Auto merge of #85707 - jam1garner:future_prelude_collision_lint, r=nikomatsakis

Add `future_prelude_collision` lint

Implements #84594. (RFC rust-lang/rfcs#3114 ([rendered](https://github.com/rust-lang/rfcs/blob/master/text/3114-prelude-2021.md))) Not entirely complete but wanted to have my progress decently available while I finish off the last little bits.

Things left to implement:

* [x] UI tests for lints
* [x] Only emit lint for 2015 and 2018 editions
* [ ] Lint name/message bikeshedding
* [x] Implement for `FromIterator` (from best I can tell, the current approach as mentioned from [this comment](#84594 (comment)) won't work due to `FromIterator` instances not using dot-call syntax, but if I'm correct about this then that would also need to be fixed for `TryFrom`/`TryInto`)*
* [x] Add to `rust-2021-migration` group? (See #85512) (added to `rust-2021-compatibility` group)
* [ ] Link to edition guide in lint docs

*edit: looked into it, `lookup_method` will also not be hit for `TryFrom`/`TryInto` for non-dotcall syntax. If anyone who is more familiar with typecheck knows the equivalent for looking up associated functions, feel free to chime in.

Author: bors

compiler/rustc_lint_defs/src/builtin.rs

@@ -3001,6 +3001,7 @@ declare_lint_pass! {
         PROC_MACRO_BACK_COMPAT,
         OR_PATTERNS_BACK_COMPAT,
         LARGE_ASSIGNMENTS,
+        FUTURE_PRELUDE_COLLISION,
     ]
 }
 
@@ -3244,3 +3245,52 @@ declare_lint! {
         edition: Some(Edition::Edition2021),
     };
 }
+
+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`, `TryInto`, and `FromIterator` into the standard library's prelude. Since this
+    /// results in an amiguity as to which method/function to call when an existing `try_into`
+    ///  method is called via dot-call syntax or a `try_from`/`from_iter` associated function
+    ///  is called directly on a type.
+    ///
+    /// [prelude changes]: https://blog.rust-lang.org/inside-rust/2021/03/04/planning-rust-2021.html#prelude-changes
+    pub FUTURE_PRELUDE_COLLISION,
+    Allow,
+    "detects the usage of trait methods which are ambiguous with traits added to the \
+        prelude in future editions",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #85684 <https://github.com/rust-lang/rust/issues/85684>",
+        edition: Some(Edition::Edition2021),
+    };
+}

compiler/rustc_span/src/symbol.rs

@@ -586,6 +586,7 @@ symbols! {
         from,
         from_desugaring,
         from_generator,
+        from_iter,
         from_method,
         from_output,
         from_residual,
@@ -1238,7 +1239,9 @@ symbols! {
         truncf32,
         truncf64,
         try_blocks,
+        try_from,
         try_from_trait,
+        try_into,
         try_into_trait,
         try_trait_v2,
         tt,

compiler/rustc_typeck/src/check/expr.rs

@@ -466,7 +466,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
-        let (res, opt_ty, segs) = self.resolve_ty_and_res_ufcs(qpath, expr.hir_id, expr.span);
+        let (res, opt_ty, segs) =
+            self.resolve_ty_and_res_fully_qualified_call(qpath, expr.hir_id, expr.span);
         let ty = match res {
             Res::Err => {
                 self.set_tainted_by_errors();
@@ -940,7 +941,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // no need to check for bot/err -- callee does that
         let rcvr_t = self.structurally_resolved_type(args[0].span, rcvr_t);
 
-        let method = match self.lookup_method(rcvr_t, segment, span, expr, rcvr) {
+        let method = match self.lookup_method(rcvr_t, segment, span, expr, rcvr, args) {
             Ok(method) => {
                 // We could add a "consider `foo::<params>`" suggestion here, but I wasn't able to
                 // trigger this codepath causing `structuraly_resolved_type` to emit an error.

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -906,13 +906,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Resolves an associated value path into a base type and associated constant, or method
     /// resolution. The newly resolved definition is written into `type_dependent_defs`.
-    pub fn resolve_ty_and_res_ufcs(
+    pub fn resolve_ty_and_res_fully_qualified_call(
         &self,
         qpath: &'tcx QPath<'tcx>,
         hir_id: hir::HirId,
         span: Span,
     ) -> (Res, Option<Ty<'tcx>>, &'tcx [hir::PathSegment<'tcx>]) {
-        debug!("resolve_ty_and_res_ufcs: qpath={:?} hir_id={:?} span={:?}", qpath, hir_id, span);
+        debug!(
+            "resolve_ty_and_res_fully_qualified_call: qpath={:?} hir_id={:?} span={:?}",
+            qpath, hir_id, span
+        );
         let (ty, qself, item_segment) = match *qpath {
             QPath::Resolved(ref opt_qself, ref path) => {
                 return (
@@ -922,7 +925,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 );
             }
             QPath::TypeRelative(ref qself, ref segment) => (self.to_ty(qself), qself, segment),
-            QPath::LangItem(..) => bug!("`resolve_ty_and_res_ufcs` called on `LangItem`"),
+            QPath::LangItem(..) => {
+                bug!("`resolve_ty_and_res_fully_qualified_call` called on `LangItem`")
+            }
         };
         if let Some(&cached_result) = self.typeck_results.borrow().type_dependent_defs().get(hir_id)
         {
@@ -932,25 +937,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return (def, Some(ty), slice::from_ref(&**item_segment));
         }
         let item_name = item_segment.ident;
-        let result = self.resolve_ufcs(span, item_name, ty, hir_id).or_else(|error| {
-            let result = match error {
-                method::MethodError::PrivateMatch(kind, def_id, _) => Ok((kind, def_id)),
-                _ => Err(ErrorReported),
-            };
-            if item_name.name != kw::Empty {
-                if let Some(mut e) = self.report_method_error(
-                    span,
-                    ty,
-                    item_name,
-                    SelfSource::QPath(qself),
-                    error,
-                    None,
-                ) {
-                    e.emit();
+        let result = self
+            .resolve_fully_qualified_call(span, item_name, ty, qself.span, hir_id)
+            .or_else(|error| {
+                let result = match error {
+                    method::MethodError::PrivateMatch(kind, def_id, _) => Ok((kind, def_id)),
+                    _ => Err(ErrorReported),
+                };
+                if item_name.name != kw::Empty {
+                    if let Some(mut e) = self.report_method_error(
+                        span,
+                        ty,
+                        item_name,
+                        SelfSource::QPath(qself),
+                        error,
+                        None,
+                    ) {
+                        e.emit();
+                    }
                 }
-            }
-            result
-        });
+                result
+            });
 
         if result.is_ok() {
             self.maybe_lint_bare_trait(qpath, hir_id);

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

@@ -3,6 +3,7 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/method-lookup.html
 
 mod confirm;
+mod prelude2021;
 pub mod probe;
 mod suggest;
 
@@ -173,14 +174,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ///
     /// # Arguments
     ///
-    /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
+    /// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
     ///
     /// * `self`:                  the surrounding `FnCtxt` (!)
     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
     /// * `span`:                  the span for the method call
     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
     /// * `self_expr`:             the self expression (`foo`)
+    /// * `args`:                  the expressions of the arguments (`a, b + 1, ...`)
     #[instrument(level = "debug", skip(self, call_expr, self_expr))]
     pub fn lookup_method(
         &self,
@@ -189,6 +191,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
         call_expr: &'tcx hir::Expr<'tcx>,
         self_expr: &'tcx hir::Expr<'tcx>,
+        args: &'tcx [hir::Expr<'tcx>],
     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
         debug!(
             "lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
@@ -198,6 +201,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let pick =
             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 
+        self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
+
         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)
@@ -417,16 +422,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         Some(InferOk { obligations, value: callee })
     }
 
+    /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
+    /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
+    /// function definition.
+    ///
+    /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
+    ///
+    /// # Arguments
+    ///
+    /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
+    ///
+    /// * `self`:                  the surrounding `FnCtxt` (!)
+    /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
+    /// * `method_name`:           the identifier of the function within the container type (`bar`)
+    /// * `self_ty`:               the type to search within (`Foo`)
+    /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
+    /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
     #[instrument(level = "debug", skip(self))]
-    pub fn resolve_ufcs(
+    pub fn resolve_fully_qualified_call(
         &self,
         span: Span,
         method_name: Ident,
         self_ty: Ty<'tcx>,
+        self_ty_span: Span,
         expr_id: hir::HirId,
     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
         debug!(
-            "resolve_ufcs: method_name={:?} self_ty={:?} expr_id={:?}",
+            "resolve_fully_qualified_call: method_name={:?} self_ty={:?} expr_id={:?}",
             method_name, self_ty, expr_id,
         );
 
@@ -463,18 +485,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             expr_id,
             ProbeScope::TraitsInScope,
         )?;
-        debug!("resolve_ufcs: pick={:?}", pick);
+
+        self.lint_fully_qualified_call_from_2018(
+            span,
+            method_name,
+            self_ty,
+            self_ty_span,
+            expr_id,
+            &pick,
+        );
+
+        debug!("resolve_fully_qualified_call: pick={:?}", pick);
         {
             let mut typeck_results = self.typeck_results.borrow_mut();
             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
             for import_id in pick.import_ids {
-                debug!("resolve_ufcs: used_trait_import: {:?}", import_id);
+                debug!("resolve_fully_qualified_call: used_trait_import: {:?}", import_id);
                 used_trait_imports.insert(import_id);
             }
         }
 
         let def_kind = pick.item.kind.as_def_kind();
-        debug!("resolve_ufcs: def_kind={:?}, def_id={:?}", def_kind, pick.item.def_id);
+        debug!(
+            "resolve_fully_qualified_call: def_kind={:?}, def_id={:?}",
+            def_kind, pick.item.def_id
+        );
         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
         Ok((def_kind, pick.item.def_id))
     }