Add future_prelude_collision lint

compiler/rustc_lint_defs/src/builtin.rs

@@ -3001,6 +3001,7 @@
         PROC_MACRO_BACK_COMPAT,
         OR_PATTERNS_BACK_COMPAT,
         LARGE_ASSIGNMENTS,
+        FUTURE_PRELUDE_COLLISION,
     ]
 }
 
@@ -3240,3 +3241,47 @@
     Allow,
     "detects usage of old versions of or-patterns",
 }
+
+declare_lint! {
+    /// The `future_prelude_collision` lint detects the usage of trait methods which are ambiguous
+    /// with traits added to the prelude in future editions.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(future_prelude_collision)]
+    ///
+    /// trait Foo {
+    ///     fn try_into(self) -> Result<String, !>;
+    /// }
+    ///
+    /// impl Foo for &str {
+    ///     fn try_into(self) -> Result<String, !> {
+    ///         Ok(String::from(self))
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     let x: String = "3".try_into().unwrap();
+    ///     //                  ^^^^^^^^
+    ///     // This call to try_into matches both Foo:try_into and TryInto::try_into as
+    ///     // `TryInto` has been added to the Rust prelude in 2021 edition.
+    ///     println!("{}", x);
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In Rust 2021, one of the important introductions is the [prelude changes], which add
+    /// `TryFrom` and `TryInto` into the standard library's prelude. Since this results in an
+    /// amiguity as to which method to call when an existing `try_from` or `try_into` method is
+    /// called via dot-call syntax.
+    ///
+    /// [prelude changes]: https://blog.rust-lang.org/inside-rust/2021/03/04/planning-rust-2021.html#prelude-changes
+    pub FUTURE_PRELUDE_COLLISION,
+    Warn,
+    "detects the usage of trait methods which are ambiguous with traits added to the \
+        prelude in future editions",
+}

compiler/rustc_span/src/symbol.rs

@@ -1236,7 +1236,9 @@
         truncf32,
         truncf64,
         try_blocks,
+        try_from,
         try_from_trait,
+        try_into,
         try_into_trait,
         try_trait_v2,
         tt,

compiler/rustc_typeck/src/check/method/mod.rs

@@ -21,7 +21,8 @@
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::GenericParamDefKind;
 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TypeFoldable, WithConstness};
-use rustc_span::symbol::Ident;
+use rustc_session::lint::builtin::FUTURE_PRELUDE_COLLISION;
+use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
@@ -198,6 +199,52 @@ pub fn lookup_method(
         let pick =
             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 
+        if let sym::try_from | sym::try_into = segment.ident.name {
+            if let probe::PickKind::TraitPick = pick.kind {
+                if !matches!(self.tcx.crate_name(pick.item.def_id.krate), sym::std | sym::core) {
+                    self.tcx.struct_span_lint_hir(
+                        FUTURE_PRELUDE_COLLISION,
+                        call_expr.hir_id,
+                        call_expr.span,
+                        |lint| {
+                            let sp = call_expr.span;
+                            let trait_name =
+                                self.tcx.def_path_str(pick.item.container.assert_trait());
+
+                            let mut lint = lint.build(&format!(
+                                "trait method `{}` will become ambiguous in Rust 2021",
+                                segment.ident.name
+                            ));
+
+                            if let Ok(self_expr) =
+                                self.sess().source_map().span_to_snippet(self_expr.span)
+                            {
+                                lint.span_suggestion(
+                                    sp,
+                                    "disambiguate the associated function",
+                                    format!(
+                                        "{}::{}({})",
+                                        trait_name, segment.ident.name, self_expr,
+                                    ),
+                                    Applicability::MachineApplicable,
+                                );
+                            } else {
+                                lint.span_help(
+                                    sp,
+                                    &format!(
+                                        "disambiguate the associated function with `{}::{}(...)`",
+                                        trait_name, segment.ident,
+                                    ),
+                                );
+                            }
+
+                            lint.emit();
+                        },
+                    );
+                }
+            }
+        }
+
         for import_id in &pick.import_ids {
             debug!("used_trait_import: {:?}", import_id);
             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)