - this field is just a cache.
+    closest_ast_or_macro: Vec<Option<ClosestAstOrMacro>>,
     syntax_context_data: Vec<SyntaxContextData>,
     syntax_context_map: FxHashMap<(SyntaxContext, ExpnId, Transparency), SyntaxContext>,
 }
@@ -162,6 +176,7 @@ impl HygieneData {
                 edition,
                 Some(DefId::local(CRATE_DEF_INDEX)),
             ))],
+            closest_ast_or_macro: vec![Some(ClosestAstOrMacro::None)],
             syntax_context_data: vec![SyntaxContextData {
                 outer_expn: ExpnId::root(),
                 outer_transparency: Transparency::Opaque,
@@ -178,9 +193,36 @@ impl HygieneData {
         GLOBALS.with(|globals| f(&mut *globals.hygiene_data.borrow_mut()))
     }
 
+    fn determine_closest_ast_or_macro(
+        &self,
+        id: ExpnId,
+        expn_data: &ExpnData,
+    ) -> ClosestAstOrMacro {
+        match expn_data.kind {
+            ExpnKind::Macro(_, _) | ExpnKind::AstPass(_) => ClosestAstOrMacro::Expn(id),
+            ExpnKind::Desugaring(_) | ExpnKind::Root => {
+                // Avoid using `HygieneData` when construction root
+                // `ExpnData`
+                if expn_data.call_site.ctxt() == SyntaxContext::root() {
+                    ClosestAstOrMacro::None
+                } else {
+                    self.closest_ast_or_macro(self.outer_expn(expn_data.call_site.ctxt()))
+                }
+            }
+        }
+    }
+
+    fn closest_ast_or_macro(&self, expn_id: ExpnId) -> ClosestAstOrMacro {
+        self.closest_ast_or_macro[expn_id.0 as usize].as_ref().copied().unwrap()
+    }
+
     fn fresh_expn(&mut self, expn_data: Option<ExpnData>) -> ExpnId {
+        let expn_id = ExpnId(self.expn_data.len() as u32);
+        self.closest_ast_or_macro.push(
+            expn_data.as_ref().map(|data| self.determine_closest_ast_or_macro(expn_id, data)),
+        );
         self.expn_data.push(expn_data);
-        ExpnId(self.expn_data.len() as u32 - 1)
+        expn_id
     }
 
     fn expn_data(&self, expn_id: ExpnId) -> &ExpnData {
@@ -684,6 +726,22 @@ pub struct ExpnData {
     pub macro_def_id: Option<DefId>,
 }
 
+/// The closest `ExpnKind::AstPass` or `ExpnKind::Macro` to an `ExpnData`.
+/// 'Closest' is determined by starting at the current `ExpnData`,
+/// and walking up the `call_site` tree. If an `ExpnData` with
+/// `Expn::AstPass` or `ExpnKind::Macro` is found, it is represented
+/// by `ClosestAstOrMacro::Expn(id)`, where `id` is the `EpxId` of
+/// the found `ExpnData`.
+///
+/// A `ClosestAstOrMacro` implies that no `ExpnKind::AstPass` or `ExpnKind::Macro`
+/// are found anywhere in the `call_site` tree - that is, there no macro
+/// expansions or ast pass expansions.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum ClosestAstOrMacro {
+    None,
+    Expn(ExpnId),
+}
+
 impl ExpnData {
     /// Constructs expansion data with default properties.
     pub fn default(