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.

Author: Noratrieb

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)`

src/tools/tidy/src/issues.txt

@@ -2758,7 +2758,6 @@ ui/lint/issue-63364.rs
 ui/lint/issue-70819-dont-override-forbid-in-same-scope.rs
 ui/lint/issue-79546-fuel-ice.rs
 ui/lint/issue-79744.rs
-ui/lint/issue-80988.rs
 ui/lint/issue-81218.rs
 ui/lint/issue-83477.rs
 ui/lint/issue-87274-paren-parent.rs

tests/ui/lint/auxiliary/deny-macro.rs

@@ -0,0 +1,7 @@
+#[macro_export]
+macro_rules! emit_deny {
+    () => {
+        #[deny(unsafe_code)]
+        let _so_safe = 0;
+    };
+}

tests/ui/lint/deny-inside-forbid-ignored.rs

@@ -0,0 +1,16 @@
+/// Ensure that using deny inside forbid is treated as a no-op,
+/// and does not override the level to deny.
+
+#[forbid(unsafe_code)] // NO UNSAFE CODE IN HERE!!
+fn main() {
+    #[deny(unsafe_code)] // m-m-maybe we can have unsafe code in here?
+    {
+        #[allow(unsafe_code)] // let's have some unsafe code in here
+        //~^ ERROR allow(unsafe_code) incompatible with previous forbid
+        //~| ERROR allow(unsafe_code) incompatible with previous forbid
+        {
+            unsafe { /* ≽^•⩊•^≼ */ }
+            //~^ ERROR usage of an `unsafe` block
+        }
+    }
+}

tests/ui/lint/deny-inside-forbid-ignored.stderr

@@ -0,0 +1,35 @@
+error[E0453]: allow(unsafe_code) incompatible with previous forbid
+  --> $DIR/deny-inside-forbid-ignored.rs:8:17
+   |
+LL | #[forbid(unsafe_code)] // NO UNSAFE CODE IN HERE!!
+   |          ----------- `forbid` level set here
+...
+LL |         #[allow(unsafe_code)] // let's have some unsafe code in here
+   |                 ^^^^^^^^^^^ overruled by previous forbid
+
+error[E0453]: allow(unsafe_code) incompatible with previous forbid
+  --> $DIR/deny-inside-forbid-ignored.rs:8:17
+   |
+LL | #[forbid(unsafe_code)] // NO UNSAFE CODE IN HERE!!
+   |          ----------- `forbid` level set here
+...
+LL |         #[allow(unsafe_code)] // let's have some unsafe code in here
+   |                 ^^^^^^^^^^^ overruled by previous forbid
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: usage of an `unsafe` block
+  --> $DIR/deny-inside-forbid-ignored.rs:12:13
+   |
+LL |             unsafe { /* ≽^•⩊•^≼ */ }
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/deny-inside-forbid-ignored.rs:4:10
+   |
+LL | #[forbid(unsafe_code)] // NO UNSAFE CODE IN HERE!!
+   |          ^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0453`.

tests/ui/lint/forbid-macro-with-deny.rs

@@ -0,0 +1,16 @@
+//@ aux-build:deny-macro.rs
+//@ check-pass
+
+// Ensure that when a macro (or normal code) does #[deny] inside a #[forbid]
+// context, no error is emitted, as both parties agree on the treatment of the lint.
+
+#![forbid(unsafe_code)]
+
+extern crate deny_macro;
+
+fn main() {
+    deny_macro::emit_deny! {}
+
+    #[deny(unsafe_code)]
+    let _ = 0;
+}

tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.rs

@@ -19,9 +19,9 @@
 fn forbid_first(num: i32) -> i32 {
     #![forbid(unused)]
     #![deny(unused)]
-    //~^ ERROR: deny(unused) incompatible with previous forbid
-    //~| WARNING being phased out
     #![warn(unused)]
+    //~^ ERROR: warn(unused) incompatible with previous forbid
+    //~| WARNING being phased out
     #![allow(unused)]
 
     num * num

tests/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr

@@ -1,9 +1,10 @@
-error: deny(unused) incompatible with previous forbid
-  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:21:13
+error: warn(unused) incompatible with previous forbid
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:22:13
    |
 LL |     #![forbid(unused)]
    |               ------ `forbid` level set here
 LL |     #![deny(unused)]
+LL |     #![warn(unused)]
    |             ^^^^^^ overruled by previous forbid
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!