Allow `#[deny(..)]` inside `#[forbid(..)]` as a no-op with a warning

Forbid cannot be overriden. When someome tries to do this anyways,
it results in a hard error. That makes sense.

Except it doesn't, because macros. Macros may reasonably use `#[deny]`
in their expansion to assert
that their expanded code follows the lint. This is doesn't work when the
output gets expanded into a `forbid()` context. This is pretty silly,
since both the macros and the code agree on the lint!

Therefore, we allow `#[deny(..)]`ing a lint that's already forbidden,
keeping the level at forbid.

2 files changed, 5 insertions, 2 deletions

diff --git a/compiler/rustc_lint/src/levels.rs b/compiler/rustc_lint/src/levels.rs
index 89a67fc0d89..95a8e7625ff 100644
--- a/compiler/rustc_lint/src/levels.rs
+++ b/compiler/rustc_lint/src/levels.rs
@@ -493,7 +493,10 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
         //
         // This means that this only errors if we're truly lowering the lint
         // level from forbid.
-        if self.lint_added_lints && level != Level::Forbid && old_level == Level::Forbid {
+        if self.lint_added_lints && level == Level::Deny && old_level == Level::Forbid {
+            // Having a deny inside a forbid is fine and is ignored, so we skip this check.
+            return;
+        } else if self.lint_added_lints && level != Level::Forbid && old_level == Level::Forbid {
             // Backwards compatibility check:
             //
             // We used to not consider `forbid(lint_group)`
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 45a5ce0ca20..c11571182fe 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -156,7 +156,7 @@ declare_lint! {
     ///
     /// ```rust
     /// #![forbid(warnings)]
-    /// #![deny(bad_style)]
+    /// #![warn(bad_style)]
     ///
     /// fn main() {}
     /// ```