Remove const eval limit and implement an exponential backoff lint instead

compiler/rustc_lint_defs/src/builtin.rs

@@ -3357,6 +3357,7 @@ declare_lint_pass! {
         LARGE_ASSIGNMENTS,
         LATE_BOUND_LIFETIME_ARGUMENTS,
         LEGACY_DERIVE_HELPERS,
+        LONG_RUNNING_CONST_EVAL,
         LOSSY_PROVENANCE_CASTS,
         MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
         MACRO_USE_EXTERN_CRATE,
@@ -3426,6 +3427,43 @@ declare_lint_pass! {
     ]
 }
 
+declare_lint! {
+    /// The `long_running_const_eval` lint is emitted when const
+    /// eval is running for a long time to ensure rustc terminates
+    /// even if you accidentally wrote an infinite loop.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// const FOO: () = loop {};
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Loops allow const evaluation to compute arbitrary code, but may also
+    /// cause infinite loops or just very long running computations.
+    /// Users can enable long running computations by allowing the lint
+    /// on individual constants or for entire crates.
+    ///
+    /// ### Unconditional warnings
+    ///
+    /// Note that regardless of whether the lint is allowed or set to warn,
+    /// the compiler will issue warnings if constant evaluation runs significantly
+    /// longer than this lint's limit. These warnings are also shown to downstream
+    /// users from crates.io or similar registries. If you are above the lint's limit,
+    /// both you and downstream users might be exposed to these warnings.
+    /// They might also appear on compiler updates, as the compiler makes minor changes
+    /// about how complexity is measured: staying below the limit ensures that there
+    /// is enough room, and given that the lint is disabled for people who use your
+    /// dependency it means you will be the only one to get the warning and can put
+    /// out an update in your own time.
+    pub LONG_RUNNING_CONST_EVAL,
+    Deny,
+    "detects long const eval operations"
+}
+
 declare_lint! {
     /// The `unused_doc_comments` lint detects doc comments that aren't used
     /// by `rustdoc`.