apply -Zcrate-attr attributes before the crate attributes

compiler/rustc_builtin_macros/src/cmdline_attrs.rs

@@ -8,7 +8,7 @@ use rustc_session::parse::ParseSess;
 use rustc_span::FileName;
 
 pub fn inject(krate: &mut ast::Crate, psess: &ParseSess, attrs: &[String]) {
-    for raw_attr in attrs {
+    let injected = attrs.iter().filter_map(|raw_attr| {
         let source = format!("#![{raw_attr}]");
         let parse = || -> Result<ast::Attribute, Vec<Diag<'_>>> {
             let tokens = source_str_to_stream(
@@ -22,16 +22,15 @@ pub fn inject(krate: &mut ast::Crate, psess: &ParseSess, attrs: &[String]) {
             })
             .map_err(|e| vec![e])
         };
-        let meta = match parse() {
-            Ok(meta) => meta,
+        match parse() {
+            Ok(meta) => Some(meta),
             Err(errs) => {
                 for err in errs {
                     err.emit();
                 }
-                continue;
+                None
             }
-        };
-
-        krate.attrs.push(meta);
-    }
+        }
+    });
+    krate.attrs.splice(0..0, injected);
 }

tests/ui/attributes/z-crate-attr/ordering-cli.rs

@@ -0,0 +1,6 @@
+// Multiple `-Zcrate-attr` flags apply in command-line order, so the later `deny` wins.
+//@ compile-flags: -Zcrate-attr=warn(unused) -Zcrate-attr=deny(unused)
+
+fn foo() {} //~ ERROR function `foo` is never used
+
+fn main() {}

tests/ui/attributes/z-crate-attr/ordering-cli.stderr

@@ -0,0 +1,15 @@
+error: function `foo` is never used
+  --> $DIR/ordering-cli.rs:4:4
+   |
+LL | fn foo() {}
+   |    ^^^
+   |
+note: the lint level is defined here
+  --> <crate attribute>:1:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: `#[deny(dead_code)]` implied by `#[deny(unused)]`
+
+error: aborting due to 1 previous error
+

tests/ui/attributes/z-crate-attr/ordering-source.rs

@@ -0,0 +1,9 @@
+// RFC 3791: `-Zcrate-attr` applies before the crate's own attributes, so `warn` here wins.
+//@ check-pass
+//@ compile-flags: -Zcrate-attr=deny(unused)
+
+#![warn(unused)]
+
+fn foo() {} //~ WARN function `foo` is never used
+
+fn main() {}

tests/ui/attributes/z-crate-attr/ordering-source.stderr

@@ -0,0 +1,15 @@
+warning: function `foo` is never used
+  --> $DIR/ordering-source.rs:7:4
+   |
+LL | fn foo() {}
+   |    ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/ordering-source.rs:5:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(dead_code)]` implied by `#[warn(unused)]`
+
+warning: 1 warning emitted
+