diff --git a/changelog.d/15940.misc b/changelog.d/15940.misc
new file mode 100644
index 0000000000..eac008eb3e
--- /dev/null
+++ b/changelog.d/15940.misc
@@ -0,0 +1 @@
+Unbreak the nix development environment by pinning the Rust version to 1.70.0.
\ No newline at end of file
diff --git a/flake.lock b/flake.lock
index d1c933e9aa..eb5a65e445 100644
--- a/flake.lock
+++ b/flake.lock
@@ -22,27 +22,6 @@
         "type": "github"
       }
     },
-    "fenix": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ],
-        "rust-analyzer-src": "rust-analyzer-src"
-      },
-      "locked": {
-        "lastModified": 1682490133,
-        "narHash": "sha256-tR2Qx0uuk97WySpSSk4rGS/oH7xb5LykbjATcw1vw1I=",
-        "owner": "nix-community",
-        "repo": "fenix",
-        "rev": "4e9412753ab75ef0e038a5fe54a062fb44c27c6a",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "repo": "fenix",
-        "type": "github"
-      }
-    },
     "flake-compat": {
       "flake": false,
       "locked": {
@@ -74,6 +53,24 @@
         "type": "github"
       }
     },
+    "flake-utils_2": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1681202837,
+        "narHash": "sha256-H+Rh19JDwRtpVPAWp64F+rlEtxUWBAQW28eAi3SRSzg=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "cfacdce06f30d2b68473a46042957675eebb3401",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "gitignore": {
       "inputs": {
         "nixpkgs": [
@@ -200,6 +197,22 @@
         "type": "github"
       }
     },
+    "nixpkgs_3": {
+      "locked": {
+        "lastModified": 1681358109,
+        "narHash": "sha256-eKyxW4OohHQx9Urxi7TQlFBTDWII+F+x2hklDOQPB50=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "96ba1c52e54e74c3197f4d43026b3f3d92e83ff9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "pre-commit-hooks": {
       "inputs": {
         "flake-compat": [
@@ -231,25 +244,27 @@
     "root": {
       "inputs": {
         "devenv": "devenv",
-        "fenix": "fenix",
         "nixpkgs": "nixpkgs_2",
-        "systems": "systems"
+        "rust-overlay": "rust-overlay",
+        "systems": "systems_2"
       }
     },
-    "rust-analyzer-src": {
-      "flake": false,
+    "rust-overlay": {
+      "inputs": {
+        "flake-utils": "flake-utils_2",
+        "nixpkgs": "nixpkgs_3"
+      },
       "locked": {
-        "lastModified": 1682426789,
-        "narHash": "sha256-UqnLmJESRZE0tTEaGbRAw05Hm19TWIPA+R3meqi5I4w=",
-        "owner": "rust-lang",
-        "repo": "rust-analyzer",
-        "rev": "943d2a8a1ca15e8b28a1f51f5a5c135e3728da04",
+        "lastModified": 1689302058,
+        "narHash": "sha256-yD74lcHTrw4niXcE9goJLbzsgyce48rQQoy5jK5ZK40=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "7b8dbbf4c67ed05a9bf3d9e658c12d4108bc24c8",
         "type": "github"
       },
       "original": {
-        "owner": "rust-lang",
-        "ref": "nightly",
-        "repo": "rust-analyzer",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
         "type": "github"
       }
     },
@@ -267,6 +282,21 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",
diff --git a/flake.nix b/flake.nix
index bb42c9ff9b..bacb70f478 100644
--- a/flake.nix
+++ b/flake.nix
@@ -46,20 +46,20 @@
     systems.url = "github:nix-systems/default";
     # A development environment manager built on Nix. See https://devenv.sh.
     devenv.url = "github:cachix/devenv/main";
-    # Rust toolchains and rust-analyzer nightly.
-    fenix = {
-      url = "github:nix-community/fenix";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
+    # Rust toolchain.
+    rust-overlay.url = "github:oxalica/rust-overlay";
   };
 
-  outputs = { self, nixpkgs, devenv, systems, ... } @ inputs:
+  outputs = { self, nixpkgs, devenv, systems, rust-overlay, ... } @ inputs:
     let
       forEachSystem = nixpkgs.lib.genAttrs (import systems);
     in {
       devShells = forEachSystem (system:
         let
-          pkgs = nixpkgs.legacyPackages.${system};
+          overlays = [ (import rust-overlay) ];
+          pkgs = import nixpkgs {
+            inherit system overlays;
+          };
         in {
           # Everything is configured via devenv - a Nix module for creating declarative
           # developer environments. See https://devenv.sh/reference/options/ for a list
@@ -76,6 +76,20 @@
                 # Configure packages to install.
                 # Search for package names at https://search.nixos.org/packages?channel=unstable
                 packages = with pkgs; [
+                  # The rust toolchain and related tools.
+                  # This will install the "default" profile of rust components.
+                  # https://rust-lang.github.io/rustup/concepts/profiles.html
+                  #
+                  # NOTE: We currently need to set the Rust version unnecessarily high
+                  # in order to work around https://github.com/matrix-org/synapse/issues/15939
+                  (rust-bin.stable."1.70.0".default.override {
+                    # Additionally install the "rust-src" extension to allow diving into the
+                    # Rust source code in an IDE (rust-analyzer will also make use of it).
+                    extensions = [ "rust-src" ];
+                  })
+                  # The rust-analyzer language server implementation.
+                  rust-analyzer
+
                   # Native dependencies for running Synapse.
                   icu
                   libffi
@@ -124,12 +138,11 @@
                 # Install dependencies for the additional programming languages
                 # involved with Synapse development.
                 #
-                # * Rust is used for developing and running Synapse.
                 # * Golang is needed to run the Complement test suite.
                 # * Perl is needed to run the SyTest test suite.
+                # * Rust is used for developing and running Synapse.
+                #   It is installed manually with `packages` above.
                 languages.go.enable = true;
-                languages.rust.enable = true;
-                languages.rust.version = "stable";
                 languages.perl.enable = true;
 
                 # Postgres is needed to run Synapse with postgres support and