Improve consistency of error message wordings

* Use contractions per Simplified Technical English guidelines
  (e.g. "Couldn't" instead of "Could not").
* Prefer using past tense for potentially transient failures
  (e.g. "Couldn't read buildpack.toml" rather than "Can't").
* Use "I/O error" instead of "IO error" or "IOError"
* Use display representations when including underlying error
  messages in the current error message where possible (ie: For
  most errors apart from `io::Error`). This also means preferring
`panic!` over `.expect()`, since the latter uses the debug
  representation instead of display.
* Use "Error ..." as the prefix for all top level error message
  (i.e. at the `panic!` or `.expect()` call sites).
* Use `#[error(transparent)]` instead of `#[error("{0}")]` per:
  #720 (comment)

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Improved the consistency of all user-facing libcnb.rs error message wordings. ([#722](https://github.com/heroku/libcnb.rs/pull/722))
+
 ### Fixed
 
 - `libcnb-test`:

examples/ruby-sample/src/util.rs

@@ -53,7 +53,7 @@ pub(crate) fn sha256_checksum(path: impl AsRef<Path>) -> Result<String, std::io:
     fs::read(path).map(|bytes| format!("{:x}", sha2::Sha256::digest(bytes)))
 }
 
-/// Helper to run very simple commands where we just need to handle IO errors and non-zero exit
+/// Helper to run very simple commands where we just need to handle I/O errors and non-zero exit
 /// codes. Not very useful in complex scenarios, but can cut down the amount of code in simple
 /// cases.
 pub(crate) fn run_simple_command<E, F: FnOnce(std::io::Error) -> E, F2: FnOnce(ExitStatus) -> E>(

libcnb-cargo/src/package/command.rs

@@ -46,7 +46,7 @@ pub(crate) fn execute(args: &PackageArgs) -> Result<(), Error> {
             CrossCompileAssistance::Configuration { cargo_env } => cargo_env,
             CrossCompileAssistance::NoAssistance => {
                 eprintln!(
-                    "Could not determine automatic cross-compile settings for target triple {}.",
+                    "Couldn't determine automatic cross-compile settings for target triple {}.",
                     args.target
                 );
                 eprintln!("This is not an error, but without proper cross-compile settings in your Cargo manifest and locally installed toolchains, compilation might fail.");

libcnb-common/src/toml_file.rs

@@ -4,7 +4,7 @@ use std::{fs, path::Path};
 /// An error that occurred during reading or writing a TOML file.
 #[derive(thiserror::Error, Debug)]
 pub enum TomlFileError {
-    #[error("IO error while reading/writing TOML file: {0}")]
+    #[error("I/O error while reading/writing TOML file: {0}")]
     IoError(#[from] std::io::Error),
 
     #[error("TOML deserialization error while reading TOML file: {0}")]
@@ -18,7 +18,7 @@ pub enum TomlFileError {
 ///
 /// # Errors
 ///
-/// Will return `Err` if the file could not be written or the value could not be serialized as a TOML string.
+/// Will return `Err` if the file couldn't be written or the value couldn't be serialized as a TOML string.
 pub fn write_toml_file(
     value: &impl Serialize,
     path: impl AsRef<Path>,
@@ -32,7 +32,7 @@ pub fn write_toml_file(
 ///
 /// # Errors
 ///
-/// Will return `Err` if the file could not be read or its contents could not be deserialized.
+/// Will return `Err` if the file couldn't be read or its contents couldn't be deserialized.
 pub fn read_toml_file<A: DeserializeOwned>(path: impl AsRef<Path>) -> Result<A, TomlFileError> {
     let contents = fs::read_to_string(path)?;
     Ok(toml::from_str(&contents)?)

libcnb-data/src/build_plan.rs

@@ -119,7 +119,7 @@ impl Require {
             Ok(())
         } else {
             Err(toml::ser::Error::custom(String::from(
-                "Could not be serialized as a TOML Table.",
+                "Couldn't be serialized as a TOML Table.",
             )))
         }
     }

libcnb-data/src/buildpack_plan.rs

@@ -25,7 +25,11 @@ impl Entry {
     where
         T: Deserialize<'de>,
     {
-        // All toml `Value`s have `try_into()` which converts them to a `T` if `Deserialize` and `Deserializer` is implemented for `T`. Sadly, the `Table` type we use in `Entry` is not a `Value` so we need to make it one by wrapping it. We cannot wrap directly in `Entry` since that would allow users to put non-table TOML values as metadata. As outlined earlier, we can't get around the clone since we're only borrowing the metadata.
+        // All toml `Value`s have `try_into()` which converts them to a `T` if `Deserialize` and
+        // `Deserializer` is implemented for `T`. Sadly, the `Table` type we use in `Entry` is not
+        // a `Value` so we need to make it one by wrapping it. We can't wrap directly in `Entry`
+        // since that would allow users to put non-table TOML values as metadata. As outlined
+        // earlier, we can't get around the clone since we're only borrowing the metadata.
         toml::Value::Table(self.metadata.clone()).try_into()
     }
 }

libcnb-package/src/build.rs

@@ -18,7 +18,7 @@ use std::process::{Command, ExitStatus};
 ///
 /// # Errors
 ///
-/// Will return `Err` if any build did not finish successfully, the configuration cannot be
+/// Will return `Err` if any build did not finish successfully, the configuration can't be
 /// read or the configured main buildpack binary does not exist.
 pub fn build_buildpack_binaries(
     project_path: impl AsRef<Path>,
@@ -167,7 +167,7 @@ pub struct BuildpackBinaries {
 
 #[derive(thiserror::Error, Debug)]
 pub enum BuildError {
-    #[error("Error while running Cargo build process: {0}")]
+    #[error("I/O error while running Cargo build process: {0}")]
     CargoProcessIoError(#[source] std::io::Error),
     #[error("Cargo unexpectedly exited with status {0}")]
     UnexpectedCargoExitStatus(ExitStatus),
@@ -179,6 +179,6 @@ pub enum BuildBinariesError {
     CannotDetermineBuildpackCargoTargetName(#[source] DetermineBuildpackCargoTargetNameError),
     #[error("Failed to build binary target {0}: {1}")]
     BuildError(String, #[source] BuildError),
-    #[error("Binary target {0} could not be found")]
+    #[error("Binary target {0} couldn't be found")]
     MissingBuildpackTarget(String),
 }

libcnb-package/src/buildpack_dependency_graph.rs

@@ -24,7 +24,7 @@ use std::path::{Path, PathBuf};
 /// # Errors
 ///
 /// Returns `Err` if a buildpack declares an invalid dependency, has an invalid buildpack.toml or
-/// package.toml or an IO error occurred while traversing the given directory.
+/// package.toml or an I/O error occurred while traversing the given directory.
 pub fn build_libcnb_buildpacks_dependency_graph(
     cargo_workspace_root: &Path,
 ) -> Result<Graph<BuildpackDependencyGraphNode, ()>, BuildBuildpackDependencyGraphError> {
@@ -86,9 +86,9 @@ fn build_libcnb_buildpack_dependency_graph_node(
 pub enum BuildBuildpackDependencyGraphError {
     #[error("Error while finding buildpack directories: {0}")]
     FindBuildpackDirectories(ignore::Error),
-    #[error("Cannot read buildpack descriptor: {0}")]
+    #[error("Couldn't read buildpack.toml: {0}")]
     ReadBuildpackDescriptorError(TomlFileError),
-    #[error("Cannot read package descriptor: {0}")]
+    #[error("Couldn't read package.toml: {0}")]
     ReadPackageDescriptorError(TomlFileError),
     #[error("Dependency uses an invalid buildpack id: {0}")]
     InvalidDependencyBuildpackId(BuildpackIdError),

libcnb-package/src/cross_compile.rs

@@ -4,7 +4,7 @@ use which::which;
 /// Provides assistance for cross-compiling from the user's host platform to the desired target platform.
 ///
 /// This function will not install required toolchains, linkers or compilers automatically. It will
-/// look for the required tools and returns a human-readable help text if they cannot be found or
+/// look for the required tools and returns a human-readable help text if they can't be found or
 /// any other issue has been detected.
 pub fn cross_compile_assistance(target_triple: impl AsRef<str>) -> CrossCompileAssistance {
     // Background: https://omarkhawaja.com/cross-compiling-rust-from-macos-to-linux/

libcnb-package/src/dependency_graph.rs

@@ -24,7 +24,7 @@ where
 /// # Errors
 ///
 /// Will return an `Err` if the graph contains references to missing dependencies or the
-/// dependencies of a [`DependencyNode`] could not be gathered.
+/// dependencies of a [`DependencyNode`] couldn't be gathered.
 pub fn create_dependency_graph<T, I, E>(
     nodes: Vec<T>,
 ) -> Result<Graph<T, ()>, CreateDependencyGraphError<I, E>>

libcnb-package/src/lib.rs

@@ -44,7 +44,7 @@ pub enum CargoProfile {
 ///
 /// # Errors
 ///
-/// Will return `Err` if the buildpack directory could not be assembled.
+/// Will return `Err` if the buildpack directory couldn't be assembled.
 pub fn assemble_buildpack_directory(
     destination_path: impl AsRef<Path>,
     buildpack_descriptor_path: impl AsRef<Path>,
@@ -132,7 +132,7 @@ pub fn find_buildpack_dirs(start_dir: &Path) -> Result<Vec<PathBuf>, ignore::Err
 ///
 /// # Errors
 ///
-/// Will return an `Err` if the root workspace directory cannot be located due to:
+/// Will return an `Err` if the root workspace directory can't be located due to:
 /// - no `CARGO` environment variable with the path to the `cargo` binary
 /// - executing this function with a directory that is not within a Cargo project
 /// - any other file or system error that might occur
@@ -167,13 +167,13 @@ pub fn find_cargo_workspace_root_dir(
 
 #[derive(thiserror::Error, Debug)]
 pub enum FindCargoWorkspaceRootError {
-    #[error("Cannot get value of CARGO environment variable: {0}")]
+    #[error("Couldn't get value of CARGO environment variable: {0}")]
     GetCargoEnv(#[source] std::env::VarError),
     #[error("Error while spawning Cargo process: {0}")]
     SpawnCommand(#[source] std::io::Error),
     #[error("Unexpected Cargo exit status ({}) while attempting to read workspace root", exit_code_or_unknown(*.0))]
     CommandFailure(std::process::ExitStatus),
-    #[error("Could not locate a Cargo workspace within {0} or its parent directories")]
+    #[error("Couldn't locate a Cargo workspace within {0} or its parent directories")]
     GetParentDirectory(PathBuf),
 }
 

libcnb-package/src/package.rs

@@ -43,9 +43,9 @@ pub fn package_buildpack(
 
 #[derive(thiserror::Error, Debug)]
 pub enum PackageBuildpackError {
-    #[error("{0}")]
+    #[error(transparent)]
     PackageCompositeBuildpackError(PackageCompositeBuildpackError),
-    #[error("{0}")]
+    #[error(transparent)]
     PackageLibcnbBuildpackError(PackageLibcnbBuildpackError),
     #[error("Buildpack is not supported to be packaged")]
     UnsupportedBuildpack,
@@ -95,7 +95,7 @@ pub fn package_libcnb_buildpack(
 pub enum PackageLibcnbBuildpackError {
     #[error("Assembling buildpack directory failed: {0}")]
     AssembleBuildpackDirectory(std::io::Error),
-    #[error("IO error while writing package descriptor: {0}")]
+    #[error("Couldn't write package.toml: {0}")]
     WritePackageDescriptor(std::io::Error),
     #[error("Building buildpack binaries failed: {0}")]
     BuildBinariesError(crate::build::BuildBinariesError),
@@ -115,7 +115,7 @@ pub enum PackageLibcnbBuildpackError {
 /// # Errors
 ///
 /// Returns `Err` if a `libcnb:` URI refers to a buildpack not in `buildpack_paths` or packaging
-/// otherwise failed (i.e. IO errors).
+/// otherwise failed (i.e. I/O errors).
 pub fn package_composite_buildpack(
     buildpack_directory: &Path,
     destination: &Path,
@@ -150,12 +150,12 @@ pub fn package_composite_buildpack(
 
 #[derive(thiserror::Error, Debug)]
 pub enum PackageCompositeBuildpackError {
-    #[error("Could not copy buildpack.toml: {0}")]
+    #[error("Couldn't copy buildpack.toml: {0}")]
     CouldNotCopyBuildpackToml(std::io::Error),
-    #[error("Could not read package.toml: {0}")]
+    #[error("Couldn't read package.toml: {0}")]
     CouldNotReadPackageDescriptor(TomlFileError),
     #[error("Error while normalizing package.toml: {0}")]
     NormalizePackageDescriptorError(NormalizePackageDescriptorError),
-    #[error("Could not write package descriptor: {0}")]
+    #[error("Couldn't write package.toml: {0}")]
     CouldNotWritePackageDescriptor(TomlFileError),
 }

libcnb-package/src/package_descriptor.rs

@@ -21,10 +21,10 @@ pub(crate) fn normalize_package_descriptor(
 
 #[derive(thiserror::Error, Debug)]
 pub enum NormalizePackageDescriptorError {
-    #[error("{0}")]
-    ReplaceLibcnbUriError(#[source] ReplaceLibcnbUriError),
-    #[error("{0}")]
-    PackageDescriptorDependencyError(#[source] PackageDescriptorDependencyError),
+    #[error(transparent)]
+    ReplaceLibcnbUriError(ReplaceLibcnbUriError),
+    #[error(transparent)]
+    PackageDescriptorDependencyError(PackageDescriptorDependencyError),
 }
 
 fn replace_libcnb_uris(

libcnb-package/src/util.rs

@@ -4,7 +4,7 @@ use std::path::{Component, Path, PathBuf};
 ///
 /// # Errors
 ///
-/// Returns `Err` if an IO error occurred during the size calculation.
+/// Returns `Err` if an I/O error occurred during the size calculation.
 pub fn calculate_dir_size(path: impl AsRef<Path>) -> std::io::Result<u64> {
     let mut size_in_bytes = 0;
 

libcnb-proc-macros/src/lib.rs

@@ -44,7 +44,7 @@ pub fn verify_regex(input: TokenStream) -> TokenStream {
         }
         Err(err) => syn::Error::new(
             input.regex.span(),
-            format!("Could not compile regular expression: {err}"),
+            format!("Couldn't compile regular expression: {err}"),
         )
         .to_compile_error(),
     };
@@ -110,12 +110,12 @@ pub fn verify_bin_target_exists(input: TokenStream) -> TokenStream {
             }
         } else {
             quote! {
-                compile_error!("Cannot read root package for this crate!")
+                compile_error!("Couldn't read root package for this crate!")
             }
         }
     } else {
         quote! {
-            compile_error!("Cannot read Cargo metadata!")
+            compile_error!("Couldn't read Cargo metadata!")
         }
     };
 

libcnb-test/src/app.rs

@@ -21,9 +21,11 @@ pub(crate) fn copy_app(app_dir: impl AsRef<Path>) -> Result<AppDir, PrepareAppEr
         })
 }
 
-#[derive(Debug)]
+#[derive(thiserror::Error, Debug)]
 pub enum PrepareAppError {
+    #[error("Couldn't create temporary directory: {0}")]
     CreateTempDirError(std::io::Error),
+    #[error("Couldn't copy directory: {0}")]
     CopyAppError(fs_extra::error::Error),
 }
 

libcnb-test/src/build.rs

@@ -113,7 +113,7 @@ pub(crate) enum PackageBuildpackError {
     BuildpackDescriptorNotFound(PathBuf),
     #[error("Couldn't find a buildpack with ID '{0}' in the workspace at {1}")]
     BuildpackIdNotFound(BuildpackId, PathBuf),
-    #[error("I/O error creating directory {0}: {1}")]
+    #[error("Couldn't create directory {0}: {1}")]
     CannotCreateDirectory(PathBuf, io::Error),
     #[error("Couldn't read buildpack.toml: {0}")]
     CannotReadBuildpackDescriptor(TomlFileError),

libcnb-test/src/container_context.rs

@@ -125,7 +125,7 @@ impl ContainerContext {
                 .stdout
                 .trim()
                 .parse()
-                .expect("Couldn't parse `docker port` output"),
+                .unwrap_or_else(|error| panic!("Error parsing `docker port` output: {error}")),
             Err(CommandError::NonZeroExitCode { log_output, .. }) => {
                 panic!(
                     "Error obtaining container port mapping:\n{}\nThis normally means that the container crashed. Container logs:\n\n{}",

libcnb-test/src/test_context.rs

@@ -75,7 +75,7 @@ impl<'a> TestContext<'a> {
     ///
     /// # Panics
     ///
-    /// Panics if there was an error starting the container, such as when the specified entrypoint/command cannot be found.
+    /// Panics if there was an error starting the container, such as when the specified entrypoint/command can't be found.
     ///
     /// Note: Does not panic if the container exits after starting (including if it crashes and exits non-zero).
     pub fn start_container<C: Borrow<ContainerConfig>, F: FnOnce(ContainerContext)>(
@@ -223,7 +223,7 @@ impl<'a> TestContext<'a> {
     /// Panics if there was an error creating the temporary directory used to store the
     /// SBOM files, or if the Pack CLI command used to download the SBOM files failed.
     pub fn download_sbom_files<R, F: Fn(SbomFiles) -> R>(&self, f: F) -> R {
-        let temp_dir = tempdir().expect("Could not create temporary directory for SBOM files");
+        let temp_dir = tempdir().expect("Couldn't create temporary directory for SBOM files");
 
         let mut command = PackSbomDownloadCommand::new(&self.image_name);
         command.output_dir(temp_dir.path());

libcnb-test/src/test_runner.rs

@@ -59,9 +59,10 @@ impl TestRunner {
     ) {
         let config = config.borrow();
 
-        let cargo_manifest_dir = env::var("CARGO_MANIFEST_DIR")
-            .map(PathBuf::from)
-            .expect("Could not determine Cargo manifest directory");
+        let cargo_manifest_dir = env::var("CARGO_MANIFEST_DIR").map_or_else(
+            |error| panic!("Error determining Cargo manifest directory: {error}"),
+            PathBuf::from,
+        );
 
         let app_dir = {
             let normalized_app_dir_path = if config.app_dir.is_relative() {
@@ -80,7 +81,7 @@ impl TestRunner {
             // preprocessor. Skip app copying if no changes to the app will be made.
             if let Some(app_dir_preprocessor) = &config.app_dir_preprocessor {
                 let temporary_app_dir = app::copy_app(&normalized_app_dir_path)
-                    .expect("Could not copy app to temporary location");
+                    .expect("Error copying app fixture to temporary location");
 
                 (app_dir_preprocessor)(temporary_app_dir.as_path().to_owned());
 
@@ -91,7 +92,7 @@ impl TestRunner {
         };
 
         let buildpacks_target_dir =
-            tempdir().expect("Could not create a temporary directory for compiled buildpacks");
+            tempdir().expect("Error creating temporary directory for compiled buildpacks");
 
         let mut pack_command = PackBuildCommand::new(&config.builder_name, &app_dir, &image_name);
 

libcnb-test/tests/integration_test.rs

@@ -179,7 +179,7 @@ fn packaging_failure_invalid_buildpack_toml() {
 #[test]
 #[ignore = "integration test"]
 #[should_panic(
-    expected = "Error packaging buildpack 'libcnb-test/composite-missing-package-toml': Could not read package.toml: IO error while reading/writing TOML file: No such file or directory (os error 2)"
+    expected = "Error packaging buildpack 'libcnb-test/composite-missing-package-toml': Couldn't read package.toml: I/O error while reading/writing TOML file: No such file or directory (os error 2)"
 )]
 fn packaging_failure_composite_buildpack_missing_package_toml() {
     TestRunner::default().build(