Test showing surprising inference of the unit type.

.envrc

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+use_nix

.gitignore

@@ -0,0 +1 @@
+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "proc-macro2"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e0704ee1a7e00d7bb417d0770ea303c1bccbabf0ef1667dae92b5967f5f8a71"
+dependencies = [
+ "unicode-xid",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "991431c3519a3f36861882da93630ce66b52918dcf1b8e2fd66b397fc96f28df"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "rustinferencetest"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "serde_derive",
+ "toml",
+]
+
+[[package]]
+name = "serde"
+version = "1.0.123"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92d5161132722baa40d802cc70b15262b98258453e85e5d1d365c757c73869ae"
+
+[[package]]
+name = "serde_derive"
+version = "1.0.123"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9391c295d64fc0abb2c556bad848f33cb8296276b1ad2677d1ae1ace4f258f31"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "syn"
+version = "1.0.60"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c700597eca8a5a762beb35753ef6b94df201c81cca676604f547495a0d7f0081"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-xid",
+]
+
+[[package]]
+name = "toml"
+version = "0.5.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a31142970826733df8241ef35dc040ef98c679ab14d7c3e54d827099b3acecaa"
+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "unicode-xid"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f7fe0bb3479651439c9112f72b6c505038574c9fbb575ed1bf3b797fa39dd564"

Cargo.toml

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+[package]
+name = "rustinferencetest"
+version = "0.1.0"
+authors = ["Bill Thiede <git@xinu.tv>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+serde = "1"
+serde_derive = "1"
+toml = "0.5.8"

default.nix

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+let
+  unstableTarball = fetchTarball
+    "https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz";
+  pkgs = import <nixpkgs> { };
+  unstable = import unstableTarball { };
+
+in with pkgs;
+pkgs.mkShell {
+  name = "rust";
+  buildInputs = [
+    openssl
+    pkg-config
+    unstable.cargo
+    unstable.rustc
+    unstable.rustfmt
+    unstable.rust-analyzer
+    postgresql
+  ];
+}

src/main.rs

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+use serde_derive::{Deserialize, Serialize};
+use std::collections::HashMap;
+
+#[derive(Debug, Serialize, Deserialize)]
+struct Config {
+    sites: HashMap<String, Site>,
+}
+#[derive(Debug, Serialize, Deserialize)]
+struct Site {
+    url: String,
+    title: String,
+    hover: Option<String>,
+    main_image: String,
+    alternate_image: Option<String>,
+}
+
+fn main() -> Result<(), Box<std::error::Error>> {
+    let c = Config {
+        sites: vec![
+            (
+                "xkcd".to_string(),
+                Site {
+                    url: "https://xkcd.com/1/".to_string(),
+                    title: "title".to_string(),
+                    hover: None,
+                    main_image: "img".to_string(),
+                    alternate_image: None,
+                },
+            ),
+            (
+                "sinfest".to_string(),
+                Site {
+                    url: "https://sinfest.net/view.php?date=2000-01-17".to_string(),
+                    title: "title".to_string(),
+                    hover: None,
+                    main_image: "img".to_string(),
+                    alternate_image: None,
+                },
+            ),
+        ]
+        .into_iter()
+        .collect(),
+    };
+    let s = toml::to_string(&c)?;
+    println!("s {}", s);
+    // c is inferred as the unit type judging by the error you get when you print with Display
+    // formatting (i.e. "{}") instead of debug formating (i.e. "{:?}").
+    let c = toml::from_str(&s)?;
+    println!("c {:?}", c);
+    Ok(())
+}