Rollup merge of #48909 - RalfJung:type_alias_bounds, r=petrochenkov

Improve lint for type alias bounds

First of all, I learned just today that I was wrong assuming that the bounds in type aliases are entirely ignored: It turns out they are used to resolve associated types in type aliases. So:
```rust
type T1<U: Bound> = U::Assoc; // compiles
type T2<U> = U::Assoc; // fails
type T3<U> = <U as Bound>::Assoc; // "correct" way to write this, maybe?
```
I am sorry for creating this mess.

This PR changes the wording of the lint accordingly. Moreover, since just removing the bound is no longer always a possible fix, I tried to detect cases like `T1` above and show a helpful message to the user:
```
warning: bounds on generic parameters are not enforced in type aliases
  --> $DIR/type-alias-bounds.rs:57:12
   |
LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
   |            ^^^^^
   |
   = help: the bound will not be checked when the type alias is used, and should be removed
help: use absolute paths (i.e., <T as Trait>::Assoc) to refer to associated types in type aliases
  --> $DIR/type-alias-bounds.rs:57:21
   |
LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
   |                     ^^^^^^^^
```
I am not sure if I got this entirely right. Ideally, we could provide a suggestion involving the correct trait and type name -- however, while I have access to the HIR in the lint, I do not know how to get access to the resolved name information, like which trait `Assoc` belongs to above. The lint does not even run if that resolution fails, so I assume that information is available *somewhere*...

This is a follow-up for (parts of) https://github.com/rust-lang/rust/pull/48326. Also see https://github.com/rust-lang/rust/issues/21903.

This changes the name of a lint, but that lint was just merged to master yesterday and has never even been on beta.

6 files changed, 164 insertions, 65 deletions

diff --git a/src/test/compile-fail/issue-17994.rs b/src/test/compile-fail/issue-17994.rs
index 0f30e2461cf..25141b9b825 100644
--- a/src/test/compile-fail/issue-17994.rs
+++ b/src/test/compile-fail/issue-17994.rs
@@ -10,5 +10,4 @@
 
 trait Tr {}
 type Huh<T> where T: Tr = isize; //~  ERROR type parameter `T` is unused
-                                 //~| WARNING where clauses are ignored in type aliases
 fn main() {}
diff --git a/src/test/compile-fail/private-in-public-warn.rs b/src/test/compile-fail/private-in-public-warn.rs
index cc9eed7e654..6eeb14638e7 100644
--- a/src/test/compile-fail/private-in-public-warn.rs
+++ b/src/test/compile-fail/private-in-public-warn.rs
@@ -58,7 +58,6 @@ mod traits {
     pub trait PubTr {}
 
     pub type Alias<T: PrivTr> = T; //~ ERROR private trait `traits::PrivTr` in public interface
-    //~^ WARNING bounds on generic parameters are ignored
     //~| WARNING hard error
     pub trait Tr1: PrivTr {} //~ ERROR private trait `traits::PrivTr` in public interface
     //~^ WARNING hard error
@@ -85,7 +84,6 @@ mod traits_where {
     pub type Alias<T> where T: PrivTr = T;
         //~^ ERROR private trait `traits_where::PrivTr` in public interface
         //~| WARNING hard error
-        //~| 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
diff --git a/src/test/ui/param-bounds-ignored.rs b/src/test/ui/higher-lifetime-bounds.rs
index 94bcdec9450..70b3b34fbd8 100644
--- a/src/test/ui/param-bounds-ignored.rs
+++ b/src/test/ui/higher-lifetime-bounds.rs
@@ -10,31 +10,7 @@
 
 #![allow(dead_code, non_camel_case_types)]
 
-use std::rc::Rc;
-
-type SVec<T: Send+Send> = Vec<T>;
-//~^ WARN bounds on generic parameters are ignored in type aliases
-type VVec<'b, 'a: 'b+'b> = Vec<&'a i32>;
-//~^ WARN bounds on generic parameters are ignored in type aliases
-type WVec<'b, T: 'b+'b> = Vec<T>;
-//~^ WARN bounds on generic parameters are ignored in type aliases
-type W2Vec<'b, T> where T: 'b, T: 'b = Vec<T>;
-//~^ WARN where clauses are ignored in type aliases
-
-fn foo<'a>(y: &'a i32) {
-    // If the bounds above would matter, the code below would be rejected.
-    let mut x : SVec<_> = Vec::new();
-    x.push(Rc::new(42));
-
-    let mut x : VVec<'static, 'a> = Vec::new();
-    x.push(y);
-
-    let mut x : WVec<'static, & 'a i32> = Vec::new();
-    x.push(y);
-
-    let mut x : W2Vec<'static, & 'a i32> = Vec::new();
-    x.push(y);
-}
+// Test that bounds on higher-kinded lifetime binders are rejected.
 
 fn bar1<'a, 'b>(
     x: &'a i32,
diff --git a/src/test/ui/param-bounds-ignored.stderr b/src/test/ui/higher-lifetime-bounds.stderr
index 657fec54f96..82c00747436 100644
--- a/src/test/ui/param-bounds-ignored.stderr
+++ b/src/test/ui/higher-lifetime-bounds.stderr
@@ -1,94 +1,68 @@
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:42:22
+  --> $DIR/higher-lifetime-bounds.rs:18:22
    |
 LL |     f: for<'xa, 'xb: 'xa+'xa> fn(&'xa i32, &'xb i32) -> &'xa i32)
    |                      ^^^ ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:50:34
+  --> $DIR/higher-lifetime-bounds.rs:26:34
    |
 LL | fn bar2<'a, 'b, F: for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32>(
    |                                  ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:65:28
+  --> $DIR/higher-lifetime-bounds.rs:41:28
    |
 LL |     where F: for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32
    |                            ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:77:25
+  --> $DIR/higher-lifetime-bounds.rs:53:25
    |
 LL |     where for<'xa, 'xb: 'xa> F: Fn(&'xa i32, &'xb i32) -> &'xa i32
    |                         ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:85:28
+  --> $DIR/higher-lifetime-bounds.rs:61:28
    |
 LL | struct S1<F: for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32>(F);
    |                            ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:87:40
+  --> $DIR/higher-lifetime-bounds.rs:63:40
    |
 LL | struct S2<F>(F) where F: for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32;
    |                                        ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:89:37
+  --> $DIR/higher-lifetime-bounds.rs:65:37
    |
 LL | struct S3<F>(F) where for<'xa, 'xb: 'xa> F: Fn(&'xa i32, &'xb i32) -> &'xa i32;
    |                                     ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:92:29
+  --> $DIR/higher-lifetime-bounds.rs:68:29
    |
 LL | struct S_fnty(for<'xa, 'xb: 'xa> fn(&'xa i32, &'xb i32) -> &'xa i32);
    |                             ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:95:29
+  --> $DIR/higher-lifetime-bounds.rs:71:29
    |
 LL | type T1 = Box<for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32>;
    |                             ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:99:34
+  --> $DIR/higher-lifetime-bounds.rs:75:34
    |
 LL |     let _ : Option<for<'xa, 'xb: 'xa> fn(&'xa i32, &'xb i32) -> &'xa i32> = None;
    |                                  ^^^
 
 error: lifetime bounds cannot be used in this context
-  --> $DIR/param-bounds-ignored.rs:101:38
+  --> $DIR/higher-lifetime-bounds.rs:77:38
    |
 LL |     let _ : Option<Box<for<'xa, 'xb: 'xa> Fn(&'xa i32, &'xb i32) -> &'xa i32>> = None;
    |                                      ^^^
 
-warning: bounds on generic parameters are ignored in type aliases
-  --> $DIR/param-bounds-ignored.rs:15:14
-   |
-LL | type SVec<T: Send+Send> = Vec<T>;
-   |              ^^^^ ^^^^
-   |
-   = note: #[warn(ignored_generic_bounds)] on by default
-
-warning: bounds on generic parameters are ignored in type aliases
-  --> $DIR/param-bounds-ignored.rs:17:19
-   |
-LL | type VVec<'b, 'a: 'b+'b> = Vec<&'a i32>;
-   |                   ^^ ^^
-
-warning: bounds on generic parameters are ignored in type aliases
-  --> $DIR/param-bounds-ignored.rs:19:18
-   |
-LL | type WVec<'b, T: 'b+'b> = Vec<T>;
-   |                  ^^ ^^
-
-warning: where clauses are ignored in type aliases
-  --> $DIR/param-bounds-ignored.rs:21:25
-   |
-LL | type W2Vec<'b, T> where T: 'b, T: 'b = Vec<T>;
-   |                         ^^^^^  ^^^^^
-
 error: aborting due to 11 previous errors
 
diff --git a/src/test/ui/type-alias-bounds.rs b/src/test/ui/type-alias-bounds.rs
new file mode 100644
index 00000000000..c1cdeef3a46
--- /dev/null
+++ b/src/test/ui/type-alias-bounds.rs
@@ -0,0 +1,69 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test ignored_generic_bounds lint warning about bounds in type aliases
+
+// must-compile-successfully
+#![allow(dead_code)]
+
+use std::rc::Rc;
+
+type SVec<T: Send+Send> = Vec<T>;
+//~^ WARN bounds on generic parameters are not enforced in type aliases [type_alias_bounds]
+type S2Vec<T> where T: Send = Vec<T>;
+//~^ WARN where clauses are not enforced in type aliases [type_alias_bounds]
+type VVec<'b, 'a: 'b+'b> = (&'b u32, Vec<&'a i32>);
+//~^ WARN bounds on generic parameters are not enforced in type aliases [type_alias_bounds]
+type WVec<'b, T: 'b+'b> = (&'b u32, Vec<T>);
+//~^ WARN bounds on generic parameters are not enforced in type aliases [type_alias_bounds]
+type W2Vec<'b, T> where T: 'b, T: 'b = (&'b u32, Vec<T>);
+//~^ WARN where clauses are not enforced in type aliases [type_alias_bounds]
+
+static STATIC : u32 = 0;
+
+fn foo<'a>(y: &'a i32) {
+    // If any of the bounds above would matter, the code below would be rejected.
+    // This can be seen when replacing the type aliases above by newtype structs.
+    // (The type aliases have no unused parameters to make that a valid transformation.)
+    let mut x : SVec<_> = Vec::new();
+    x.push(Rc::new(42)); // is not send
+
+    let mut x : S2Vec<_> = Vec::new();
+    x.push(Rc::new(42)); // is not send
+
+    let mut x : VVec<'static, 'a> = (&STATIC, Vec::new());
+    x.1.push(y); // 'a: 'static does not hold
+
+    let mut x : WVec<'static, &'a i32> = (&STATIC, Vec::new());
+    x.1.push(y); // &'a i32: 'static does not hold
+
+    let mut x : W2Vec<'static, &'a i32> = (&STATIC, Vec::new());
+    x.1.push(y); // &'a i32: 'static does not hold
+}
+
+// Bounds are not checked either, i.e. the definition is not necessarily well-formed
+struct Sendable<T: Send>(T);
+type MySendable<T> = Sendable<T>; // no error here!
+
+// However, bounds *are* taken into account when accessing associated types
+trait Bound { type Assoc; }
+type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
+type T2<U> where U: Bound = U::Assoc;  //~ WARN not enforced in type aliases
+
+// This errors
+// type T3<U> = U::Assoc;
+// Do this instead
+type T4<U> = <U as Bound>::Assoc;
+
+// Make sure the help about associatd types is not shown incorrectly
+type T5<U: Bound> = <U as Bound>::Assoc;  //~ WARN not enforced in type aliases
+type T6<U: Bound> = ::std::vec::Vec<U>;  //~ WARN not enforced in type aliases
+
+fn main() {}
diff --git a/src/test/ui/type-alias-bounds.stderr b/src/test/ui/type-alias-bounds.stderr
new file mode 100644
index 00000000000..2a2b0b0f26e
--- /dev/null
+++ b/src/test/ui/type-alias-bounds.stderr
@@ -0,0 +1,83 @@
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:18:14
+   |
+LL | type SVec<T: Send+Send> = Vec<T>;
+   |              ^^^^ ^^^^
+   |
+   = note: #[warn(type_alias_bounds)] on by default
+   = help: the bound will not be checked when the type alias is used, and should be removed
+
+warning: where clauses are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:20:21
+   |
+LL | type S2Vec<T> where T: Send = Vec<T>;
+   |                     ^^^^^^^
+   |
+   = help: the clause will not be checked when the type alias is used, and should be removed
+
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:22:19
+   |
+LL | type VVec<'b, 'a: 'b+'b> = (&'b u32, Vec<&'a i32>);
+   |                   ^^ ^^
+   |
+   = help: the bound will not be checked when the type alias is used, and should be removed
+
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:24:18
+   |
+LL | type WVec<'b, T: 'b+'b> = (&'b u32, Vec<T>);
+   |                  ^^ ^^
+   |
+   = help: the bound will not be checked when the type alias is used, and should be removed
+
+warning: where clauses are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:26:25
+   |
+LL | type W2Vec<'b, T> where T: 'b, T: 'b = (&'b u32, Vec<T>);
+   |                         ^^^^^  ^^^^^
+   |
+   = help: the clause will not be checked when the type alias is used, and should be removed
+
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:57:12
+   |
+LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
+   |            ^^^^^
+   |
+   = help: the bound will not be checked when the type alias is used, and should be removed
+help: use fully disambiguated paths (i.e., `<T as Trait>::Assoc`) to refer to associated types in type aliases
+  --> $DIR/type-alias-bounds.rs:57:21
+   |
+LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
+   |                     ^^^^^^^^
+
+warning: where clauses are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:58:18
+   |
+LL | type T2<U> where U: Bound = U::Assoc;  //~ WARN not enforced in type aliases
+   |                  ^^^^^^^^
+   |
+   = help: the clause will not be checked when the type alias is used, and should be removed
+help: use fully disambiguated paths (i.e., `<T as Trait>::Assoc`) to refer to associated types in type aliases
+  --> $DIR/type-alias-bounds.rs:58:29
+   |
+LL | type T2<U> where U: Bound = U::Assoc;  //~ WARN not enforced in type aliases
+   |                             ^^^^^^^^
+
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:66:12
+   |
+LL | type T5<U: Bound> = <U as Bound>::Assoc;  //~ WARN not enforced in type aliases
+   |            ^^^^^
+   |
+   = help: the bound will not be checked when the type alias is used, and should be removed
+
+warning: bounds on generic parameters are not enforced in type aliases
+  --> $DIR/type-alias-bounds.rs:67:12
+   |
+LL | type T6<U: Bound> = ::std::vec::Vec<U>;  //~ WARN not enforced in type aliases
+   |            ^^^^^
+   |
+   = help: the bound will not be checked when the type alias is used, and should be removed
+