Rollup merge of #24966 - ruud-v-a:explain, r=pnkfelix

The error message was misleading, so I adjusted it, and I also added the long diagnostics for this error (resolves one point in #24407).

I was unsure about how to phrase the error message. Is “generic parameter binding” the correct term for this?

src/librustc/diagnostics.rs

@@ -419,6 +419,74 @@ enum Method { GET, POST }
 be taken.
 "##,
 
+E0282: r##"
+This error indicates that type inference did not result in one unique possible
+type, and extra information is required. In most cases this can be provided
+by adding a type annotation. Sometimes you need to specify a generic type
+parameter manually.
+
+A common example is the `collect` method on `Iterator`. It has a generic type
+parameter with a `FromIterator` bound, which for a `char` iterator is
+implemented by `Vec` and `String` among others. Consider the following snippet
+that reverses the characters of a string:
+
+```
+let x = "hello".chars().rev().collect();
+```
+
+In this case, the compiler cannot infer what the type of `x` should be:
+`Vec<char>` and `String` are both suitable candidates. To specify which type to
+use, you can use a type annotation on `x`:
+
+```
+let x: Vec<char> = "hello".chars().rev().collect();
+```
+
+It is not necessary to annotate the full type. Once the ambiguity is resolved,
+the compiler can infer the rest:
+
+```
+let x: Vec<_> = "hello".chars().rev().collect();
+```
+
+Another way to provide the compiler with enough information, is to specify the
+generic type parameter:
+
+```
+let x = "hello".chars().rev().collect::<Vec<char>>();
+```
+
+Again, you need not specify the full type if the compiler can infer it:
+
+```
+let x = "hello".chars().rev().collect::<Vec<_>>();
+```
+
+Apart from a method or function with a generic type parameter, this error can
+occur when a type parameter of a struct or trait cannot be inferred. In that
+case it is not always possible to use a type annotation, because all candidates
+have the same return type. For instance:
+
+```
+struct Foo<T> {
+    // Some fields omitted.
+}
+
+impl<T> Foo<T> {
+    fn bar() -> i32 {
+        0
+    }
+
+    fn baz() {
+        let number = Foo::bar();
+    }
+}
+```
+
+This will fail because the compiler does not know which instance of `Foo` to
+call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
+"##,
+
 E0296: r##"
 This error indicates that the given recursion limit could not be parsed. Ensure
 that the value provided is a positive integer between quotes, like so:
@@ -617,7 +685,6 @@ struct Foo<T: 'static> {
     E0279, // requirement is not satisfied
     E0280, // requirement is not satisfied
     E0281, // type implements trait but other trait is required
-    E0282, // unable to infer enough type information about
     E0283, // cannot resolve type
     E0284, // cannot resolve type
     E0285, // overflow evaluation builtin bounds

src/librustc/middle/traits/error_reporting.rs

@@ -290,7 +290,7 @@ pub fn maybe_report_ambiguity<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
                     {
                         span_err!(infcx.tcx.sess, obligation.cause.span, E0282,
                                 "unable to infer enough type information about `{}`; \
-                                 type annotations required",
+                                 type annotations or generic parameter binding required",
                                 self_ty.user_string(infcx.tcx));
                     } else {
                         span_err!(infcx.tcx.sess, obligation.cause.span, E0283,

src/test/compile-fail/issue-12187-1.rs

-// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -14,5 +14,5 @@ fn new<T>() -> &'static T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/issue-12187-2.rs

-// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -14,5 +14,5 @@ fn new<'r, T>() -> &'r T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/issue-5062.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -9,4 +9,4 @@
 // except according to those terms.
 
 fn main() { format!("{:?}", None); }
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required

src/test/compile-fail/issue-6458-2.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -11,5 +11,5 @@
 fn main() {
     // Unconstrained type:
     format!("{:?}", None);
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/issue-6458-3.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -12,5 +12,5 @@
 
 fn main() {
     mem::transmute(0);
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/issue-6458-4.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -10,7 +10,7 @@
 
 fn foo(b: bool) -> Result<bool,String> {
     Err("bar".to_string());
-    //~^ ERROR type annotations required
+    //~^ ERROR type annotations or generic parameter binding required
 }
 
 fn main() {

src/test/compile-fail/issue-6458.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -16,7 +16,8 @@
 pub fn foo<State>(_: TypeWithState<State>) {}
 
 pub fn bar() {
-   foo(TypeWithState(marker::PhantomData));  //~ ERROR type annotations required
+   foo(TypeWithState(marker::PhantomData));
+   //~^ ERROR type annotations or generic parameter binding required
 }
 
 fn main() {

src/test/compile-fail/issue-7813.rs

-// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -10,5 +10,5 @@
 
 fn main() {
     let v = &[];
-    let it = v.iter(); //~ ERROR type annotations required
+    let it = v.iter(); //~ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/method-ambig-one-trait-unknown-int-type.rs

-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -31,7 +31,8 @@ fn foo(&self) -> isize {2}
 
 fn m1() {
     // we couldn't infer the type of the vector just based on calling foo()...
-    let mut x = Vec::new(); //~ ERROR type annotations required
+    let mut x = Vec::new();
+    //~^ ERROR type annotations or generic parameter binding required
     x.foo();
 }
 

src/test/compile-fail/traits-multidispatch-convert-ambig-dest.rs

-// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -33,7 +33,8 @@ fn test<T,U>(_: T, _: U)
 }
 
 fn a() {
-    test(22, std::default::Default::default()); //~ ERROR type annotations required
+    test(22, std::default::Default::default());
+    //~^ ERROR type annotations or generic parameter binding required
 }
 
 fn main() {}

src/test/compile-fail/unconstrained-none.rs

-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -11,5 +11,5 @@
 // Issue #5062
 
 fn main() {
-    None; //~ ERROR type annotations required
+    None; //~ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/unconstrained-ref.rs

-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -13,5 +13,5 @@ struct S<'a, T:'a> {
 }
 
 fn main() {
-    S { o: &None }; //~ ERROR type annotations required
+    S { o: &None }; //~ ERROR type annotations or generic parameter binding required
 }

src/test/compile-fail/vector-no-ann.rs

-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -10,5 +10,6 @@
 
 
 fn main() {
-    let _foo = Vec::new(); //~ ERROR type annotations required
+    let _foo = Vec::new();
+    //~^ ERROR type annotations or generic parameter binding required
 }