Auto merge of #48326 - RalfJung:generic-bounds, r=<try>

Warn about ignored generic bounds in `for`

This adds a new lint to fix #42181. For consistency and to avoid code duplication, I also moved the existing "bounds in type aliases are ignored" here.

Questions to the reviewer:
* Is it okay to just remove a diagnostic error code like this? Should I instead keep the warning about type aliases where it is? The old code provided a detailed explanation of what's going on when asked, that information is now lost. On the other hand, `span_warn!` seems deprecated (after this patch, it has exactly one user left!).
* Did I miss any syntactic construct that can appear as `for` in the surface syntax? I covered function types (`for<'a> fn(...)`), generic traits (`for <'a> Fn(...)`, can appear both as bounds as as trait objects) and bounds (`for<'a> F: ...`).
* For the sake of backwards compatibility, this adds a warning, not an error. @nikomatsakis suggested an error in #42181 (comment), but I feel that can only happen in a new epoch -- right?

Cc @eddyb

Author: bors

src/librustc/lint/context.rs

@@ -793,6 +793,17 @@ impl<'a, 'tcx> hir_visit::Visitor<'tcx> for LateContext<'a, 'tcx> {
         hir_visit::walk_generics(self, g);
     }
 
+    fn visit_where_predicate(&mut self, p: &'tcx hir::WherePredicate) {
+        run_lints!(self, check_where_predicate, late_passes, p);
+        hir_visit::walk_where_predicate(self, p);
+    }
+
+    fn visit_poly_trait_ref(&mut self, t: &'tcx hir::PolyTraitRef,
+                            m: hir::TraitBoundModifier) {
+        run_lints!(self, check_poly_trait_ref, late_passes, t, m);
+        hir_visit::walk_poly_trait_ref(self, t, m);
+    }
+
     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
         let generics = self.generics.take();
         self.generics = Some(&trait_item.generics);
@@ -955,6 +966,16 @@ impl<'a> ast_visit::Visitor<'a> for EarlyContext<'a> {
         ast_visit::walk_generics(self, g);
     }
 
+    fn visit_where_predicate(&mut self, p: &'a ast::WherePredicate) {
+        run_lints!(self, check_where_predicate, early_passes, p);
+        ast_visit::walk_where_predicate(self, p);
+    }
+
+    fn visit_poly_trait_ref(&mut self, t: &'a ast::PolyTraitRef, m: &'a ast::TraitBoundModifier) {
+        run_lints!(self, check_poly_trait_ref, early_passes, t, m);
+        ast_visit::walk_poly_trait_ref(self, t, m);
+    }
+
     fn visit_trait_item(&mut self, trait_item: &'a ast::TraitItem) {
         self.with_lint_attrs(trait_item.id, &trait_item.attrs, |cx| {
             run_lints!(cx, check_trait_item, early_passes, trait_item);

src/librustc/lint/mod.rs

@@ -158,6 +158,9 @@ pub trait LateLintPass<'a, 'tcx>: LintPass {
     fn check_ty(&mut self, _: &LateContext<'a, 'tcx>, _: &'tcx hir::Ty) { }
     fn check_generic_param(&mut self, _: &LateContext<'a, 'tcx>, _: &'tcx hir::GenericParam) { }
     fn check_generics(&mut self, _: &LateContext<'a, 'tcx>, _: &'tcx hir::Generics) { }
+    fn check_where_predicate(&mut self, _: &LateContext<'a, 'tcx>, _: &'tcx hir::WherePredicate) { }
+    fn check_poly_trait_ref(&mut self, _: &LateContext<'a, 'tcx>, _: &'tcx hir::PolyTraitRef,
+                            _: hir::TraitBoundModifier) { }
     fn check_fn(&mut self,
                 _: &LateContext<'a, 'tcx>,
                 _: FnKind<'tcx>,
@@ -230,6 +233,9 @@ pub trait EarlyLintPass: LintPass {
     fn check_ty(&mut self, _: &EarlyContext, _: &ast::Ty) { }
     fn check_generic_param(&mut self, _: &EarlyContext, _: &ast::GenericParam) { }
     fn check_generics(&mut self, _: &EarlyContext, _: &ast::Generics) { }
+    fn check_where_predicate(&mut self, _: &EarlyContext, _: &ast::WherePredicate) { }
+    fn check_poly_trait_ref(&mut self, _: &EarlyContext, _: &ast::PolyTraitRef,
+                            _: &ast::TraitBoundModifier) { }
     fn check_fn(&mut self, _: &EarlyContext,
         _: ast_visit::FnKind, _: &ast::FnDecl, _: Span, _: ast::NodeId) { }
     fn check_fn_post(&mut self, _: &EarlyContext,

src/librustc_lint/builtin.rs

@@ -1386,3 +1386,97 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnreachablePub {
         self.perform_lint(cx, "item", impl_item.id, &impl_item.vis, impl_item.span, false);
     }
 }
+
+/// Lint for trait and lifetime bounds that are (accidentally) accepted by the parser, but
+/// ignored later.
+
+pub struct IgnoredGenericBounds;
+
+declare_lint! {
+    IGNORED_GENERIC_BOUNDS,
+    Deny,
+    "these generic bounds are ignored"
+}
+
+impl LintPass for IgnoredGenericBounds {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(IGNORED_GENERIC_BOUNDS)
+    }
+}
+
+impl IgnoredGenericBounds {
+    fn ensure_no_param_bounds(
+        cx: &EarlyContext,
+        generics: &Vec<ast::GenericParam>,
+        thing: &'static str,
+    ) {
+        for param in generics.iter() {
+            match param {
+                &ast::GenericParam::Lifetime(ref lifetime) => {
+                    if !lifetime.bounds.is_empty() {
+                        let spans : Vec<_> = lifetime.bounds.iter().map(|b| b.span).collect();
+                        cx.span_lint(
+                            IGNORED_GENERIC_BOUNDS,
+                            spans,
+                            format!("bounds on generic lifetime parameters are ignored in {}",
+                                thing).as_ref()
+                        );
+                    }
+                }
+                &ast::GenericParam::Type(ref ty) => {
+                    if !ty.bounds.is_empty() {
+                        let spans : Vec<_> = ty.bounds.iter().map(|b| b.span()).collect();
+                        cx.span_lint(
+                            IGNORED_GENERIC_BOUNDS,
+                            spans,
+                            format!("bounds on generic type parameters are ignored in {}", thing)
+                                .as_ref()
+                        );
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl EarlyLintPass for IgnoredGenericBounds {
+    fn check_item(&mut self, cx: &EarlyContext, item: &ast::Item) {
+        match item.node {
+            ast::ItemKind::Ty(_, ref generics) => {
+                if !generics.where_clause.predicates.is_empty() {
+                    let spans : Vec<_> = generics.where_clause.predicates.iter()
+                        .map(|pred| pred.span()).collect();
+                    cx.span_lint(IGNORED_GENERIC_BOUNDS, spans,
+                        "where clauses are ignored in type aliases");
+                }
+                IgnoredGenericBounds::ensure_no_param_bounds(cx, &generics.params,
+                    "type aliases");
+            }
+            _ => {}
+        }
+    }
+
+    fn check_where_predicate(&mut self, cx: &EarlyContext, p: &ast::WherePredicate) {
+        if let &ast::WherePredicate::BoundPredicate(ref bound_predicate) = p {
+            // A type binding, eg `for<'c> Foo: Send+Clone+'c`
+            IgnoredGenericBounds::ensure_no_param_bounds(cx,
+                &bound_predicate.bound_generic_params, "higher-ranked trait bounds (i.e., `for`)");
+        }
+    }
+
+    fn check_poly_trait_ref(&mut self, cx: &EarlyContext, t: &ast::PolyTraitRef,
+                            _: &ast::TraitBoundModifier) {
+        IgnoredGenericBounds::ensure_no_param_bounds(cx, &t.bound_generic_params,
+            "higher-ranked trait bounds (i.e., `for`)");
+    }
+
+    fn check_ty(&mut self, cx: &EarlyContext, ty: &ast::Ty) {
+        match ty.node {
+            ast::TyKind::BareFn(ref fn_ty) => {
+                IgnoredGenericBounds::ensure_no_param_bounds(cx, &fn_ty.generic_params,
+                    "higher-ranked function types (i.e., `for`)");
+            }
+            _ => {}
+        }
+    }
+}

src/librustc_lint/lib.rs

@@ -109,6 +109,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                        AnonymousParameters,
                        IllegalFloatLiteralPattern,
                        UnusedDocComment,
+                       IgnoredGenericBounds,
                        );
 
     add_early_builtin_with_new!(sess,

src/librustc_typeck/collect.rs

@@ -355,39 +355,6 @@ fn is_param<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 }
 
-fn ensure_no_param_bounds(tcx: TyCtxt,
-                          span: Span,
-                          generics: &hir::Generics,
-                          thing: &'static str) {
-    let mut warn = false;
-
-    for ty_param in generics.ty_params() {
-        if !ty_param.bounds.is_empty() {
-            warn = true;
-        }
-    }
-
-    for lft_param in generics.lifetimes() {
-        if !lft_param.bounds.is_empty() {
-            warn = true;
-        }
-    }
-
-    if !generics.where_clause.predicates.is_empty() {
-        warn = true;
-    }
-
-    if warn {
-        // According to accepted RFC #XXX, we should
-        // eventually accept these, but it will not be
-        // part of this PR. Still, convert to warning to
-        // make bootstrapping easier.
-        span_warn!(tcx.sess, span, E0122,
-                   "generic bounds are ignored in {}",
-                   thing);
-    }
-}
-
 fn convert_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_id: ast::NodeId) {
     let it = tcx.hir.expect_item(item_id);
     debug!("convert: item {} with id {}", it.name, it.id);
@@ -448,13 +415,7 @@ fn convert_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_id: ast::NodeId) {
                 convert_variant_ctor(tcx, struct_def.id());
             }
         },
-        hir::ItemTy(_, ref generics) => {
-            ensure_no_param_bounds(tcx, it.span, generics, "type aliases");
-            tcx.generics_of(def_id);
-            tcx.type_of(def_id);
-            tcx.predicates_of(def_id);
-        }
-        hir::ItemStatic(..) | hir::ItemConst(..) | hir::ItemFn(..) => {
+        hir::ItemTy(..) | hir::ItemStatic(..) | hir::ItemConst(..) | hir::ItemFn(..) => {
             tcx.generics_of(def_id);
             tcx.type_of(def_id);
             tcx.predicates_of(def_id);

src/librustc_typeck/diagnostics.rs

@@ -1524,26 +1524,6 @@ static BAR: _ = "test"; // error, explicitly write out the type instead
 ```
 "##,
 
-E0122: r##"
-An attempt was made to add a generic constraint to a type alias. This constraint
-is entirely ignored. For backwards compatibility, Rust still allows this with a
-warning. Consider the example below:
-
-```
-trait Foo{}
-
-type MyType<R: Foo> = (R, ());
-
-fn main() {
-    let t: MyType<u32>;
-}
-```
-
-We're able to declare a variable of type `MyType<u32>`, despite the fact that
-`u32` does not implement `Foo`. As a result, one should avoid using generic
-constraints in concert with type aliases.
-"##,
-
 E0124: r##"
 You declared two fields of a struct with the same name. Erroneous code
 example:
@@ -4757,6 +4737,7 @@ register_diagnostics! {
 //  E0086,
 //  E0103,
 //  E0104,
+//  E0122, // bounds in type aliases are ignored, turned into proper lint
 //  E0123,
 //  E0127,
 //  E0129,

src/libsyntax/ast.rs

@@ -294,6 +294,15 @@ pub enum TyParamBound {
     RegionTyParamBound(Lifetime)
 }
 
+impl TyParamBound {
+    pub fn span(&self) -> Span {
+        match self {
+            &TraitTyParamBound(ref t, ..) => t.span,
+            &RegionTyParamBound(ref l) => l.span,
+        }
+    }
+}
+
 /// A modifier on a bound, currently this is only used for `?Sized`, where the
 /// modifier is `Maybe`. Negative bounds should also be handled here.
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
@@ -404,6 +413,16 @@ pub enum WherePredicate {
     EqPredicate(WhereEqPredicate),
 }
 
+impl WherePredicate {
+    pub fn span(&self) -> Span {
+        match self {
+            &WherePredicate::BoundPredicate(ref p) => p.span,
+            &WherePredicate::RegionPredicate(ref p) => p.span,
+            &WherePredicate::EqPredicate(ref p) => p.span,
+        }
+    }
+}
+
 /// A type bound.
 ///
 /// E.g. `for<'c> Foo: Send+Clone+'c`

src/test/compile-fail/dst-bad-assign-3.rs

@@ -12,8 +12,7 @@
 
 #![feature(unsized_tuple_coercion)]
 
-type Fat<T: ?Sized> = (isize, &'static str, T);
-//~^ WARNING bounds are ignored
+type Fat<T> = (isize, &'static str, T);
 
 #[derive(PartialEq,Eq)]
 struct Bar;

src/test/compile-fail/issue-17994.rs

@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![warn(ignored_generic_bounds)]
+
 trait Tr {}
 type Huh<T> where T: Tr = isize; //~  ERROR type parameter `T` is unused
-                                 //~| WARNING E0122
+                                 //~| WARNING where clauses are ignored in type aliases
 fn main() {}

src/test/compile-fail/issue-23046.rs

@@ -10,15 +10,15 @@
 
 pub enum Expr<'var, VAR> {
     Let(Box<Expr<'var, VAR>>,
-        Box<for<'v: 'var> Fn(Expr<'v, VAR>) -> Expr<'v, VAR> + 'var>)
+        Box<for<'v> Fn(Expr<'v, VAR>) -> Expr<'v, VAR> + 'var>)
 }
 
 pub fn add<'var, VAR>
                       (a: Expr<'var, VAR>, b: Expr<'var, VAR>) -> Expr<'var, VAR> {
     loop {}
 }
 
-pub fn let_<'var, VAR, F: for<'v: 'var> Fn(Expr<'v, VAR>) -> Expr<'v, VAR>>
+pub fn let_<'var, VAR, F: for<'v> Fn(Expr<'v, VAR>) -> Expr<'v, VAR>>
                        (a: Expr<'var, VAR>, b: F) -> Expr<'var, VAR> {
     loop {}
 }

src/test/compile-fail/issue-39122.rs

@@ -8,6 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-type Foo<T: std::ops::Add> = T; //~ WARNING E0122
+type Foo<T: std::ops::Add> = T; //~ ERROR bounds on generic type parameters are ignored
 
-type Bar<T> where T: std::ops::Add = T; //~ WARNING E0122
+type Bar<T> where T: std::ops::Add = T; //~ ERROR where clauses are ignored

src/test/compile-fail/private-in-public-warn.rs

@@ -14,6 +14,7 @@
 #![feature(associated_type_defaults)]
 #![deny(private_in_public)]
 #![allow(improper_ctypes)]
+#![warn(ignored_generic_bounds)]
 
 mod types {
     struct Priv;
@@ -58,7 +59,7 @@ mod traits {
     pub trait PubTr {}
 
     pub type Alias<T: PrivTr> = T; //~ ERROR private trait `traits::PrivTr` in public interface
-    //~^ WARN bounds are ignored in type aliases
+    //~^ WARNING bounds on generic type parameters are ignored
     //~| WARNING hard error
     pub trait Tr1: PrivTr {} //~ ERROR private trait `traits::PrivTr` in public interface
     //~^ WARNING hard error
@@ -85,7 +86,7 @@ mod traits_where {
     pub type Alias<T> where T: PrivTr = T;
         //~^ ERROR private trait `traits_where::PrivTr` in public interface
         //~| WARNING hard error
-        //~| WARNING E0122
+        //~| WARNING where clauses are ignored in type aliases
     pub trait Tr2<T> where T: PrivTr {}
         //~^ ERROR private trait `traits_where::PrivTr` in public interface
         //~| WARNING hard error

src/test/compile-fail/rfc1623.rs

@@ -22,7 +22,7 @@ static NON_ELIDABLE_FN: &fn(&u8, &u8) -> &u8 =
 struct SomeStruct<'x, 'y, 'z: 'x> {
     foo: &'x Foo<'z>,
     bar: &'x Bar<'z>,
-    f: &'y for<'a, 'b: 'a> Fn(&'a Foo<'b>) -> &'a Bar<'b>,
+    f: &'y for<'a, 'b> Fn(&'a Foo<'b>) -> &'a Bar<'b>,
 }
 
 fn id<T>(t: T) -> T {