diff --git a/Cargo.lock b/Cargo.lock
index 7337901bc3a59..412a4e5a2b3d7 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4236,6 +4236,7 @@ dependencies = [
  "rustc_fluent_macro",
  "rustc_fs_util",
  "rustc_hir",
+ "rustc_lexer",
  "rustc_lint_defs",
  "rustc_macros",
  "rustc_serialize",
diff --git a/compiler/rustc_session/Cargo.toml b/compiler/rustc_session/Cargo.toml
index e26d25d9a4123..5b91d75a2c974 100644
--- a/compiler/rustc_session/Cargo.toml
+++ b/compiler/rustc_session/Cargo.toml
@@ -19,6 +19,7 @@ rustc_span = { path = "../rustc_span" }
 rustc_fs_util = { path = "../rustc_fs_util" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_lint_defs = { path = "../rustc_lint_defs" }
+rustc_lexer = { path = "../rustc_lexer" }
 smallvec = "1.8.1"
 termize = "0.1.1"
 
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index f00472f181d8e..eee9ac54b3a16 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2451,6 +2451,19 @@ pub fn parse_externs(
             Some((opts, name)) => (Some(opts), name.to_string()),
         };
 
+        if !rustc_lexer::is_ident(&name) {
+            let mut error = handler.early_struct_error(format!(
+                "crate name `{name}` passed to `--extern` is not a valid identifier"
+            ));
+            let adjusted_name = name.replace("-", "_");
+            if rustc_lexer::is_ident(&adjusted_name) {
+                error.help(format!(
+                    "consider replacing the dashes with underscores: `{adjusted_name}`"
+                ));
+            }
+            error.emit();
+        }
+
         let path = path.map(|p| CanonicalizedPath::new(p));
 
         let entry = externs.entry(name.to_owned());
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 086ce4e69646a..889d7e730108f 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1710,6 +1710,15 @@ impl EarlyErrorHandler {
         self.handler.struct_fatal(msg).emit()
     }
 
+    #[allow(rustc::untranslatable_diagnostic)]
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    pub(crate) fn early_struct_error(
+        &self,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, !> {
+        self.handler.struct_fatal(msg)
+    }
+
     #[allow(rustc::untranslatable_diagnostic)]
     #[allow(rustc::diagnostic_outside_of_impl)]
     pub fn early_warn(&self, msg: impl Into<DiagnosticMessage>) {
diff --git a/tests/ui/extern-flag/invalid-crate-name-dashed.rs b/tests/ui/extern-flag/invalid-crate-name-dashed.rs
new file mode 100644
index 0000000000000..bb1f4b0a4dfd4
--- /dev/null
+++ b/tests/ui/extern-flag/invalid-crate-name-dashed.rs
@@ -0,0 +1,7 @@
+// compile-flags: --extern=my-awesome-library=libawesome.rlib
+// error-pattern: crate name `my-awesome-library` passed to `--extern` is not a valid identifier
+// error-pattern: consider replacing the dashes with underscores: `my_awesome_library`
+
+pub use my_awesome_library::*;
+
+fn main() {}
diff --git a/tests/ui/extern-flag/invalid-crate-name-dashed.stderr b/tests/ui/extern-flag/invalid-crate-name-dashed.stderr
new file mode 100644
index 0000000000000..00deaf332ea5d
--- /dev/null
+++ b/tests/ui/extern-flag/invalid-crate-name-dashed.stderr
@@ -0,0 +1,4 @@
+error: crate name `my-awesome-library` passed to `--extern` is not a valid identifier
+  |
+  = help: consider replacing the dashes with underscores: `my_awesome_library`
+
diff --git a/tests/ui/extern-flag/invalid-crate-name.rs b/tests/ui/extern-flag/invalid-crate-name.rs
new file mode 100644
index 0000000000000..8be3f1a32ce90
--- /dev/null
+++ b/tests/ui/extern-flag/invalid-crate-name.rs
@@ -0,0 +1,4 @@
+// compile-flags: --extern=?#1%$
+// error-pattern: crate name `?#1%$` passed to `--extern` is not a valid identifier
+
+fn main() {}
diff --git a/tests/ui/extern-flag/invalid-crate-name.stderr b/tests/ui/extern-flag/invalid-crate-name.stderr
new file mode 100644
index 0000000000000..798b88c5fdadb
--- /dev/null
+++ b/tests/ui/extern-flag/invalid-crate-name.stderr
@@ -0,0 +1,2 @@
+error: crate name `?#1%$` passed to `--extern` is not a valid identifier
+
diff --git a/tests/ui/rust-2018/trait-import-suggestions.rs b/tests/ui/rust-2018/trait-import-suggestions.rs
index 9c67c3f4b4b2e..900b3d09334d0 100644
--- a/tests/ui/rust-2018/trait-import-suggestions.rs
+++ b/tests/ui/rust-2018/trait-import-suggestions.rs
@@ -1,6 +1,6 @@
 // edition:2018
 // aux-build:trait-import-suggestions.rs
-// compile-flags:--extern trait-import-suggestions
+// compile-flags:--extern trait_import_suggestions
 
 mod foo {
     mod foobar {