Add structured suggestions for proc macro use imports

Author: oli-obk

src/librustc_resolve/lib.rs

@@ -1290,6 +1290,8 @@ pub struct Resolver<'a> {
     ambiguity_errors: Vec<AmbiguityError<'a>>,
     /// `use` injections are delayed for better placement and deduplication
     use_injections: Vec<UseError<'a>>,
+    /// `use` injections for proc macros wrongly imported with #[macro_use]
+    proc_mac_errors: Vec<macros::ProcMacError>,
 
     gated_errors: FxHashSet<Span>,
     disallowed_shadowing: Vec<&'a LegacyBinding<'a>>,
@@ -1498,6 +1500,7 @@ impl<'a> Resolver<'a> {
             privacy_errors: Vec::new(),
             ambiguity_errors: Vec::new(),
             use_injections: Vec::new(),
+            proc_mac_errors: Vec::new(),
             gated_errors: FxHashSet(),
             disallowed_shadowing: Vec::new(),
 
@@ -3551,6 +3554,7 @@ impl<'a> Resolver<'a> {
     fn report_errors(&mut self, krate: &Crate) {
         self.report_shadowing_errors();
         self.report_with_use_injections(krate);
+        self.report_proc_macro_import(krate);
         let mut reported_spans = FxHashSet();
 
         for &AmbiguityError { span, name, b1, b2, lexical, legacy } in &self.ambiguity_errors {

src/librustc_resolve/macros.rs

@@ -83,6 +83,14 @@ pub struct LegacyBinding<'a> {
     pub span: Span,
 }
 
+pub struct ProcMacError {
+    crate_name: Symbol,
+    name: Symbol,
+    module: ast::NodeId,
+    use_span: Span,
+    warn_msg: &'static str,
+}
+
 #[derive(Copy, Clone)]
 pub enum MacroBinding<'a> {
     Legacy(&'a LegacyBinding<'a>),
@@ -779,12 +787,37 @@ impl<'a> Resolver<'a> {
             _ => return,
         };
 
-        let crate_name = self.cstore.crate_name_untracked(krate);
+        let def_id = self.current_module.normal_ancestor_id;
+        let node_id = self.definitions.as_local_node_id(def_id).unwrap();
+
+        self.proc_mac_errors.push(ProcMacError {
+            crate_name: self.cstore.crate_name_untracked(krate),
+            name,
+            module: node_id,
+            use_span,
+            warn_msg,
+        });
+    }
+
+    pub fn report_proc_macro_import(&mut self, krate: &ast::Crate) {
+        for err in self.proc_mac_errors.drain(..) {
+            let (span, found_use) = ::UsePlacementFinder::check(krate, err.module);
 
-        self.session.struct_span_err(use_span, warn_msg)
-            .help(&format!("instead, import the procedural macro like any other item: \
-                             `use {}::{};`", crate_name, name))
-            .emit();
+            if let Some(span) = span {
+                let found_use = if found_use { "" } else { "\n" };
+                self.session.struct_span_err(err.use_span, err.warn_msg)
+                    .span_suggestion(
+                        span,
+                        "instead, import the procedural macro like any other item",
+                        format!("use {}::{};{}", err.crate_name, err.name, found_use),
+                    ).emit();
+            } else {
+                self.session.struct_span_err(err.use_span, err.warn_msg)
+                    .help(&format!("instead, import the procedural macro like any other item: \
+                                    `use {}::{};`", err.crate_name, err.name))
+                    .emit();
+            }
+        }
     }
 
     fn gate_legacy_custom_derive(&mut self, name: Symbol, span: Span) {

src/test/ui/issue-35976.stderr

@@ -3,7 +3,6 @@ error: the `wait` method cannot be invoked on a trait object
    |
 24 |     arg.wait();
    |         ^^^^
-   |
 help: another candidate was found in the following trait, perhaps add a `use` for it:
    |
 11 | use private::Future;