Use caret (^) as the default required_version comparator

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## [Unreleased]
 
+### Changed
+- The unstable `required_version` configuration option now defaults to a caret (`^`) semver requirement instead of an exact match when no comparator is given, matching how Cargo treats dependency version requirements. Use the explicit `=` operator (e.g. `required_version = "=1.2.3"`) to opt back into exact matching [#6729](https://github.com/rust-lang/rustfmt/issues/6729)
+
 
 ## [1.10.0] 2026-07-21
 

Configurations.md

@@ -2465,14 +2465,28 @@ specific version of rustfmt is used in your CI, use this option.
 
 - **Default value**: `CARGO_PKG_VERSION`
 - **Possible values**: `semver` compliant values, such as defined on [semver.org](https://semver.org/).
+  A value with no comparator, such as `"1.0.0"`, defaults to a caret (`^`)
+  requirement, matching how Cargo treats dependency version requirements.
 - **Stable**: No (tracking issue: [#3386](https://github.com/rust-lang/rustfmt/issues/3386))
 
-#### Match on exact version:
+#### New minor or patch versions (default behavior):
+
+```toml
+required_version="^1.0.0"
+```
+
+This is the default behavior, so that is equivalent to:
 
 ```toml
 required_version="1.0.0"
 ```
 
+#### Match on exact version:
+
+```toml
+required_version="=1.0.0"
+```
+
 #### Higher or equal to:
 
 ```toml

src/config/mod.rs

@@ -242,7 +242,10 @@ impl PartialConfig {
 }
 
 fn check_semver_version(range_requirement: &str, actual: &str) -> bool {
-    let mut version_req = match semver::VersionReq::parse(range_requirement) {
+    // A bare version (no comparator prefix) is left as `semver`'s own default,
+    // which is a caret requirement, matching how Cargo treats dependency version
+    // requirements.
+    let version_req = match semver::VersionReq::parse(range_requirement) {
         Ok(r) => r,
         Err(e) => {
             eprintln!("Error: failed to parse required version {range_requirement:?}: {e}");
@@ -257,24 +260,6 @@ fn check_semver_version(range_requirement: &str, actual: &str) -> bool {
         }
     };
 
-    range_requirement
-        .split(',')
-        .enumerate()
-        .for_each(|(i, label)| {
-            // the label refers to the current comparator
-            let Some(comparator) = version_req.comparators.get_mut(i) else {
-                return;
-            };
-
-            // semver crate handles "1.0.0" as "^1.0.0", and we want to treat it as "=1.0.0"
-            // because of this, we need to iterate over the comparators, and change each one
-            // that has "default caret operator" to an exact operator
-            // this condition overrides the "default caret operator" of semver create.
-            if !label.starts_with('^') && comparator.op == semver::Op::Caret {
-                comparator.op = semver::Op::Exact;
-            }
-        });
-
     version_req.matches(&actual_version)
 }
 
@@ -1552,15 +1537,27 @@ make_backup = false
         use super::*;
 
         #[test]
-        fn test_exact_version_match() {
+        fn test_default_caret_match() {
+            // A bare version (no operator) defaults to a caret requirement,
+            // matching how Cargo treats dependency version requirements.
             assert!(check_semver_version("1.0.0", "1.0.0"));
-            assert!(!check_semver_version("1.0.0", "1.1.0"));
-            assert!(!check_semver_version("1.0.0", "1.0.1"));
+            assert!(check_semver_version("1.0.0", "1.1.0"));
+            assert!(check_semver_version("1.0.0", "1.0.1"));
             assert!(!check_semver_version("1.0.0", "2.1.0"));
             assert!(!check_semver_version("1.0.0", "0.1.0"));
             assert!(!check_semver_version("1.0.0", "0.0.1"));
         }
 
+        #[test]
+        fn test_explicit_exact_match() {
+            assert!(check_semver_version("=1.0.0", "1.0.0"));
+            assert!(!check_semver_version("=1.0.0", "1.1.0"));
+            assert!(!check_semver_version("=1.0.0", "1.0.1"));
+            assert!(!check_semver_version("=1.0.0", "2.1.0"));
+            assert!(!check_semver_version("=1.0.0", "0.1.0"));
+            assert!(!check_semver_version("=1.0.0", "0.0.1"));
+        }
+
         #[test]
         fn test_version_mismatch() {
             assert!(!check_semver_version("2.0.0", "1.0.0"));