Auto merge of #94584 - pnkfelix:inject-use-suggestion-sites, r=ekuber

More robust fallback for `use` suggestion

Our old way to suggest where to add `use`s would first look for pre-existing `use`s in the relevant crate/module, and if there are *no* uses, it would fallback on trying to use another item as the basis for the suggestion.

But this was fragile, as illustrated in issue #87613

This PR instead identifies span of the first token after any inner attributes, and uses *that* as the fallback for the `use` suggestion.

Fix #87613

compiler/rustc_ast/src/ast.rs

@@ -510,7 +510,7 @@ pub struct WhereEqPredicate {
 pub struct Crate {
     pub attrs: Vec<Attribute>,
     pub items: Vec<P<Item>>,
-    pub span: Span,
+    pub spans: ModSpans,
     /// Must be equal to `CRATE_NODE_ID` after the crate root is expanded, but may hold
     /// expansion placeholders or an unassigned value (`DUMMY_NODE_ID`) before that.
     pub id: NodeId,
@@ -2317,11 +2317,25 @@ pub enum ModKind {
     /// or with definition outlined to a separate file `mod foo;` and already loaded from it.
     /// The inner span is from the first token past `{` to the last token until `}`,
     /// or from the first to the last token in the loaded file.
-    Loaded(Vec<P<Item>>, Inline, Span),
+    Loaded(Vec<P<Item>>, Inline, ModSpans),
     /// Module with definition outlined to a separate file `mod foo;` but not yet loaded from it.
     Unloaded,
 }
 
+#[derive(Copy, Clone, Encodable, Decodable, Debug)]
+pub struct ModSpans {
+    /// `inner_span` covers the body of the module; for a file module, its the whole file.
+    /// For an inline module, its the span inside the `{ ... }`, not including the curly braces.
+    pub inner_span: Span,
+    pub inject_use_span: Span,
+}
+
+impl Default for ModSpans {
+    fn default() -> ModSpans {
+        ModSpans { inner_span: Default::default(), inject_use_span: Default::default() }
+    }
+}
+
 /// Foreign module declaration.
 ///
 /// E.g., `extern { .. }` or `extern "C" { .. }`.

compiler/rustc_ast/src/mut_visit.rs

@@ -1009,8 +1009,9 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
         ItemKind::Mod(unsafety, mod_kind) => {
             visit_unsafety(unsafety, vis);
             match mod_kind {
-                ModKind::Loaded(items, _inline, inner_span) => {
+                ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
                     vis.visit_span(inner_span);
+                    vis.visit_span(inject_use_span);
                     items.flat_map_in_place(|item| vis.flat_map_item(item));
                 }
                 ModKind::Unloaded => {}
@@ -1121,11 +1122,13 @@ pub fn noop_visit_fn_header<T: MutVisitor>(header: &mut FnHeader, vis: &mut T) {
 }
 
 pub fn noop_visit_crate<T: MutVisitor>(krate: &mut Crate, vis: &mut T) {
-    let Crate { attrs, items, span, id, is_placeholder: _ } = krate;
+    let Crate { attrs, items, spans, id, is_placeholder: _ } = krate;
     vis.visit_id(id);
     visit_attrs(attrs, vis);
     items.flat_map_in_place(|item| vis.flat_map_item(item));
-    vis.visit_span(span);
+    let ModSpans { inner_span, inject_use_span } = spans;
+    vis.visit_span(inner_span);
+    vis.visit_span(inject_use_span);
 }
 
 // Mutates one item into possibly many items.
@@ -1558,7 +1561,7 @@ impl DummyAstNode for Crate {
         Crate {
             attrs: Default::default(),
             items: Default::default(),
-            span: Default::default(),
+            spans: Default::default(),
             id: DUMMY_NODE_ID,
             is_placeholder: Default::default(),
         }

compiler/rustc_ast_lowering/src/item.rs

@@ -291,7 +291,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 })
             }
             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
-                ModKind::Loaded(items, _, inner_span) => {
+                ModKind::Loaded(items, _, ModSpans { inner_span, inject_use_span: _ }) => {
                     hir::ItemKind::Mod(self.lower_mod(items, *inner_span))
                 }
                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),

compiler/rustc_ast_lowering/src/lib.rs

@@ -452,7 +452,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         visit::walk_crate(&mut item::ItemLowerer { lctx: &mut self }, c);
 
         self.with_hir_id_owner(CRATE_NODE_ID, |lctx| {
-            let module = lctx.lower_mod(&c.items, c.span);
+            let module = lctx.lower_mod(&c.items, c.spans.inner_span);
             lctx.lower_attrs(hir::CRATE_HIR_ID, &c.attrs);
             hir::OwnerNode::Crate(lctx.arena.alloc(module))
         });

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -112,7 +112,7 @@ impl<'a> MutVisitor for TestHarnessGenerator<'a> {
     fn visit_crate(&mut self, c: &mut ast::Crate) {
         let prev_tests = mem::take(&mut self.tests);
         noop_visit_crate(c, self);
-        self.add_test_cases(ast::CRATE_NODE_ID, c.span, prev_tests);
+        self.add_test_cases(ast::CRATE_NODE_ID, c.spans.inner_span, prev_tests);
 
         // Create a main function to run our tests
         c.items.push(mk_main(&mut self.cx));
@@ -129,7 +129,8 @@ impl<'a> MutVisitor for TestHarnessGenerator<'a> {
 
         // We don't want to recurse into anything other than mods, since
         // mods or tests inside of functions will break things
-        if let ast::ItemKind::Mod(_, ModKind::Loaded(.., span)) = item.kind {
+        if let ast::ItemKind::Mod(_, ModKind::Loaded(.., ref spans)) = item.kind {
+            let ast::ModSpans { inner_span: span, inject_use_span: _ } = *spans;
             let prev_tests = mem::take(&mut self.tests);
             noop_visit_item_kind(&mut item.kind, self);
             self.add_test_cases(item.id, span, prev_tests);

compiler/rustc_expand/src/base.rs

@@ -67,7 +67,7 @@ impl Annotatable {
             Annotatable::Param(ref p) => p.span,
             Annotatable::FieldDef(ref sf) => sf.span,
             Annotatable::Variant(ref v) => v.span,
-            Annotatable::Crate(ref c) => c.span,
+            Annotatable::Crate(ref c) => c.spans.inner_span,
         }
     }
 

compiler/rustc_expand/src/expand.rs

@@ -12,8 +12,8 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::visit::{self, AssocCtxt, Visitor};
 use rustc_ast::{AssocItemKind, AstLike, AstLikeWrapper, AttrStyle, ExprKind, ForeignItemKind};
-use rustc_ast::{Inline, ItemKind, MacArgs, MacStmtStyle, MetaItemKind, ModKind, NestedMetaItem};
-use rustc_ast::{NodeId, PatKind, StmtKind, TyKind};
+use rustc_ast::{Inline, ItemKind, MacArgs, MacStmtStyle, MetaItemKind, ModKind};
+use rustc_ast::{NestedMetaItem, NodeId, PatKind, StmtKind, TyKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::map_in_place::MapInPlace;
 use rustc_data_structures::sync::Lrc;
@@ -364,7 +364,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
     }
 
     pub fn expand_crate(&mut self, krate: ast::Crate) -> ast::Crate {
-        let file_path = match self.cx.source_map().span_to_filename(krate.span) {
+        let file_path = match self.cx.source_map().span_to_filename(krate.spans.inner_span) {
             FileName::Real(name) => name
                 .into_local_path()
                 .expect("attempting to resolve a file path in an external file"),
@@ -1091,7 +1091,7 @@ impl InvocationCollectorNode for P<ast::Item> {
             ModKind::Unloaded => {
                 // We have an outline `mod foo;` so we need to parse the file.
                 let old_attrs_len = attrs.len();
-                let ParsedExternalMod { items, inner_span, file_path, dir_path, dir_ownership } =
+                let ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership } =
                     parse_external_mod(
                         &ecx.sess,
                         ident,
@@ -1112,7 +1112,7 @@ impl InvocationCollectorNode for P<ast::Item> {
                     );
                 }
 
-                *mod_kind = ModKind::Loaded(items, Inline::No, inner_span);
+                *mod_kind = ModKind::Loaded(items, Inline::No, spans);
                 node.attrs = attrs;
                 if node.attrs.len() > old_attrs_len {
                     // If we loaded an out-of-line module and added some inner attributes,

compiler/rustc_expand/src/module.rs

@@ -1,6 +1,6 @@
 use crate::base::ModuleData;
 use rustc_ast::ptr::P;
-use rustc_ast::{token, Attribute, Inline, Item};
+use rustc_ast::{token, Attribute, Inline, Item, ModSpans};
 use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_parse::new_parser_from_file;
 use rustc_parse::validate_attr;
@@ -28,7 +28,7 @@ pub struct ModulePathSuccess {
 
 crate struct ParsedExternalMod {
     pub items: Vec<P<Item>>,
-    pub inner_span: Span,
+    pub spans: ModSpans,
     pub file_path: PathBuf,
     pub dir_path: PathBuf,
     pub dir_ownership: DirOwnership,
@@ -69,13 +69,13 @@ crate fn parse_external_mod(
         (items, inner_span, mp.file_path)
     };
     // (1) ...instead, we return a dummy module.
-    let (items, inner_span, file_path) =
+    let (items, spans, file_path) =
         result.map_err(|err| err.report(sess, span)).unwrap_or_default();
 
     // Extract the directory path for submodules of the module.
     let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
 
-    ParsedExternalMod { items, inner_span, file_path, dir_path, dir_ownership }
+    ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership }
 }
 
 crate fn mod_dir_path(

compiler/rustc_expand/src/placeholders.rs

@@ -49,7 +49,7 @@ pub fn placeholder(
         AstFragmentKind::Crate => AstFragment::Crate(ast::Crate {
             attrs: Default::default(),
             items: Default::default(),
-            span,
+            spans: ast::ModSpans { inner_span: span, ..Default::default() },
             id,
             is_placeholder: true,
         }),

compiler/rustc_metadata/src/creader.rs

@@ -899,7 +899,7 @@ impl<'a> CrateLoader<'a> {
 
     fn report_unused_deps(&mut self, krate: &ast::Crate) {
         // Make a point span rather than covering the whole file
-        let span = krate.span.shrink_to_lo();
+        let span = krate.spans.inner_span.shrink_to_lo();
         // Complain about anything left over
         for (name, entry) in self.sess.opts.externs.iter() {
             if let ExternLocation::FoundInLibrarySearchDirectories = entry.location {

compiler/rustc_parse/src/lib.rs

@@ -331,7 +331,7 @@ pub fn fake_token_stream(sess: &ParseSess, nt: &Nonterminal) -> TokenStream {
 pub fn fake_token_stream_for_crate(sess: &ParseSess, krate: &ast::Crate) -> TokenStream {
     let source = pprust::crate_to_string_for_macros(krate);
     let filename = FileName::macro_expansion_source_code(&source);
-    parse_stream_from_source_str(filename, source, sess, Some(krate.span))
+    parse_stream_from_source_str(filename, source, sess, Some(krate.spans.inner_span))
 }
 
 pub fn parse_cfg_attr(

compiler/rustc_parse/src/parser/item.rs

@@ -27,8 +27,8 @@ use tracing::debug;
 impl<'a> Parser<'a> {
     /// Parses a source module as a crate. This is the main entry point for the parser.
     pub fn parse_crate_mod(&mut self) -> PResult<'a, ast::Crate> {
-        let (attrs, items, span) = self.parse_mod(&token::Eof)?;
-        Ok(ast::Crate { attrs, items, span, id: DUMMY_NODE_ID, is_placeholder: false })
+        let (attrs, items, spans) = self.parse_mod(&token::Eof)?;
+        Ok(ast::Crate { attrs, items, spans, id: DUMMY_NODE_ID, is_placeholder: false })
     }
 
     /// Parses a `mod <foo> { ... }` or `mod <foo>;` item.
@@ -52,10 +52,11 @@ impl<'a> Parser<'a> {
     pub fn parse_mod(
         &mut self,
         term: &TokenKind,
-    ) -> PResult<'a, (Vec<Attribute>, Vec<P<Item>>, Span)> {
+    ) -> PResult<'a, (Vec<Attribute>, Vec<P<Item>>, ModSpans)> {
         let lo = self.token.span;
         let attrs = self.parse_inner_attributes()?;
 
+        let post_attr_lo = self.token.span;
         let mut items = vec![];
         while let Some(item) = self.parse_item(ForceCollect::No)? {
             items.push(item);
@@ -72,7 +73,9 @@ impl<'a> Parser<'a> {
             }
         }
 
-        Ok((attrs, items, lo.to(self.prev_token.span)))
+        let inject_use_span = post_attr_lo.data().with_hi(post_attr_lo.lo());
+        let mod_spans = ModSpans { inner_span: lo.to(self.prev_token.span), inject_use_span };
+        Ok((attrs, items, mod_spans))
     }
 }
 

compiler/rustc_resolve/src/lib.rs

@@ -71,7 +71,6 @@ use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 use std::cell::{Cell, RefCell};
 use std::collections::BTreeSet;
-use std::ops::ControlFlow;
 use std::{cmp, fmt, iter, mem, ptr};
 use tracing::debug;
 
@@ -315,74 +314,70 @@ impl<'a> From<&'a ast::PathSegment> for Segment {
     }
 }
 
+#[derive(Debug)]
 struct UsePlacementFinder {
     target_module: NodeId,
-    span: Option<Span>,
-    found_use: bool,
+    first_legal_span: Option<Span>,
+    first_use_span: Option<Span>,
 }
 
 impl UsePlacementFinder {
     fn check(krate: &Crate, target_module: NodeId) -> (Option<Span>, bool) {
-        let mut finder = UsePlacementFinder { target_module, span: None, found_use: false };
-        if let ControlFlow::Continue(..) = finder.check_mod(&krate.items, CRATE_NODE_ID) {
-            visit::walk_crate(&mut finder, krate);
-        }
-        (finder.span, finder.found_use)
-    }
-
-    fn check_mod(&mut self, items: &[P<ast::Item>], node_id: NodeId) -> ControlFlow<()> {
-        if self.span.is_some() {
-            return ControlFlow::Break(());
-        }
-        if node_id != self.target_module {
-            return ControlFlow::Continue(());
-        }
-        // find a use statement
-        for item in items {
-            match item.kind {
-                ItemKind::Use(..) => {
-                    // don't suggest placing a use before the prelude
-                    // import or other generated ones
-                    if !item.span.from_expansion() {
-                        self.span = Some(item.span.shrink_to_lo());
-                        self.found_use = true;
-                        return ControlFlow::Break(());
-                    }
-                }
-                // don't place use before extern crate
-                ItemKind::ExternCrate(_) => {}
-                // but place them before the first other item
-                _ => {
-                    if self.span.map_or(true, |span| item.span < span)
-                        && !item.span.from_expansion()
-                    {
-                        self.span = Some(item.span.shrink_to_lo());
-                        // don't insert between attributes and an item
-                        // find the first attribute on the item
-                        // FIXME: This is broken for active attributes.
-                        for attr in &item.attrs {
-                            if !attr.span.is_dummy()
-                                && self.span.map_or(true, |span| attr.span < span)
-                            {
-                                self.span = Some(attr.span.shrink_to_lo());
-                            }
-                        }
-                    }
-                }
+        let mut finder =
+            UsePlacementFinder { target_module, first_legal_span: None, first_use_span: None };
+        finder.visit_crate(krate);
+        if let Some(use_span) = finder.first_use_span {
+            (Some(use_span), true)
+        } else {
+            (finder.first_legal_span, false)
+        }
+    }
+}
+
+fn is_span_suitable_for_use_injection(s: Span) -> bool {
+    // don't suggest placing a use before the prelude
+    // import or other generated ones
+    !s.from_expansion()
+}
+
+fn search_for_any_use_in_items(items: &[P<ast::Item>]) -> Option<Span> {
+    for item in items {
+        if let ItemKind::Use(..) = item.kind {
+            if is_span_suitable_for_use_injection(item.span) {
+                return Some(item.span.shrink_to_lo());
             }
         }
-        ControlFlow::Continue(())
     }
+    return None;
 }
 
 impl<'tcx> Visitor<'tcx> for UsePlacementFinder {
+    fn visit_crate(&mut self, c: &Crate) {
+        if self.target_module == CRATE_NODE_ID {
+            let inject = c.spans.inject_use_span;
+            if is_span_suitable_for_use_injection(inject) {
+                self.first_legal_span = Some(inject);
+            }
+            self.first_use_span = search_for_any_use_in_items(&c.items);
+            return;
+        } else {
+            visit::walk_crate(self, c);
+        }
+    }
+
     fn visit_item(&mut self, item: &'tcx ast::Item) {
-        if let ItemKind::Mod(_, ModKind::Loaded(items, ..)) = &item.kind {
-            if let ControlFlow::Break(..) = self.check_mod(items, item.id) {
+        if self.target_module == item.id {
+            if let ItemKind::Mod(_, ModKind::Loaded(items, _inline, mod_spans)) = &item.kind {
+                let inject = mod_spans.inject_use_span;
+                if is_span_suitable_for_use_injection(inject) {
+                    self.first_legal_span = Some(inject);
+                }
+                self.first_use_span = search_for_any_use_in_items(items);
                 return;
             }
+        } else {
+            visit::walk_item(self, item);
         }
-        visit::walk_item(self, item);
     }
 }
 
@@ -1282,7 +1277,7 @@ impl<'a> Resolver<'a> {
             None,
             ModuleKind::Def(DefKind::Mod, root_def_id, kw::Empty),
             ExpnId::root(),
-            krate.span,
+            krate.spans.inner_span,
             session.contains_name(&krate.attrs, sym::no_implicit_prelude),
             &mut module_map,
         );
@@ -1295,7 +1290,7 @@ impl<'a> Resolver<'a> {
             &mut FxHashMap::default(),
         );
 
-        let definitions = Definitions::new(session.local_stable_crate_id(), krate.span);
+        let definitions = Definitions::new(session.local_stable_crate_id(), krate.spans.inner_span);
         let root = definitions.get_root_def();
 
         let mut visibilities = FxHashMap::default();