From 36363e535a6efb56cc7ef57e5231f526843d3c68 Mon Sep 17 00:00:00 2001
From: Daiki Ihara <sasurau4@gmail.com>
Date: Fri, 4 Dec 2020 21:18:01 +0900
Subject: [PATCH 1/3] Refine E0212 error message

---
 compiler/rustc_typeck/src/collect.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 0ff10abb60a..38da1e5ea03 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -359,8 +359,8 @@ impl AstConv<'tcx> for ItemCtxt<'tcx> {
                 self.tcx().sess,
                 span,
                 E0212,
-                "cannot extract an associated type from a higher-ranked trait bound \
-                 in this context"
+                "cannot use the associated type of a trait \
+                 with uninferred generic parameters"
             );
 
             match self.node() {

From a1e94cdcd509779163eff165bd346f7f40607542 Mon Sep 17 00:00:00 2001
From: Daiki Ihara <sasurau4@gmail.com>
Date: Thu, 3 Dec 2020 00:23:01 +0900
Subject: [PATCH 2/3] Add long explanation for E0212

Update compiler/rustc_error_codes/src/error_codes/E0212.md

Co-authored-by: Joshua Nelson <joshua@yottadb.com>
---
 compiler/rustc_error_codes/src/error_codes.rs |  2 +-
 .../src/error_codes/E0212.md                  | 35 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 compiler/rustc_error_codes/src/error_codes/E0212.md

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 636c37142ad..fef6602b9cc 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -111,6 +111,7 @@ E0206: include_str!("./error_codes/E0206.md"),
 E0207: include_str!("./error_codes/E0207.md"),
 E0210: include_str!("./error_codes/E0210.md"),
 E0211: include_str!("./error_codes/E0211.md"),
+E0212: include_str!("./error_codes/E0212.md"),
 E0214: include_str!("./error_codes/E0214.md"),
 E0220: include_str!("./error_codes/E0220.md"),
 E0221: include_str!("./error_codes/E0221.md"),
@@ -503,7 +504,6 @@ E0779: include_str!("./error_codes/E0779.md"),
 //  E0196, // cannot determine a type for this closure
     E0208,
 //  E0209, // builtin traits can only be implemented on structs or enums
-    E0212, // cannot extract an associated type from a higher-ranked trait bound
 //  E0213, // associated types are not accepted in this context
 //  E0215, // angle-bracket notation is not stable with `Fn`
 //  E0216, // parenthetical notation is only stable with `Fn`
diff --git a/compiler/rustc_error_codes/src/error_codes/E0212.md b/compiler/rustc_error_codes/src/error_codes/E0212.md
new file mode 100644
index 00000000000..17465414650
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0212.md
@@ -0,0 +1,35 @@
+Cannot use the associated type of
+a trait with uninferred generic parameters.
+
+Erroneous code example:
+
+```compile_fail,E0212
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+fn foo2<I : for<'x> Foo<&'x isize>>(
+    field: I::A) {} // error!
+```
+
+In this example, we have to instantiate `'x`, and
+we don't know what lifetime to instantiate it with.
+To fix this, spell out the precise lifetimes involved.
+Example:
+
+```
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+fn foo3<I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&isize>>::A) {} // ok!
+
+
+fn foo4<'a, I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&'a isize>>::A) {} // ok!
+```

From 87c621690a00c96e9f2874ba318ee46db98b7ab5 Mon Sep 17 00:00:00 2001
From: Daiki Ihara <sasurau4@gmail.com>
Date: Thu, 3 Dec 2020 00:38:02 +0900
Subject: [PATCH 3/3] Update some associated-types ui test suites

---
 .../associated-types-project-from-hrtb-in-fn.fixed       | 2 +-
 .../associated-types-project-from-hrtb-in-fn.rs          | 2 +-
 .../associated-types-project-from-hrtb-in-fn.stderr      | 3 ++-
 .../associated-types-project-from-hrtb-in-struct.rs      | 8 ++++----
 .../associated-types-project-from-hrtb-in-struct.stderr  | 9 +++++----
 ...ociated-types-project-from-hrtb-in-trait-method.fixed | 4 ++--
 ...associated-types-project-from-hrtb-in-trait-method.rs | 4 ++--
 ...ciated-types-project-from-hrtb-in-trait-method.stderr | 5 +++--
 8 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
index 760d2b433c8..bca69a97677 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 fn foo2<I : for<'x> Foo<&'x isize>>(
     x: <I as Foo<&isize>>::A)
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 {
     // This case is illegal because we have to instantiate `'x`, and
     // we don't know what region to instantiate it with.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
index 6eb584ea645..1e23dd8890b 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 fn foo2<I : for<'x> Foo<&'x isize>>(
     x: I::A)
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 {
     // This case is illegal because we have to instantiate `'x`, and
     // we don't know what region to instantiate it with.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
index f2137f68665..989624bdd93 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr
@@ -1,4 +1,4 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-fn.rs:13:8
    |
 LL |     x: I::A)
@@ -6,3 +6,4 @@ LL |     x: I::A)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0212`.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
index 58f186d7775..ed30d86cb5b 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs
@@ -9,14 +9,14 @@ pub trait Foo<T> {
 
 struct SomeStruct<I: for<'x> Foo<&'x isize>> {
     field: I::A
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 enum SomeEnum<'b, I: for<'a> Foo<&'a isize>> {
     TupleVariant(I::A),
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
     StructVariant { field: I::A },
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
     OkVariant(&'b usize),
 }
 
@@ -33,7 +33,7 @@ struct YetAnotherStruct<'a, I: for<'x> Foo<&'x isize>> {
 struct Why<'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'n, 'o, 'p, 'q, 'r, 's, 't, 'u, 'v, 'w, 'x,
     'y, 'z, 'aa, I: for<'l, 'm> Foo<&'l &'m isize>> {
     field: I::A,
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 pub fn main() {}
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
index e3fd2860ebc..cadc3e9eab1 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr
@@ -1,4 +1,4 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:11:12
    |
 LL |     field: I::A
@@ -10,7 +10,7 @@ LL | struct SomeStruct<'a, I: for<'x> Foo<&'x isize>> {
 LL |     field: <I as Foo<&'a isize>>::A
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:16:18
    |
 LL |     TupleVariant(I::A),
@@ -22,7 +22,7 @@ LL | enum SomeEnum<'c, 'b, I: for<'a> Foo<&'a isize>> {
 LL |     TupleVariant(<I as Foo<&'c isize>>::A),
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:18:28
    |
 LL |     StructVariant { field: I::A },
@@ -36,7 +36,7 @@ LL |
 LL |     StructVariant { field: <I as Foo<&'c isize>>::A },
    |
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-struct.rs:35:12
    |
 LL |     field: I::A,
@@ -51,3 +51,4 @@ LL |     field: <I as Foo<&'bb &'bb isize>>::A,
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0212`.
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
index acf32bccbec..66d8613f184 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 trait SomeTrait<I : for<'x> Foo<&'x isize>> {
     fn some_method(&self, arg: <I as Foo<&isize>>::A);
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 trait AnotherTrait<I : for<'x> Foo<&'x isize>> {
@@ -30,7 +30,7 @@ struct Peach<X>(std::marker::PhantomData<X>);
 
 impl<X: for<'a> Banana<'a>> Peach<X> {
     fn mango(&self) -> <X as Banana<'_>>::Assoc {
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
         Default::default()
     }
 }
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
index a249f89685e..0a1b29de19e 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs
@@ -11,7 +11,7 @@ pub trait Foo<T> {
 
 trait SomeTrait<I : for<'x> Foo<&'x isize>> {
     fn some_method(&self, arg: I::A);
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
 }
 
 trait AnotherTrait<I : for<'x> Foo<&'x isize>> {
@@ -30,7 +30,7 @@ struct Peach<X>(std::marker::PhantomData<X>);
 
 impl<X: for<'a> Banana<'a>> Peach<X> {
     fn mango(&self) -> X::Assoc {
-    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    //~^ ERROR cannot use the associated type of a trait with uninferred generic parameters
         Default::default()
     }
 }
diff --git a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
index a37fec24493..d457f9f8468 100644
--- a/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
+++ b/src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr
@@ -1,10 +1,10 @@
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:13:32
    |
 LL |     fn some_method(&self, arg: I::A);
    |                                ^^^^ help: use a fully qualified path with inferred lifetimes: `<I as Foo<&isize>>::A`
 
-error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+error[E0212]: cannot use the associated type of a trait with uninferred generic parameters
   --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:32:24
    |
 LL |     fn mango(&self) -> X::Assoc {
@@ -12,3 +12,4 @@ LL |     fn mango(&self) -> X::Assoc {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0212`.