defatalize ProcMacroDerive::expand

Also remove ExtCtxt::struct_span_fatal.

src/test/ui/proc-macro/derive-bad.rs

@@ -3,11 +3,9 @@
 #[macro_use]
 extern crate derive_bad;
 
-#[derive(
-    A
-)]
-//~^^ ERROR proc-macro derive produced unparseable tokens
+#[derive(A)]
+//~^ ERROR proc-macro derive produced unparseable tokens
 //~| ERROR expected `:`, found `}`
-struct A;
+struct A; //~ ERROR the name `A` is defined multiple times
 
 fn main() {}

src/test/ui/proc-macro/derive-bad.stderr

@@ -1,16 +1,28 @@
 error: expected `:`, found `}`
-  --> $DIR/derive-bad.rs:7:5
+  --> $DIR/derive-bad.rs:6:10
    |
-LL |     A
-   |     ^ expected `:`
+LL | #[derive(A)]
+   |          ^ expected `:`
    |
    = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: proc-macro derive produced unparseable tokens
-  --> $DIR/derive-bad.rs:7:5
+  --> $DIR/derive-bad.rs:6:10
    |
-LL |     A
-   |     ^
+LL | #[derive(A)]
+   |          ^
 
-error: aborting due to 2 previous errors
+error[E0428]: the name `A` is defined multiple times
+  --> $DIR/derive-bad.rs:9:1
+   |
+LL | #[derive(A)]
+   |          - previous definition of the type `A` here
+...
+LL | struct A;
+   | ^^^^^^^^^ `A` redefined here
+   |
+   = note: `A` must be defined only once in the type namespace of this module
 
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0428`.

src/test/ui/proc-macro/issue-36935.rs

@@ -5,6 +5,7 @@ extern crate test_macros;
 
 #[derive(Identity, Panic)] //~ ERROR proc-macro derive panicked
 struct Baz {
+    //~^ ERROR the name `Baz` is defined multiple times
     a: i32,
     b: i32,
 }

src/test/ui/proc-macro/issue-36935.stderr

@@ -6,5 +6,17 @@ LL | #[derive(Identity, Panic)]
    |
    = help: message: panic-derive
 
-error: aborting due to previous error
+error[E0428]: the name `Baz` is defined multiple times
+  --> $DIR/issue-36935.rs:7:1
+   |
+LL | struct Baz {
+   | ^^^^^^^^^^
+   | |
+   | `Baz` redefined here
+   | previous definition of the type `Baz` here
+   |
+   = note: `Baz` must be defined only once in the type namespace of this module
 
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0428`.