diff --git a/README.md b/README.md
index 6e2ada95893..fbfcab72288 100644
--- a/README.md
+++ b/README.md
@@ -87,9 +87,9 @@ Here's an example of using it in nixos configuration.
     distRoot = "https://static.rust-lang.org/dist";
 
     # Select a toolchain and aggregate components by rustup's `rust-toolchain` file format.
-    # See: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
+    # See: https://github.com/ebroto/rustup/blob/c2db7dac6b38c99538eec472db9d23d18f918409/README.md#the-toolchain-file
     fromRustupToolchain = { channel, components ? [], targets ? [] }: «derivation»;
-    # Same as `fromRustupToolchain` but read from a `rust-toolchain` TOML file.
+    # Same as `fromRustupToolchain` but read from a `rust-toolchain` file (legacy one-line string or in TOML).
     fromRustupToolchainFile = rust-toolchain-file-path: «derivation»;
 
     stable = {
diff --git a/flake.nix b/flake.nix
index 53562958cce..685b6e10abc 100644
--- a/flake.nix
+++ b/flake.nix
@@ -96,12 +96,16 @@
             extensions = [ "rustfmt-preview" "rustc-dev" ];
             targets = [ "wasm32-unknown-unknown" "aarch64-unknown-linux-gnu" ];
           });
-        rustup-toolchain-customization-file = assertEq
-          (fromRustupToolchainFile ./tests/rust-toolchain)
+
+        rustup-toolchain-file-toml = assertEq
+          (fromRustupToolchainFile ./tests/rust-toolchain-toml)
           (nightly."2020-07-10".rust.override {
             extensions = [ "rustfmt-preview" "rustc-dev" ];
             targets = [ "wasm32-unknown-unknown" "aarch64-unknown-linux-gnu" ];
           });
+        rustup-toolchain-file-legacy = assertEq
+          (fromRustupToolchainFile ./tests/rust-toolchain-legacy)
+          nightly."2020-07-10".rust;
       };
 
       checkDrvs = {};
diff --git a/rust-overlay.nix b/rust-overlay.nix
index 737b9e7681f..2943532c786 100644
--- a/rust-overlay.nix
+++ b/rust-overlay.nix
@@ -34,13 +34,22 @@ let
     else throw "Unknown channel: ${channel}";
 
   # Select a toolchain and aggregate components by rustup's `rust-toolchain` file format.
-  # See: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
+  # See: https://github.com/ebroto/rustup/blob/c2db7dac6b38c99538eec472db9d23d18f918409/README.md#the-toolchain-file
   fromRustupToolchain = { channel, components ? [], targets ? [] }:
     (toolchainFromManifest (selectManifest { inherit channel; })).rust.override {
       extensions = components;
       inherit targets;
     };
 
+  # Same as `fromRustupToolchain` but read from a `rust-toolchain` file (legacy one-line string or in TOML).
+  fromRustupToolchainFile = path: let
+    inherit (builtins) readFile match fromTOML head;
+    content = readFile path;
+    legacy = match "([^\r\n]+)\r?\n?" content;
+  in if legacy != null
+    then fromRustupToolchain { channel = head legacy; }
+    else fromRustupToolchain (fromTOML content).toolchain;
+
   getComponentsWithFixedPlatform = pkgs: pkgname: stdenv:
     let
       pkg = pkgs.${pkgname};
@@ -315,8 +324,7 @@ in {
     (super.rust-bin or {}) //
     mapAttrs (channel: mapAttrs (version: toolchainFromManifest)) super.rust-bin.manifests //
     {
-      fromRustupToolchainFile = path: fromRustupToolchain (fromTOML (readFile path)).toolchain;
-      inherit fromRustupToolchain;
+      inherit fromRustupToolchain fromRustupToolchainFile;
     };
 
   # All attributes below are for compatiblity with mozilla overlay.
diff --git a/tests/rust-toolchain-legacy b/tests/rust-toolchain-legacy
new file mode 100644
index 00000000000..95a7229aa56
--- /dev/null
+++ b/tests/rust-toolchain-legacy
@@ -0,0 +1 @@
+nightly-2020-07-10
diff --git a/tests/rust-toolchain b/tests/rust-toolchain-toml
similarity index 100%
rename from tests/rust-toolchain
rename to tests/rust-toolchain-toml