add long error explanation for E0625

compiler/rustc_error_codes/src/error_codes.rs

@@ -359,6 +359,7 @@ E0621: include_str!("./error_codes/E0621.md"),
 E0622: include_str!("./error_codes/E0622.md"),
 E0623: include_str!("./error_codes/E0623.md"),
 E0624: include_str!("./error_codes/E0624.md"),
+E0625: include_str!("./error_codes/E0625.md"),
 E0626: include_str!("./error_codes/E0626.md"),
 E0627: include_str!("./error_codes/E0627.md"),
 E0628: include_str!("./error_codes/E0628.md"),
@@ -622,7 +623,6 @@ E0783: include_str!("./error_codes/E0783.md"),
 //  E0611, // merged into E0616
 //  E0612, // merged into E0609
 //  E0613, // Removed (merged with E0609)
-    E0625, // thread-local statics cannot be accessed at compile-time
 //  E0629, // missing 'feature' (rustc_const_unstable)
 //  E0630, // rustc_const_unstable attribute must be paired with stable/unstable
            // attribute

compiler/rustc_error_codes/src/error_codes/E0625.md

@@ -0,0 +1,28 @@
+Static and const variables can refer to other const variables. But a const
+variable cannot refer to a thread-local static variable.
+
+Erroneous code example:
+
+```compile_fail,E0625
+#![feature(thread_local)]
+
+#[thread_local]
+static X: usize = 12;
+
+const Y: usize = 2 * X;
+```
+
+In this example, `Y` cannot refer to `X`. To fix this, the value can be
+extracted as a const and then used:
+
+```
+#![feature(thread_local)]
+
+const C: usize = 12;
+
+#[thread_local]
+static X: usize = C;
+
+const Y: usize = 2 * C;
+```
+

src/test/ui/thread-local-in-ctfe.stderr

@@ -30,3 +30,4 @@ LL |     A
 
 error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0625`.

src/test/ui/thread-local-static.stderr

@@ -40,5 +40,5 @@ LL |     std::mem::swap(x, &mut STATIC_VAR_2)
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0013, E0133, E0658.
+Some errors have detailed explanations: E0013, E0133, E0625, E0658.
 For more information about an error, try `rustc --explain E0013`.