Add lint warning for inner function marked as #[test]

text/2471-lint-test-inner-function.md

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+- Feature Name: `lint_test_inner_function`
+- Start Date: 2018-06-10
+- RFC PR: [rust-lang/rfcs#2471](https://github.com/rust-lang/rfcs/pull/2471)
+- Rust Issue: [rust-lang/rust#53911](https://github.com/rust-lang/rust/issues/53911)
+
+# Summary
+[summary]: #summary
+
+Add a lint that warns when marking an inner function as `#[test]`.
+
+# Motivation
+[motivation]: #motivation
+
+`#[test]` is used to mark funcitons to be run as part of a test suite. The
+functions being marked need to be addressable for this to work. Currently,
+marking an inner function as `#[test]` will not raise any errors or warnings,
+but the test will silently not be run. By adding a lint that identifies these
+cases, users are less likely to fail to notice.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This is a lint that triggers when a `#[test]` annotation is found in a non
+addressable function, warning that that function cannot be tested.
+
+For example, in the following code, `bar` will never be called as part of a
+test run:
+
+```rust
+fn foo() {
+    #[test]
+    fn bar() {
+        assert!(true);
+    }
+}
+```
+
+The output should resemble the following:
+
+```
+error: cannot test inner function
+  --> $DIR/test-inner-fn.rs:15:5
+   |
+LL |     #[test] //~ ERROR cannot test inner function [untestable_method]
+   |     ^^^^^^^
+   |
+   = note: requested on the command line with `-D untestable-method`
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+This is a new lint that shouldn't interact with others. Due to the interaction
+with `cfg` attributes, the lint might only warn when run as part of a `--test`
+compilation. This would be acceptable.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Can't think of any reason not to do this.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+Adding as a lint allows users to silence the error if they so wish.
+
+Not addressing this issue will let this problem continue happening without
+warning to end users.
+
+# Prior art
+[prior-art]: #prior-art
+
+This would act in the same way as other lints warning for potentially
+problematic valid code.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.