diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index 69de9be3a2f..9a3a2c42211 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/src/cargo/util/toml/mod.rs
@@ -180,6 +180,15 @@ pub fn parse_document(
         .map_err(|e| anyhow::Error::from(e).context("could not parse input as TOML"))
 }
 
+/// Warn about paths that have been deprecated and may conflict.
+fn warn_on_deprecated(new_path: &str, name: &str, kind: &str, warnings: &mut Vec<String>) {
+    let old_path = new_path.replace("-", "_");
+    warnings.push(format!(
+        "conflicting between `{new_path}` and `{old_path}` in the `{name}` {kind}.\n
+        `{old_path}` is ignored and not recommended for use in the future"
+    ))
+}
+
 type TomlLibTarget = TomlTarget;
 type TomlBinTarget = TomlTarget;
 type TomlExampleTarget = TomlTarget;
@@ -1264,11 +1273,17 @@ impl TomlManifest {
 
             // Collect the dependencies.
             process_dependencies(&mut cx, me.dependencies.as_ref(), None)?;
+            if me.dev_dependencies.is_some() && me.dev_dependencies2.is_some() {
+                warn_on_deprecated("dev-dependencies", package_name, "package", cx.warnings);
+            }
             let dev_deps = me
                 .dev_dependencies
                 .as_ref()
                 .or_else(|| me.dev_dependencies2.as_ref());
             process_dependencies(&mut cx, dev_deps, Some(DepKind::Development))?;
+            if me.build_dependencies.is_some() && me.build_dependencies2.is_some() {
+                warn_on_deprecated("build-dependencies", package_name, "package", cx.warnings);
+            }
             let build_deps = me
                 .build_dependencies
                 .as_ref()
@@ -1282,11 +1297,17 @@ impl TomlManifest {
                     Some(platform)
                 };
                 process_dependencies(&mut cx, platform.dependencies.as_ref(), None)?;
+                if platform.build_dependencies.is_some() && platform.build_dependencies2.is_some() {
+                    warn_on_deprecated("build-dependencies", name, "platform target", cx.warnings);
+                }
                 let build_deps = platform
                     .build_dependencies
                     .as_ref()
                     .or_else(|| platform.build_dependencies2.as_ref());
                 process_dependencies(&mut cx, build_deps, Some(DepKind::Build))?;
+                if platform.dev_dependencies.is_some() && platform.dev_dependencies2.is_some() {
+                    warn_on_deprecated("dev-dependencies", name, "platform target", cx.warnings);
+                }
                 let dev_deps = platform
                     .dev_dependencies
                     .as_ref()
@@ -1908,6 +1929,9 @@ impl<P: ResolveToPath> DetailedTomlDependency<P> {
 
         let version = self.version.as_deref();
         let mut dep = Dependency::parse(pkg_name, version, new_source_id)?;
+        if self.default_features.is_some() && self.default_features2.is_some() {
+            warn_on_deprecated("default-features", name_in_toml, "dependency", cx.warnings);
+        }
         dep.set_features(self.features.iter().flatten())
             .set_default_features(
                 self.default_features
@@ -2057,6 +2081,17 @@ impl TomlTarget {
         }
     }
 
+    fn validate_proc_macro(&self, warnings: &mut Vec<String>) {
+        if self.proc_macro_raw.is_some() && self.proc_macro_raw2.is_some() {
+            warn_on_deprecated(
+                "proc-macro",
+                self.name().as_str(),
+                "library target",
+                warnings,
+            );
+        }
+    }
+
     fn proc_macro(&self) -> Option<bool> {
         self.proc_macro_raw.or(self.proc_macro_raw2).or_else(|| {
             if let Some(types) = self.crate_types() {
@@ -2068,6 +2103,17 @@ impl TomlTarget {
         })
     }
 
+    fn validate_crate_types(&self, target_kind_human: &str, warnings: &mut Vec<String>) {
+        if self.crate_type.is_some() && self.crate_type2.is_some() {
+            warn_on_deprecated(
+                "crate-type",
+                self.name().as_str(),
+                format!("{target_kind_human} target").as_str(),
+                warnings,
+            );
+        }
+    }
+
     fn crate_types(&self) -> Option<&Vec<String>> {
         self.crate_type
             .as_ref()
diff --git a/src/cargo/util/toml/targets.rs b/src/cargo/util/toml/targets.rs
index 13aec29a4e8..ac9bf468af5 100644
--- a/src/cargo/util/toml/targets.rs
+++ b/src/cargo/util/toml/targets.rs
@@ -174,6 +174,8 @@ fn clean_lib(
         Some(ref lib) => lib,
         None => return Ok(None),
     };
+    lib.validate_proc_macro(warnings);
+    lib.validate_crate_types("library", warnings);
 
     validate_target_name(lib, "library", "lib", warnings)?;
 
@@ -398,6 +400,7 @@ fn clean_examples(
 
     let mut result = Vec::new();
     for (path, toml) in targets {
+        toml.validate_crate_types("example", warnings);
         let crate_types = match toml.crate_types() {
             Some(kinds) => kinds.iter().map(|s| s.into()).collect(),
             None => Vec::new(),
diff --git a/tests/testsuite/build.rs b/tests/testsuite/build.rs
index 70550431a35..c6ab943e4ba 100644
--- a/tests/testsuite/build.rs
+++ b/tests/testsuite/build.rs
@@ -1676,6 +1676,153 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn dev_dependencies_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [dev-dependencies]
+                a = {path = "a"}
+                [dev_dependencies]
+                a = {path = "a"}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.0.1"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .build();
+    p.cargo("build")
+        .with_stderr_contains(
+"[WARNING] conflicting between `dev-dependencies` and `dev_dependencies` in the `foo` package.\n
+        `dev_dependencies` is ignored and not recommended for use in the future"
+        )
+        .run();
+}
+
+#[cargo_test]
+fn build_dependencies_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+
+                [build-dependencies]
+                a = {path = "a"}
+                [build_dependencies]
+                a = {path = "a"}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.0.1"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .build();
+    p.cargo("build")
+        .with_stderr_contains(
+"[WARNING] conflicting between `build-dependencies` and `build_dependencies` in the `foo` package.\n
+        `build_dependencies` is ignored and not recommended for use in the future"
+        )
+        .run();
+}
+
+#[cargo_test]
+fn lib_crate_types_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.5.0"
+                authors = ["wycats@example.com"]
+
+                [lib]
+                name = "foo"
+                crate-type = ["rlib", "dylib"]
+                crate_type = ["staticlib", "dylib"]
+            "#,
+        )
+        .file("src/lib.rs", "pub fn foo() {}")
+        .build();
+    p.cargo("build")
+        .with_stderr_contains(
+"[WARNING] conflicting between `crate-type` and `crate_type` in the `foo` library target.\n
+        `crate_type` is ignored and not recommended for use in the future",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn examples_crate_types_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.5.0"
+                authors = ["wycats@example.com"]
+
+                [[example]]
+                name = "ex"
+                path = "examples/ex.rs"
+                crate-type = ["rlib", "dylib"]
+                crate_type = ["proc_macro"]
+                [[example]]
+                name = "goodbye"
+                path = "examples/ex-goodbye.rs"
+                crate-type = ["rlib", "dylib"]
+                crate_type = ["rlib", "staticlib"]
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "examples/ex.rs",
+            r#"
+                fn main() { println!("ex"); }
+            "#,
+        )
+        .file(
+            "examples/ex-goodbye.rs",
+            r#"
+                fn main() { println!("goodbye"); }
+            "#,
+        )
+        .build();
+    p.cargo("build")
+        .with_stderr_contains(
+            "\
+[WARNING] conflicting between `crate-type` and `crate_type` in the `ex` example target.\n
+        `crate_type` is ignored and not recommended for use in the future
+[WARNING] conflicting between `crate-type` and `crate_type` in the `goodbye` example target.\n
+        `crate_type` is ignored and not recommended for use in the future",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn self_dependency() {
     let p = project()
@@ -2841,6 +2988,88 @@ fn cargo_platform_specific_dependency() {
     p.cargo("test").run();
 }
 
+#[cargo_test]
+fn cargo_platform_specific_dependency_build_dependencies_conflicting_warning() {
+    let host = rustc_host();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [project]
+                    name = "foo"
+                    version = "0.5.0"
+                    authors = ["wycats@example.com"]
+                    build = "build.rs"
+
+                    [target.{host}.build-dependencies]
+                    build = {{ path = "build" }}
+                    [target.{host}.build_dependencies]
+                    build = {{ path = "build" }}
+                "#,
+                host = host
+            ),
+        )
+        .file("src/main.rs", "fn main() { }")
+        .file(
+            "build.rs",
+            "extern crate build; fn main() { build::build(); }",
+        )
+        .file("build/Cargo.toml", &basic_manifest("build", "0.5.0"))
+        .file("build/src/lib.rs", "pub fn build() {}")
+        .build();
+
+    p.cargo("build")
+        .with_stderr_contains(
+        format!("[WARNING] conflicting between `build-dependencies` and `build_dependencies` in the `{}` platform target.\n
+        `build_dependencies` is ignored and not recommended for use in the future", host)
+        )
+        .run();
+
+    assert!(p.bin("foo").is_file());
+}
+
+#[cargo_test]
+fn cargo_platform_specific_dependency_dev_dependencies_conflicting_warning() {
+    let host = rustc_host();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [project]
+                    name = "foo"
+                    version = "0.5.0"
+                    authors = ["wycats@example.com"]
+
+                    [target.{host}.dev-dependencies]
+                    dev = {{ path = "dev" }}
+                    [target.{host}.dev_dependencies]
+                    dev = {{ path = "dev" }}
+                "#,
+                host = host
+            ),
+        )
+        .file("src/main.rs", "fn main() { }")
+        .file(
+            "tests/foo.rs",
+            "extern crate dev; #[test] fn foo() { dev::dev() }",
+        )
+        .file("dev/Cargo.toml", &basic_manifest("dev", "0.5.0"))
+        .file("dev/src/lib.rs", "pub fn dev() {}")
+        .build();
+
+    p.cargo("build")
+        .with_stderr_contains(
+        format!("[WARNING] conflicting between `dev-dependencies` and `dev_dependencies` in the `{}` platform target.\n
+        `dev_dependencies` is ignored and not recommended for use in the future", host)
+        )
+        .run();
+
+    assert!(p.bin("foo").is_file());
+    p.cargo("test").run();
+}
+
 #[cargo_test]
 fn bad_platform_specific_dependency() {
     let p = project()
diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index 8a8ed78e595..cc5d10e4045 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2067,3 +2067,43 @@ Caused by:
         )
         .run();
 }
+
+#[cargo_test]
+fn default_features_conflicting_warning() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                authors = []
+
+                [dependencies]
+                a = { path = "a", features = ["f1"], default-features = false, default_features = false }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.1.0"
+                authors = []
+
+                [features]
+                default = ["f1"]
+                f1 = []
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .build();
+
+    p.cargo("build")
+        .with_stderr_contains(
+"[WARNING] conflicting between `default-features` and `default_features` in the `a` dependency.\n
+        `default_features` is ignored and not recommended for use in the future"
+        )
+        .run();
+}
diff --git a/tests/testsuite/proc_macro.rs b/tests/testsuite/proc_macro.rs
index 12ff56284a8..50e432a19c8 100644
--- a/tests/testsuite/proc_macro.rs
+++ b/tests/testsuite/proc_macro.rs
@@ -387,6 +387,31 @@ fn proc_macro_crate_type_warning() {
         .run();
 }
 
+#[cargo_test]
+fn proc_macro_conflicting_warning() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                [lib]
+                proc-macro = false
+                proc_macro = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("build")
+        .with_stderr_contains(
+"[WARNING] conflicting between `proc-macro` and `proc_macro` in the `foo` library target.\n
+        `proc_macro` is ignored and not recommended for use in the future",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn proc_macro_crate_type_warning_plugin() {
     let foo = project()