Reject contract attributes without arguments

compiler/rustc_builtin_macros/src/contracts.rs

@@ -137,6 +137,21 @@ fn expand_contract_clause_tts(
     annotated: TokenStream,
     clause_keyword: rustc_span::Symbol,
 ) -> Result<TokenStream, ErrorGuaranteed> {
+    if annotation.is_empty() {
+        let (name, example) = if clause_keyword == kw::ContractRequires {
+            ("requires", "condition")
+        } else {
+            ("ensures", "|result: &T| condition")
+        };
+        ecx.sess.dcx().span_err(
+            attr_span,
+            format!("`{name}` attribute requires an argument, e.g., `#[{name}({example})]`"),
+        );
+        // Returning `Err` would replace it with a dummy fragment and cause cascading name-resolution errors.
+        // Instead, we return the original token stream so that there is no later noises.
+        return Ok(annotated);
+    }
+
     let feature_span = ecx.with_def_site_ctxt(attr_span);
     expand_contract_clause(ecx, attr_span, annotated, |new_tts| {
         new_tts.push(TokenTree::Token(

tests/ui/contracts/empty-ensures.rs

@@ -6,7 +6,7 @@ extern crate core;
 use core::contracts::ensures;
 
 #[ensures()]
-//~^ ERROR expected an `Fn(&_)` closure, found `()` [E0277]
+//~^ ERROR `ensures` attribute requires an argument
 fn foo(x: u32) -> u32 {
     x * 2
 }

tests/ui/contracts/empty-ensures.stderr

@@ -1,16 +1,8 @@
-error[E0277]: expected an `Fn(&_)` closure, found `()`
+error: `ensures` attribute requires an argument, e.g., `#[ensures(|result: &T| condition)]`
   --> $DIR/empty-ensures.rs:8:1
    |
 LL | #[ensures()]
    | ^^^^^^^^^^^^
-   | |
-   | expected an `Fn(&_)` closure, found `()`
-   | required by a bound introduced by this call
-   |
-   = help: the trait `for<'a> Fn(&'a _)` is not implemented for `()`
-note: required by a bound in `build_check_ensures`
-  --> $SRC_DIR/core/src/contracts.rs:LL:COL
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.

tests/ui/contracts/empty-requires.rs

@@ -1,4 +1,3 @@
-//@ dont-require-annotations: NOTE
 //@ compile-flags: -Zcontract-checks=yes
 #![expect(incomplete_features)]
 #![feature(contracts)]
@@ -7,8 +6,7 @@ extern crate core;
 use core::contracts::requires;
 
 #[requires()]
-//~^ ERROR mismatched types [E0308]
-//~| NOTE expected `bool`, found `()`
+//~^ ERROR `requires` attribute requires an argument
 fn foo(x: u32) -> u32 {
     x * 2
 }

tests/ui/contracts/empty-requires.stderr

@@ -1,9 +1,8 @@
-error[E0308]: mismatched types
-  --> $DIR/empty-requires.rs:9:1
+error: `requires` attribute requires an argument, e.g., `#[requires(condition)]`
+  --> $DIR/empty-requires.rs:8:1
    |
 LL | #[requires()]
-   | ^^^^^^^^^^^^^ expected `bool`, found `()`
+   | ^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0308`.

tests/ui/lint/single-use-lifetimes-issue-146834.rs

@@ -0,0 +1,18 @@
+//! Regression test for https://github.com/rust-lang/rust/issues/146834.
+
+//@ compile-flags: -Wsingle-use-lifetimes
+//@ edition: 2024
+
+#![expect(incomplete_features)]
+#![feature(contracts)]
+
+#[core::contracts::ensures]
+//~^ ERROR `ensures` attribute requires an argument
+fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+    //~^ ERROR missing lifetime specifiers
+    //~| WARN lifetime parameter `'a` only used once
+    //~| WARN lifetime parameter `'b` only used once
+    loop {}
+}
+
+fn main() {}

tests/ui/lint/single-use-lifetimes-issue-146834.stderr

@@ -0,0 +1,55 @@
+error: `ensures` attribute requires an argument, e.g., `#[ensures(|result: &T| condition)]`
+  --> $DIR/single-use-lifetimes-issue-146834.rs:9:1
+   |
+LL | #[core::contracts::ensures]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0106]: missing lifetime specifiers
+  --> $DIR/single-use-lifetimes-issue-146834.rs:11:42
+   |
+LL | fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+   |                 -------     -------      ^     ^ expected named lifetime parameter
+   |                                          |
+   |                                          expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+note: these named lifetimes are available to use
+  --> $DIR/single-use-lifetimes-issue-146834.rs:11:6
+   |
+LL | fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+   |      ^^  ^^
+help: consider using one of the available lifetimes here
+   |
+LL | fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&'lifetime i32, &'lifetime i32) {
+   |                                           +++++++++       +++++++++
+
+warning: lifetime parameter `'a` only used once
+  --> $DIR/single-use-lifetimes-issue-146834.rs:11:6
+   |
+LL | fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+   |      ^^          -- ...is used only here
+   |      |
+   |      this lifetime...
+   |
+   = note: requested on the command line with `-W single-use-lifetimes`
+help: elide the single-use lifetime
+   |
+LL - fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+LL + fn f<'b>(a: &i32, b: &'b i32) -> (&i32, &i32) {
+   |
+
+warning: lifetime parameter `'b` only used once
+  --> $DIR/single-use-lifetimes-issue-146834.rs:11:10
+   |
+LL | fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+   |          ^^ this lifetime... -- ...is used only here
+   |
+help: elide the single-use lifetime
+   |
+LL - fn f<'a, 'b>(a: &'a i32, b: &'b i32) -> (&i32, &i32) {
+LL + fn f<'a>(a: &'a i32, b: &i32) -> (&i32, &i32) {
+   |
+
+error: aborting due to 2 previous errors; 2 warnings emitted
+
+For more information about this error, try `rustc --explain E0106`.

tests/ui/macros/ice-in-tokenstream-for-contracts-issue-140683.rs

@@ -4,7 +4,7 @@
 struct T;
 
 impl T {
-    #[core::contracts::ensures] //~ ERROR expected an `Fn(&_)` closure, found `()`
+    #[core::contracts::ensures] //~ ERROR `ensures` attribute requires an argument
     fn b() {(loop)}
     //~^ ERROR expected `{`, found `)`
     //~| ERROR expected `{`, found `)`

tests/ui/macros/ice-in-tokenstream-for-contracts-issue-140683.stderr

@@ -6,6 +6,12 @@ LL |     fn b() {(loop)}
    |              |
    |              while parsing this `loop` expression
 
+error: `ensures` attribute requires an argument, e.g., `#[ensures(|result: &T| condition)]`
+  --> $DIR/ice-in-tokenstream-for-contracts-issue-140683.rs:7:5
+   |
+LL |     #[core::contracts::ensures]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error: expected `{`, found `)`
   --> $DIR/ice-in-tokenstream-for-contracts-issue-140683.rs:8:18
    |
@@ -16,19 +22,5 @@ LL |     fn b() {(loop)}
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error[E0277]: expected an `Fn(&_)` closure, found `()`
-  --> $DIR/ice-in-tokenstream-for-contracts-issue-140683.rs:7:5
-   |
-LL |     #[core::contracts::ensures]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |     |
-   |     expected an `Fn(&_)` closure, found `()`
-   |     required by a bound introduced by this call
-   |
-   = help: the trait `for<'a> Fn(&'a _)` is not implemented for `()`
-note: required by a bound in `build_check_ensures`
-  --> $SRC_DIR/core/src/contracts.rs:LL:COL
-
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.