Warn on always false contract predicates

src/implementation/parse.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use proc_macro2::{Spacing, TokenStream, TokenTree};
-use syn::{Expr, ExprLit, Lit};
+use syn::{spanned::Spanned, Expr, ExprLit, Lit};
 
 /// Parse attributes into a list of expression and an optional description of
 /// the assert
@@ -42,9 +42,39 @@ pub(crate) fn parse_attributes(
         segments_stream.pop();
     }
 
+    for cond in &mut conds {
+        warn_on_false_literal(cond);
+    }
+
     (conds, segments_stream, desc)
 }
 
+fn warn_on_false_literal(expr: &mut Expr) {
+    let Expr::Lit(ExprLit {
+        lit: Lit::Bool(lit),
+        ..
+    }) = expr
+    else {
+        return;
+    };
+
+    if lit.value {
+        return;
+    }
+
+    let span = expr.span();
+    let toks = quote::quote_spanned! { span=>
+        {
+            #[deprecated(note = "contract predicate is always false")]
+            const __CONTRACTS_FALSE_PREDICATE: bool = false;
+
+            __CONTRACTS_FALSE_PREDICATE
+        }
+    };
+
+    *expr = syn::parse2(toks).unwrap();
+}
+
 // This function rewrites a list of TokenTrees so that the "pseudooperator" for
 // implication `==>` gets transformed into an `if` expression.
 //

tests/ui/fail/always_false_predicate.rs

@@ -0,0 +1,10 @@
+#![deny(deprecated)]
+
+use contracts::ensures;
+
+#[ensures(false)]
+fn checked() {}
+
+fn main() {
+    checked();
+}

tests/ui/fail/always_false_predicate.stderr

@@ -0,0 +1,11 @@
+error: use of deprecated constant `checked::__CONTRACTS_FALSE_PREDICATE`: contract predicate is always false
+ --> tests/ui/fail/always_false_predicate.rs:5:11
+  |
+5 | #[ensures(false)]
+  |           ^^^^^
+  |
+note: the lint level is defined here
+ --> tests/ui/fail/always_false_predicate.rs:1:9
+  |
+1 | #![deny(deprecated)]
+  |         ^^^^^^^^^^