Detect duplicates

compiler/rustc_ast_passes/messages.ftl

@@ -14,8 +14,6 @@ ast_passes_assoc_type_without_body =
     associated type in `impl` without body
     .suggestion = provide a definition for the type
 
-ast_passes_precise_capturing_not_allowed_here = `use<...>` precise capturing syntax not allowed in {$loc}
-
 ast_passes_at_least_one_trait = at least one trait must be specified
 
 ast_passes_auto_generic = auto traits cannot have generic parameters
@@ -212,6 +210,11 @@ ast_passes_pattern_in_fn_pointer = patterns aren't allowed in function pointer t
 ast_passes_pattern_in_foreign = patterns aren't allowed in foreign function declarations
     .label = pattern not allowed in foreign function
 
+ast_passes_precise_capturing_duplicated = duplicate `use<...>` precise capturing syntax
+    .label = second `use<...>` here
+
+ast_passes_precise_capturing_not_allowed_here = `use<...>` precise capturing syntax not allowed in {$loc}
+
 ast_passes_show_span = {$msg}
 
 ast_passes_stability_outside_std = stability attributes may not be used outside of the standard library

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -183,8 +183,24 @@ impl<'a> AstValidator<'a> {
     // Mirrors `visit::walk_ty`, but tracks relevant state.
     fn walk_ty(&mut self, t: &'a Ty) {
         match &t.kind {
-            TyKind::ImplTrait(..) => {
-                self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
+            TyKind::ImplTrait(_, bounds) => {
+                self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t));
+
+                // FIXME(precise_capturing): If we were to allow `use` in other positions
+                // (e.g. GATs), then we must validate those as well. However, we don't have
+                // a good way of doing this with the current `Visitor` structure.
+                let mut use_bounds = bounds
+                    .iter()
+                    .filter_map(|bound| match bound {
+                        GenericBound::Use(_, span) => Some(span),
+                        _ => None,
+                    })
+                    .copied();
+                if let Some(bound1) = use_bounds.next()
+                    && let Some(bound2) = use_bounds.next()
+                {
+                    self.dcx().emit_err(errors::DuplicatePreciseCapturing { bound1, bound2 });
+                }
             }
             TyKind::TraitObject(..) => self
                 .with_tilde_const(Some(DisallowTildeConstContext::TraitObject), |this| {

compiler/rustc_ast_passes/src/errors.rs

@@ -836,3 +836,12 @@ pub struct PreciseCapturingNotAllowedHere {
     pub span: Span,
     pub loc: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_precise_capturing_duplicated)]
+pub struct DuplicatePreciseCapturing {
+    #[primary_span]
+    pub bound1: Span,
+    #[label]
+    pub bound2: Span,
+}

tests/ui/impl-trait/precise-capturing/duplicated-use.rs

@@ -0,0 +1,7 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn hello<'a>() -> impl Sized + use<'a> + use<'a> {}
+//~^ ERROR duplicate `use<...>` precise capturing syntax
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/duplicated-use.stderr

@@ -0,0 +1,17 @@
+error: duplicate `use<...>` precise capturing syntax
+  --> $DIR/duplicated-use.rs:4:32
+   |
+LL | fn hello<'a>() -> impl Sized + use<'a> + use<'a> {}
+   |                                ^^^^^^^   ------- second `use<...>` here
+
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/duplicated-use.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 1 previous error; 1 warning emitted
+