Expand E0738 to cover different cases.

src/librustc_typeck/check/wfcheck.rs

@@ -180,7 +180,7 @@ pub fn check_trait_item(tcx: TyCtxt<'_>, def_id: DefId) {
                 tcx.sess,
                 attr.span,
                 E0738,
-                "`#[track_caller]` is not supported for trait items yet."
+                "`#[track_caller]` is not supported in trait declarations."
             ).emit();
         }
     }

src/librustc_typeck/error_codes.rs

@@ -4926,12 +4926,11 @@ extern "C" fn foo() {}
 "##,
 
 E0738: r##"
-#[track_caller] cannot be applied to trait methods.
+#[track_caller] cannot be used in traits yet.  This is due to limitations in the
+compiler which are likely to be temporary. See [RFC 2091] for details on this
+and other restrictions.
 
-This is due to limitations in the compiler which are likely to be temporary.
-See [RFC 2091] for details on this and other restrictions.
-
-Erroneous code example:
+Erroneous example with a trait method implementation:
 
 ```compile_fail,E0738
 #![feature(track_caller)]
@@ -4940,14 +4939,40 @@ trait Foo {
     fn bar(&self);
 }
 
-struct Bar;
-
-impl Foo for Bar {
+impl Foo for u64 {
     #[track_caller]
     fn bar(&self) {}
 }
 ```
 
+Erroneous example with a blanket trait method implementation:
+
+```compile_fail,E0738
+#![feature(track_caller)]
+
+trait Foo {
+    #[track_caller]
+    fn bar(&self) {}
+    fn baz(&self);
+}
+```
+
+Erroneous example with a trait method declaration:
+
+```compile_fail,E0738
+#[!feature(track_caller)]
+
+trait Foo {
+    fn bar(&self) {}
+
+    #[track_caller]
+    fn baz(&self);
+}
+```
+
+Note that while the compiler may be able to support the attribute in traits in
+the future, [RFC 2091] prohibits their implementation without a follow-up RFC.
+
 [RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md
 "##,
 

src/test/ui/rfc-2091-track-caller/error-with-trait-decl.rs

@@ -3,7 +3,7 @@
 trait Trait {
     #[track_caller]
     fn unwrap(&self);
-    //~^^ ERROR: `#[track_caller]` is not supported for trait items yet.
+    //~^^ ERROR: `#[track_caller]` is not supported in traits yet.
 }
 
 impl Trait for u64 {

src/test/ui/rfc-2091-track-caller/error-with-trait-decl.stderr

@@ -0,0 +1,17 @@
+warning: the feature `track_caller` is incomplete and may cause the compiler to crash
+  --> $DIR/error-with-trait-decl.rs:1:12
+   |
+LL | #![feature(track_caller)]
+   |            ^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0738]: `#[track_caller]` is not supported in trait declarations.
+  --> $DIR/error-with-trait-decl.rs:4:5
+   |
+LL |     #[track_caller]
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0738`.

src/test/ui/rfc-2091-track-caller/error-with-trait-default-impl.rs

@@ -0,0 +1,9 @@
+#![feature(track_caller)] //~ WARN the feature `track_caller` is incomplete
+
+trait Trait {
+    #[track_caller]
+    fn unwrap(&self) {}
+    //~^^ ERROR: `#[track_caller]` is not supported in trait declarations.
+}
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-with-trait-default-impl.stderr

@@ -0,0 +1,17 @@
+warning: the feature `track_caller` is incomplete and may cause the compiler to crash
+  --> $DIR/error-with-trait-default-impl.rs:1:12
+   |
+LL | #![feature(track_caller)]
+   |            ^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0738]: `#[track_caller]` is not supported in traits yet.
+  --> $DIR/error-with-trait-default-impl.rs:4:5
+   |
+LL |     #[track_caller]
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0738`.

src/test/ui/rfc-2091-track-caller/error-with-trait-fn-impl.rs

@@ -0,0 +1,13 @@
+#![feature(track_caller)] //~ WARN the feature `track_caller` is incomplete
+
+trait Trait {
+    fn unwrap(&self);
+}
+
+impl Trait for u64 {
+    #[track_caller]
+    fn unwrap(&self) {}
+    //~^^ ERROR: `#[track_caller]` is not supported in traits yet.
+}
+
+fn main() {}

src/test/ui/rfc-2091-track-caller/error-with-trait-fn-impl.stderr

@@ -1,13 +1,13 @@
 warning: the feature `track_caller` is incomplete and may cause the compiler to crash
-  --> $DIR/error-with-trait-fns.rs:1:12
+  --> $DIR/error-with-trait-fn-impl.rs:1:12
    |
 LL | #![feature(track_caller)]
    |            ^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0738]: `#[track_caller]` is not supported for trait items yet.
-  --> $DIR/error-with-trait-fns.rs:4:5
+error[E0738]: `#[track_caller]` is not supported in traits yet.
+  --> $DIR/error-with-trait-fn-impl.rs:4:5
    |
 LL |     #[track_caller]
    |     ^^^^^^^^^^^^^^^