Add code

2025-03-26 12:41:58 -04:00
parent 637b26a0e9
commit 416dd617e6
36 changed files with 732 additions and 0 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -0,0 +1,345 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "anyhow"
+version = "1.0.86"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b3d1d046238990b9cf5bcde22a3fb3584ee5cf65fb2765f454ed428c7a0063da"
+
+[[package]]
+name = "autocfg"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0c4b4d0bd25bd0b74681c0ad21497610ce1b7c91b1022cd21c80c6fbdd9476b0"
+
+[[package]]
+name = "bitflags"
+version = "2.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b048fb63fd8b5923fc5aa7b340d8e156aec7ec02f0c78fa8a6ddc2613f6f71de"
+
+[[package]]
+name = "cfg-if"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
+
+[[package]]
+name = "crossbeam"
+version = "0.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1137cd7e7fc0fb5d3c5a8678be38ec56e819125d8d7907411fe24ccb943faca8"
+dependencies = [
+ "crossbeam-channel",
+ "crossbeam-deque",
+ "crossbeam-epoch",
+ "crossbeam-queue",
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-channel"
+version = "0.5.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "33480d6946193aa8033910124896ca395333cae7e2d1113d1fef6c3272217df2"
+dependencies = [
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-deque"
+version = "0.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "613f8cc01fe9cf1a3eb3d7f488fd2fa8388403e97039e2f73692932e291a770d"
+dependencies = [
+ "crossbeam-epoch",
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-epoch"
+version = "0.9.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b82ac4a3c2ca9c3460964f020e1402edd5753411d7737aa39c3714ad1b5420e"
+dependencies = [
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-queue"
+version = "0.3.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "df0346b5d5e76ac2fe4e327c5fd1118d6be7c51dfb18f9b7922923f287471e35"
+dependencies = [
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-utils"
+version = "0.8.20"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22ec99545bb0ed0ea7bb9b8e1e9122ea386ff8a48c0922e43f36d45ab09e0e80"
+
+[[package]]
+name = "either"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
+
+[[package]]
+name = "example_1"
+version = "0.1.0"
+
+[[package]]
+name = "example_10"
+version = "0.1.0"
+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "example_11"
+version = "0.1.0"
+
+[[package]]
+name = "example_2"
+version = "0.1.0"
+dependencies = [
+ "crossbeam",
+ "parking_lot",
+]
+
+[[package]]
+name = "example_3"
+version = "0.1.0"
+dependencies = [
+ "rayon",
+]
+
+[[package]]
+name = "example_4"
+version = "0.1.0"
+
+[[package]]
+name = "example_5"
+version = "0.1.0"
+
+[[package]]
+name = "example_6"
+version = "0.1.0"
+
+[[package]]
+name = "example_7"
+version = "0.1.0"
+
+[[package]]
+name = "example_8"
+version = "0.1.0"
+
+[[package]]
+name = "example_9"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]
+
+[[package]]
+name = "libc"
+version = "0.2.155"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "97b3888a4aecf77e811145cadf6eef5901f4782c53886191b2f693f24761847c"
+
+[[package]]
+name = "lock_api"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "07af8b9cdd281b7915f413fa73f29ebd5d55d0d3f0155584dade1ff18cea1b17"
+dependencies = [
+ "autocfg",
+ "scopeguard",
+]
+
+[[package]]
+name = "parking_lot"
+version = "0.12.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1bf18183cf54e8d6059647fc3063646a1801cf30896933ec2311622cc4b9a27"
+dependencies = [
+ "lock_api",
+ "parking_lot_core",
+]
+
+[[package]]
+name = "parking_lot_core"
+version = "0.9.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e401f977ab385c9e4e3ab30627d6f26d00e2c73eef317493c4ec6d468726cf8"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "redox_syscall",
+ "smallvec",
+ "windows-targets",
+]
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.86"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5e719e8df665df0d1c8fbfd238015744736151d4445ec0836b8e628aae103b77"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.36"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0fa76aaf39101c457836aec0ce2316dbdc3ab723cdda1c6bd4e6ad4208acaca7"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "rayon"
+version = "1.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b418a60154510ca1a002a752ca9714984e21e4241e804d32555251faf8b78ffa"
+dependencies = [
+ "either",
+ "rayon-core",
+]
+
+[[package]]
+name = "rayon-core"
+version = "1.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1465873a3dfdaa8ae7cb14b4383657caab0b3e8a0aa9ae8e04b044854c8dfce2"
+dependencies = [
+ "crossbeam-deque",
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "redox_syscall"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2a908a6e00f1fdd0dfd9c0eb08ce85126f6d8bbda50017e74bc4a4b7d4a926a4"
+dependencies = [
+ "bitflags",
+]
+
+[[package]]
+name = "scopeguard"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
+
+[[package]]
+name = "serde"
+version = "1.0.207"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5665e14a49a4ea1b91029ba7d3bca9f299e1f7cfa194388ccc20f14743e784f2"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.207"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6aea2634c86b0e8ef2cfdc0c340baede54ec27b1e46febd7f80dffb2aa44a00e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "smallvec"
+version = "1.13.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c5e1a9a646d36c3599cd173a41282daf47c44583ad367b8e6837255952e5c67"
+
+[[package]]
+name = "syn"
+version = "2.0.74"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1fceb41e3d546d0bd83421d3409b1460cc7444cd389341a4c880fe7a042cb3d7"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
+
+[[package]]
+name = "windows-targets"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9b724f72796e036ab90c1021d4780d4d3d648aca59e491e6b98e725b84e99973"
+dependencies = [
+ "windows_aarch64_gnullvm",
+ "windows_aarch64_msvc",
+ "windows_i686_gnu",
+ "windows_i686_gnullvm",
+ "windows_i686_msvc",
+ "windows_x86_64_gnu",
+ "windows_x86_64_gnullvm",
+ "windows_x86_64_msvc",
+]
+
+[[package]]
+name = "windows_aarch64_gnullvm"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32a4622180e7a0ec044bb555404c800bc9fd9ec262ec147edd5989ccd0c02cd3"
+
+[[package]]
+name = "windows_aarch64_msvc"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09ec2a7bb152e2252b53fa7803150007879548bc709c039df7627cabbd05d469"
+
+[[package]]
+name = "windows_i686_gnu"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8e9b5ad5ab802e97eb8e295ac6720e509ee4c243f69d781394014ebfe8bbfa0b"
+
+[[package]]
+name = "windows_i686_gnullvm"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0eee52d38c090b3caa76c563b86c3a4bd71ef1a819287c19d586d7334ae8ed66"
+
+[[package]]
+name = "windows_i686_msvc"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "240948bc05c5e7c6dabba28bf89d89ffce3e303022809e73deaefe4f6ec56c66"
+
+[[package]]
+name = "windows_x86_64_gnu"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "147a5c80aabfbf0c7d901cb5895d1de30ef2907eb21fbbab29ca94c5b08b1a78"
+
+[[package]]
+name = "windows_x86_64_gnullvm"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "24d5b23dc417412679681396f2b49f3de8c1473deb516bd34410872eff51ed0d"
+
+[[package]]
+name = "windows_x86_64_msvc"
+version = "0.52.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -0,0 +1,16 @@
+[workspace]
+
+resolver = "2"
+members = [
+    "example_1",
+    "example_10",
+    "example_11",
+    "example_2",
+    "example_3",
+    "example_4",
+    "example_5",
+    "example_6",
+    "example_7",
+    "example_8",
+    "example_9",
+]
--- a/example_1/.gitignore
+++ b/example_1/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_1/Cargo.lock
+++ b/example_1/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "example_1"
+version = "0.1.0"
--- a/example_1/Cargo.toml
+++ b/example_1/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_1"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_1/src/main.rs
+++ b/example_1/src/main.rs
@@ -0,0 +1,6 @@
+fn main() {
+    let mut vec = vec![1, 2, 3];
+    let first: &i32 = &vec[0];
+    vec.push(92);
+    println!("{}", *first);
+}
--- a/example_10/.gitignore
+++ b/example_10/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_10/Cargo.toml
+++ b/example_10/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "example_10"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+
+[features]
+default = ["test"]
+
+test = []
--- a/example_10/src/main.rs
+++ b/example_10/src/main.rs
@@ -0,0 +1,28 @@
+use serde::Deserialize;
+use serde::Serialize;
+
+macro_rules! assert_eq {
+    ($left:expr, $right:expr) => {
+        let left = $left;
+        let right = $right;
+        if left != right {
+            panic!("assertion failed: {:?} != {:?}", left, right);
+        }
+    };
+}
+
+#[derive(Serialize, Deserialize)]
+struct Person {
+    name: String,
+    age: u32,
+}
+
+fn main() {
+    assert_eq!(1 + 1, 2);
+
+    if cfg!(feature = "test") {
+        println!("yes")
+    } else {
+        println!("no")
+    }
+}
--- a/example_11/.gitignore
+++ b/example_11/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_11/Cargo.toml
+++ b/example_11/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_11"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_11/src/main.rs
+++ b/example_11/src/main.rs
@@ -0,0 +1,14 @@
+use std::slice;
+
+fn main() {
+    let some_vector = vec![1, 2, 3, 4];
+
+    let pointer = some_vector.as_ptr();
+    let length = some_vector.len();
+
+    unsafe {
+        let my_slice: &[u32] = slice::from_raw_parts(pointer, length);
+
+        assert_eq!(some_vector.as_slice(), my_slice);
+    }
+}
--- a/example_2/.gitignore
+++ b/example_2/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_2/Cargo.toml
+++ b/example_2/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "example_2"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+parking_lot = "0.12.3"
+crossbeam = "0.8.4"
--- a/example_2/src/main.rs
+++ b/example_2/src/main.rs
@@ -0,0 +1,21 @@
+use crossbeam::scope;
+use parking_lot::{Mutex, MutexGuard};
+
+fn main() {
+    let mut counter = Mutex::new(0);
+
+    scope(|s| {
+        for _ in 0..10 {
+            s.spawn(|_| {
+                for _ in 0..10 {
+                    let mut guard: MutexGuard<i32> = counter.lock();
+                    *guard += 1;
+                }
+            });
+        }
+    })
+    .unwrap();
+
+    let total: &mut i32 = counter.get_mut();
+    println!("total = {}", *total)
+}
--- a/example_3/.gitignore
+++ b/example_3/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_3/Cargo.toml
+++ b/example_3/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "example_3"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+rayon = "1.10.0"
--- a/example_3/src/main.rs
+++ b/example_3/src/main.rs
@@ -0,0 +1,19 @@
+use rayon::iter::{IntoParallelIterator, IntoParallelRefIterator, ParallelIterator};
+use std::fs::read_to_string;
+
+fn main() {
+    // read a csv file to a string
+    let my_string = read_to_string("my_file.txt").unwrap();
+
+    let my_vec = my_string.lines().collect::<Vec<&str>>();
+    let my_vec: Vec<String> = my_vec
+        //.into_iter()
+        .into_par_iter()
+        .filter(|x| x.contains("ERROR"))
+        .map(|x| x.to_owned())
+        .collect();
+
+    // this should now print a vec of vecs
+    // where every single value is the "Hello world!" string
+    println!("{:?}", my_vec);
+}
--- a/example_4/.gitignore
+++ b/example_4/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_4/Cargo.toml
+++ b/example_4/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_4"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_4/src/main.rs
+++ b/example_4/src/main.rs
@@ -0,0 +1,15 @@
+#[derive(Debug)]
+enum Tree<T> {
+    Empty,
+    Node(T, Box<Tree<T>>, Box<Tree<T>>),
+}
+
+fn main() {
+    let tree = Tree::Node(
+        42,
+        Box::new(Tree::Node(0, Box::new(Tree::Empty), Box::new(Tree::Empty))),
+        Box::new(Tree::Empty),
+    );
+
+    println!("{:?}", tree); // prints Node(42, Node(0, Empty, Empty), Empty)
+}
--- a/example_5/.gitignore
+++ b/example_5/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_5/Cargo.toml
+++ b/example_5/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_5"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_5/src/main.rs
+++ b/example_5/src/main.rs
@@ -0,0 +1,27 @@
+struct Years(i64);
+struct Days(i64);
+
+impl Years {
+    pub fn to_days(&self) -> Days {
+        Days(self.0 * 365)
+    }
+}
+
+impl Days {
+    /// truncates partial years
+    pub fn to_years(&self) -> Years {
+        Years(self.0 / 365)
+    }
+}
+
+fn is_adult(age: &Years) -> bool {
+    age.0 >= 18
+}
+
+fn main() {
+    let age = Years(25);
+    let age_days = age.to_days();
+    println!("Is an adult? {}", is_adult(&age));
+    println!("Is an adult? {}", is_adult(&age_days.to_years()));
+    // println!("Is an adult? {}", is_adult(&age_days));
+}
--- a/example_6/.gitignore
+++ b/example_6/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_6/Cargo.toml
+++ b/example_6/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_6"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_6/src/main.rs
+++ b/example_6/src/main.rs
@@ -0,0 +1,46 @@
+use std::fmt::Display;
+
+struct GameScore<T> {
+    name: String,
+    scores: Vec<T>,
+}
+
+struct Example {
+    name: String,
+}
+
+impl Example {
+    fn new(name: String) -> Self {
+        Self { name }
+    }
+}
+
+impl<T: Ord> GameScore<T> {
+    fn high_score(&self) -> Option<&T> {
+        self.scores.iter().max()
+    }
+}
+
+impl<T: Display> GameScore<T> {
+    fn reverse_print_scores(&mut self) {
+        while let Some(last) = self.scores.pop() {
+            println!("{}", last);
+        }
+    }
+}
+
+fn main() {
+    let mut taylor_score = GameScore {
+        name: "taylor".to_string(),
+        scores: vec![Example::new("test".to_string())],
+    };
+
+    let mut bad_score = GameScore {
+        name: "taylor".to_string(),
+        scores: vec![10, 200, 3],
+    };
+
+    println!("{:?}", taylor_score.high_score());
+
+    taylor_score.reverse_print_scores();
+}
--- a/example_7/.gitignore
+++ b/example_7/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_7/Cargo.toml
+++ b/example_7/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_7"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_7/src/main.rs
+++ b/example_7/src/main.rs
@@ -0,0 +1,27 @@
+fn double_free() {
+    // every value has an owner, responsible for destructor (RAII).
+    // compiler checks:
+
+    // only ever one owner:
+    // no double-free
+
+    let x: Vec<u32> = Vec::new();
+    let y = x;
+    drop(x); // illegal, y is now owner
+}
+
+fn main() {
+    double_free();
+    immutable();
+}
+
+// Functions and structs can be declared any order (looking at you c++ and python!)
+fn immutable() {
+    let v = Vec::new();
+
+    // this compiles just fine:
+    println!("len: {}", v.len());
+
+    // this will not compile; would need mutable access
+    v.push(42);
+}
--- a/example_8/.gitignore
+++ b/example_8/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_8/Cargo.toml
+++ b/example_8/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "example_8"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
--- a/example_8/src/main.rs
+++ b/example_8/src/main.rs
@@ -0,0 +1,23 @@
+use std::rc::Rc; // reference-counted, non atomic
+use std::sync::Arc; // reference-counted, atomic
+
+fn main() {
+    // this will not compile:
+    let rc = Rc::new("not thread safe");
+    std::thread::spawn(move || {
+        println!("I have an rc with: {}", rc);
+    });
+
+    // this compiles fine:
+    let arc = Arc::new("thread safe");
+    std::thread::spawn(move || {
+        println!("I have an arc with: {}", arc);
+    });
+
+    // this will also not compile:
+    let mut v = Vec::new();
+    std::thread::spawn(|| {
+        v.push(42);
+    });
+    let _ = v.pop();
+}
--- a/example_9/.gitignore
+++ b/example_9/.gitignore
@@ -0,0 +1 @@
+/target
--- a/example_9/Cargo.toml
+++ b/example_9/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "example_9"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.86"
--- a/example_9/src/main.rs
+++ b/example_9/src/main.rs
@@ -0,0 +1,51 @@
+use anyhow::{Context, Result};
+use std::num::NonZero;
+
+enum CustomOption<T> {
+    Some(T),
+    None,
+}
+enum CustomResult<T, E> {
+    Ok(T),
+    Err(E),
+}
+
+struct CustomVec {}
+
+impl CustomVec {
+    fn new() -> Self {
+        Self {}
+    }
+}
+
+pub trait Findable<T> {
+    fn find(&self, fun: T) -> Option<u32>;
+}
+
+impl<F> Findable<F> for CustomVec
+where
+    F: Fn(i32) -> bool,
+{
+    fn find(&self, fun: F) -> Option<u32> {
+        todo!()
+    }
+}
+fn do_stuff() -> Result<NonZero<i32>> {
+    let my_vec = CustomVec::new();
+
+    // v is Option<&T>, not &T -- cannot use without checking for None
+    let v = my_vec.find(|t| t >= 42);
+
+    // n is Result<i32, ParseIntError> -- cannot use without checking for Err
+    let n = "42".parse::<u32>();
+
+    // ? suffix is "return Err if Err, otherwise unwrap Ok"
+    let n = "42".parse::<u32>()?;
+
+    let n = NonZero::new(43).context("oopie")?;
+    Ok(n)
+}
+
+fn main() {
+    do_stuff();
+}