error when a command line flag lowers a -F forbid

compiler/rustc_lint/src/diagnostics.rs

@@ -28,9 +28,9 @@ use crate::lifetime_syntax::LifetimeSyntaxCategories;
 #[diag("{$lint_level}({$lint_name}) incompatible with previous forbid", code = E0453)]
 pub(crate) struct OverruledAttribute<'a> {
     #[primary_span]
-    pub span: Span,
+    pub span: Option<Span>,
     #[label("overruled by previous forbid")]
-    pub overruled: Span,
+    pub overruled: Option<Span>,
     pub lint_level: &'a str,
     pub lint_name: LintName,
     #[subdiagnostic]

compiler/rustc_lint/src/levels.rs

@@ -539,16 +539,16 @@ where
                 continue;
             };
             for &id in ids {
-                // ForceWarn and Forbid cannot be overridden
+                // ForceWarn cannot be overridden
                 if let Some(level_spec) = self.provider.current_specs().get(&id)
-                    && matches!(level_spec.level(), Level::ForceWarn | Level::Forbid)
+                    && matches!(level_spec.level(), Level::ForceWarn)
                 {
                     continue;
                 }
 
                 if self.check_gated_lint(id, DUMMY_SP, true) {
                     let src = LintLevelSource::CommandLine(lint_name, level);
-                    self.provider.insert(id, LevelSpec::new(level, None, src));
+                    self.insert_spec(id, LevelSpec::new(level, None, src));
                 }
             }
         }
@@ -582,11 +582,15 @@ where
             // as preventing `allow(lint)` for some lint `lint` in
             // `lint_group`. For now, issue a future-compatibility
             // warning for this case.
-            let fcw_warning = match old_src {
-                LintLevelSource::Default => false,
-                LintLevelSource::Node { name, .. } => self.store.is_lint_group(name),
-                LintLevelSource::CommandLine(name, _) => self.store.is_lint_group(name),
-            };
+            //
+            // Command-line flags were never able to lower a forbid, so there is
+            // nothing to stay compatible with there.
+            let fcw_warning = !matches!(src, LintLevelSource::CommandLine(..))
+                && match old_src {
+                    LintLevelSource::Default => false,
+                    LintLevelSource::Node { name, .. } => self.store.is_lint_group(name),
+                    LintLevelSource::CommandLine(name, _) => self.store.is_lint_group(name),
+                };
             debug!(
                 "fcw_warning={:?}, specs.get(&id) = {:?}, old_src={:?}, id_name={:?}",
                 fcw_warning,
@@ -606,9 +610,13 @@ where
                 }
             };
             if !fcw_warning {
+                let span = match src {
+                    LintLevelSource::CommandLine(..) => None,
+                    _ => Some(src.span()),
+                };
                 self.sess.dcx().emit_err(OverruledAttribute {
-                    span: src.span(),
-                    overruled: src.span(),
+                    span,
+                    overruled: span,
                     lint_level: level.as_str(),
                     lint_name: src.name(),
                     sub,

src/doc/rustc/src/lints/levels.md

@@ -214,15 +214,16 @@ The order of these command line arguments is taken into account. The following a
 $ rustc lib.rs --crate-type=lib -D unused-variables -A unused-variables
 ```
 
-You can make use of this behavior by overriding the level of one specific lint out of a group of lints. The following example denies all the lints in the `unused` group, but explicitly allows the `unused-variables` lint in that group (forbid still trumps everything regardless of ordering):
+You can make use of this behavior by overriding the level of one specific lint out of a group of lints. The following example denies all the lints in the `unused` group, but explicitly allows the `unused-variables` lint in that group (this does not work for `forbid`, see below):
 
 ```bash
 $ rustc lib.rs --crate-type=lib -D unused -A unused-variables
 ```
 
-Since `force-warn` and `forbid` cannot be overridden, setting
-one of them will prevent any later level for the same lint from
-taking effect.
+Since `force-warn` cannot be overridden, setting it will prevent
+any later level for the same lint from taking effect. A later `-A`,
+`-W` or `--force-warn` for a lint set to `forbid` is an error, and a
+later `-D` is ignored.
 
 ### Via an attribute
 

tests/ui/lint/forbid-always-trumps-cli.allow-first-group.stderr

@@ -1,5 +1,5 @@
 error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+  --> $DIR/forbid-always-trumps-cli.rs:18:9
    |
 LL |     let x = 1;
    |         ^ help: if this is intentional, prefix it with an underscore: `_x`

tests/ui/lint/forbid-always-trumps-cli.allow-first-lint.stderr

@@ -1,5 +1,5 @@
 error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+  --> $DIR/forbid-always-trumps-cli.rs:18:9
    |
 LL |     let x = 1;
    |         ^ help: if this is intentional, prefix it with an underscore: `_x`

tests/ui/lint/forbid-always-trumps-cli.allow-first-mix1.stderr

@@ -1,5 +1,5 @@
 error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+  --> $DIR/forbid-always-trumps-cli.rs:18:9
    |
 LL |     let x = 1;
    |         ^ help: if this is intentional, prefix it with an underscore: `_x`

tests/ui/lint/forbid-always-trumps-cli.allow-first-mix2.stderr

@@ -1,5 +1,5 @@
 error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+  --> $DIR/forbid-always-trumps-cli.rs:18:9
    |
 LL |     let x = 1;
    |         ^ help: if this is intentional, prefix it with an underscore: `_x`

tests/ui/lint/forbid-always-trumps-cli.forbid-first-group.stderr

@@ -1,10 +1,7 @@
-error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+error[E0453]: allow(unused) incompatible with previous forbid
    |
-LL |     let x = 1;
-   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
-   |
-   = note: `-F unused-variables` implied by `-F unused`
+   = note: `forbid` lint level was set on command line (`-F unused`)
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0453`.

tests/ui/lint/forbid-always-trumps-cli.forbid-first-lint.stderr

@@ -1,10 +1,7 @@
-error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+error[E0453]: allow(unused_variables) incompatible with previous forbid
    |
-LL |     let x = 1;
-   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
-   |
-   = note: requested on the command line with `-F unused-variables`
+   = note: `forbid` lint level was set on command line (`-F unused_variables`)
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0453`.

tests/ui/lint/forbid-always-trumps-cli.forbid-first-mix1.stderr

@@ -1,10 +1,7 @@
-error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+error[E0453]: allow(unused_variables) incompatible with previous forbid
    |
-LL |     let x = 1;
-   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
-   |
-   = note: `-F unused-variables` implied by `-F unused`
+   = note: `forbid` lint level was set on command line (`-F unused`)
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0453`.

tests/ui/lint/forbid-always-trumps-cli.forbid-first-mix2.stderr

@@ -1,10 +1,7 @@
-error: unused variable: `x`
-  --> $DIR/forbid-always-trumps-cli.rs:15:9
+error[E0453]: allow(unused) incompatible with previous forbid
    |
-LL |     let x = 1;
-   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
-   |
-   = note: requested on the command line with `-F unused-variables`
+   = note: `forbid` lint level was set on command line (`-F unused_variables`)
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0453`.

tests/ui/lint/forbid-always-trumps-cli.rs

@@ -10,8 +10,11 @@
 //@[allow-first-mix1] compile-flags: -A unused_variables -F unused
 //@[forbid-first-mix2] compile-flags: -F unused_variables -A unused
 //@[allow-first-mix2] compile-flags: -A unused -F unused_variables
+//@ compile-flags: -Zdeduplicate-diagnostics=yes
+
+//[forbid-first-group,forbid-first-lint,forbid-first-mix1,forbid-first-mix2]~? ERROR incompatible with previous forbid
 
 fn main() {
     let x = 1;
-    //~^ ERROR unused variable: `x`
+    //[allow-first-group,allow-first-lint,allow-first-mix1,allow-first-mix2]~^ ERROR unused variable: `x`
 }