feat: no-std support #455

huitseeker · 2025-10-21T10:04:38Z

Adds support for no-std.

This is a slight modernization of the miette fork by @bitwalker mentioned in #443, found at bitwalker/miette@main...no-std

All the credit for the contribution goes to @bitwalker, I'm just doing janitorial work around the edges.

This commit finishes full no_std support to miette.

…implementation - Add dedicated no-std CI job that builds for wasm32-unknown-unknown target - Fix Infallible StdError implementation for no-std environments The CI job validates: 1. Core no-std functionality (no default features) 2. fancy-no-syscall feature set compatible with no-std environments

miette-derive/src/code.rs

cgettys-microsoft · 2025-11-14T23:27:58Z

src/error.rs

        }
    }

+    #[cfg(not(feature = "std"))]


Curiosity: why not a different struct, instead of using one in two configs.
If the answer is "won't compile", then this is infectious / not additive, no?

cgettys-microsoft · 2025-11-14T23:28:39Z

src/eyreish/context.rs

    }

    impl Diag for Report {
+        #[cfg_attr(track_caller, track_caller)]


is track_caller needed for the no_std implementation, or is that a separate change?

src/eyreish/error.rs

cgettys-microsoft · 2025-11-14T23:30:23Z

src/eyreish/into_diagnostic.rs

-use std::{error::Error, fmt::Display};
+extern crate alloc;
+
+use crate::StdError as Error;


why re-name StdError to Error here, but not elsewehre?

cgettys-microsoft · 2025-11-14T23:31:18Z

src/eyreish/kind.rs

 //     let error = $msg;
 //     (&error).miette_kind().new(error)

+#[cfg(not(feature = "std"))]


can all these repeated declarations live in a single imported prelude instead, or nah?

cgettys-microsoft · 2025-11-14T23:35:36Z

src/eyreish/mod.rs

-    let hook = HOOK.get_or_init(|| Box::new(get_default_printer)).as_ref();
+pub(crate) fn capture_handler_with_location(
+    error: &(dyn Diagnostic + 'static),
+) -> Box<dyn ReportHandler> {


Why the Once? can't it just be OnceLock?
I'm having a hard time following what this change is achieving, which means more comments might be needed here. I don't see why two different once's are needed.

cgettys-microsoft · 2025-11-14T23:36:00Z

src/eyreish/wrapper.rs

    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        self.0.source()
    }
-


This is a breaking change, no?

cgettys-microsoft · 2025-11-14T23:36:42Z

src/eyreish/wrapper.rs

+        boxed::Box,
+        string::{String, ToString},
+    };
+    #[cfg(feature = "fancy")]


shouldn't many of these still be alloc:: qualified types instead, in case someone wants to make fancy no-std too in future?

cgettys-microsoft · 2025-11-14T23:37:59Z

src/handler.rs

+            if #[cfg(feature = "fancy-no-backtrace")] {
+                supports_color::on(supports_color::Stream::Stderr).is_some()
+            } else if #[cfg(feature = "fancy-no-syscall")] {
+                // In no-std environment without color support, default to no color support


comment not quite true. fancy-no-syscall can be enabled w/o no-std, right?

cgettys-microsoft · 2025-11-14T23:40:30Z

src/handler.rs

    pub(super) fn supports_color() -> bool {
        cfg_if! {
-            if #[cfg(feature = "fancy-no-syscall")] {
+            if #[cfg(feature = "fancy-no-backtrace")] {


this reordering seems like it might be a behavior change.
Shouldn't it be something like:
if fancy-no-syscall, false
maybe else if no-std, false
else if fancy-no-backtrace, check
maybe else, false?
These negated features are so hard to think through. Feels like some of them should be flipped next time there's a major version?

cgettys-microsoft · 2025-11-14T23:41:06Z

src/handler.rs

-            if #[cfg(feature = "fancy-no-syscall")] {
-                None
-            } else {
+            if #[cfg(feature = "fancy-no-backtrace")] {


again, seems like this may change behavior in with std scenarios?

cgettys-microsoft · 2025-11-14T23:44:11Z

src/handlers/theme.rs

-        match std::env::var("NO_COLOR") {
-            _ if !std::io::stdout().is_terminal() || !std::io::stderr().is_terminal() => {
-                Self::none()
+        #[cfg(feature = "fancy-no-syscall")]


this seems ok, though again, the negated features are really tricky...

cgettys-microsoft · 2025-11-15T00:02:49Z

src/highlighters/mod.rs

-                Self(Arc::new(SyntectHighlighter::default()))
+        #[cfg(all(feature = "syntect-highlighter", not(feature = "fancy-no-syscall")))]
+        {
+            use std::io::IsTerminal;


It looks like there's some deduplication that should be done vs src/handlers/theme.rs
It's already pretty complicated and presumably the two should be kept in sync.
Maybe a helper function? Ideally that memo-izes it to avoid many redundant syscalls if necessary

Note: the double or triple negation in the old version was making my head hurt, so I made it exhaustive instead

enum StyleDetection { NoColor, Color } fn detect_color_support() -> { #[cfg(any(feature = "fancy-no-syscall", not(feature = "std")))] { StyleDetection::NoColor } #[cfg(all(not(feature = "fancy-no-syscall"), feature = "std"))] { match std::env::var("NO_COLOR") { if !std::io::stdout().is_terminal() || !std::io::stderr().is_terminal() => { // TODO: address todo by making this StyleDetection::NoColor? //TODO: should use ANSI styling instead of 24-bit truecolor here StyleDetection::Color } Ok(string) => { if string == "0" { // NO_COLOR = "0" means enable color StyleDetection::Color } else { // any other value means disable color. StyleDetection::NoColor } } // This could be NotPresent (color since disablement env var not set) or NotUnicode (hopefully never happens, but we'llt reat it the same) Err(_) => StyleDetection::Color } } }

cgettys-microsoft · 2025-11-15T00:06:13Z

src/lib.rs

@@ -1,6 +1,8 @@
+#![no_std]
 #![deny(missing_docs, missing_debug_implementations, nonstandard_style)]


If this is going to be merged, I think at least some of these should probably be enabled?

![deny(clippy::alloc_instead_of_core,clippy::std_instead_of_core,clippy::std_instead_of_alloc)]

cgettys-microsoft · 2025-11-15T00:07:38Z

src/lib.rs

+#[cfg(feature = "std")]
+pub use std::error::Error as StdError;
+
+#[cfg(not(feature = "std"))]


As written, this isn't additive. Can't there be an enum MietteErr with two enum variants, one of which is only enabled when std is? or would that be breaking? not saying it should change, just seeking to understand

cgettys-microsoft · 2025-11-15T00:09:05Z

src/protocol.rs

+    /// as a [`SourceOffset`].
+    ///
+    /// In no_std environments, this is not supported and will return an error.
+    #[cfg(not(feature = "std"))]


This doesn't make sense to me, why not just compile it out entirely? why add a useless implementation?

cgettys-microsoft · 2025-11-15T00:09:52Z

tests/test_boxed.rs


    let error = MyError {
-        source: io::Error::new(io::ErrorKind::Other, "oh no!!!!"),
+        source: io::Error::other("oh no!!!!"),


Might be worth pulling all the io::Error changes out into another PR to keep the diff down, but idk.

cgettys-microsoft · 2025-11-15T00:11:28Z

Cargo.toml

 exclude = ["images/", "tests/", "miette-derive/"]

 [dependencies]
+thiserror = "2.0.11"


Why is this needed?

cgettys-microsoft · 2025-11-15T00:12:01Z

Cargo.toml

-unicode-width = "0.2.0"
+unicode-width = { version = "0.2.0", default-features = false }
 cfg-if = "1.0.0"
+spin = { version = "0.9", default-features = false, features = ["mutex", "spin_mutex", "lazy"] }


Doesn't seem like it should be a dependency for std environments. I see the simplicity benefit, but adding a dependency for consumers for whom OnceLock is an option, doesn't seem like it makes sense.
Also, do you need all those features?

cgettys-microsoft · 2025-11-15T00:15:21Z

Cargo.toml

+thiserror = "2.0.11"
 miette-derive = { path = "miette-derive", version = "=7.6.0", optional = true }
-unicode-width = "0.2.0"
+unicode-width = { version = "0.2.0", default-features = false }


This is a size optimization? If so, maybe default features should have an option that turns this back on? As written any consumers with cjk character requirements will be broken unless they otherwise include unicode-width with the relevant featur eon

build.rs

cgettys-microsoft · 2025-11-15T00:20:12Z

To be clear, my comments are just my opinion - I'm neither the author nor am I a maintainer of this library, I'm just a person who has submitted a few contributions previously and was curious. Hope the feedback is helpful.

- Fixed compilation errors in no-std mode (TestError Debug derive, ToString imports) - Addressed consistency issues: Option qualification, StdError naming, Borrow impl - Fixed feature flag logic for supports_color function with proper priority ordering - Added conditional compilation for std-dependent features in panic.rs, handler.rs, theme.rs - Made syntect-highlighter feature explicitly require std - Consolidated duplicate Borrow implementations using core::borrow::Borrow

- Add #[allow(unused_assignments)] to test structs/enums with unused fields - Fix Miri test failures caused by derive macro feature interactions - Miri tests now pass: 52 passed, 0 failed, 6 ignored - Issue occurs when fancy feature changes how derive macros process fields - Suppression is appropriate as fields are only unused under specific feature combinations

Cargo.toml

cgettys-microsoft · 2025-11-17T01:59:49Z

src/handler.rs

                    true
                }
            }
+            #[cfg(all(not(feature = "fancy-no-syscall"), not(feature = "std")))]


These two are not opposites. Above is !A && B. this is !A && !B
This should probably be A || !B?

cgettys-microsoft · 2025-11-17T02:01:54Z

src/panic.rs

+#[cfg(feature = "std")]
 use std::boxed::Box;
+#[cfg(feature = "std")]
 use std::{


maybe make the whole module conditional? I don't think a fake no-op set_panic_hook should be provided when std is not set, it's correct to not have it there and will not break any program that depends on miette with std support feature

- Remove rustc_version build dependency that was no longer needed - Remove nightly detection since nightly cfg flag wasn't used anywhere - Simplify build.rs to only set track_caller cfg flag

bitwalker and others added 3 commits October 18, 2025 07:25

fix: support no-std environments

243ea53

feat(no_std): Implement comprehensive no_std support

79fbd0a

This commit finishes full no_std support to miette.

huitseeker force-pushed the no-std branch from d60ac69 to 18c217f Compare October 21, 2025 11:19

huitseeker marked this pull request as ready for review October 21, 2025 11:19

cgettys-microsoft reviewed Nov 14, 2025

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Cargo.toml

exclude = ["images/", "tests/", "miette-derive/"]

[dependencies]

thiserror = "2.0.11"

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Why is this needed?

cgettys-microsoft reviewed Nov 15, 2025

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huitseeker force-pushed the no-std branch from 957342d to 3768f73 Compare November 16, 2025 12:44

huitseeker force-pushed the no-std branch from 3768f73 to 9af0988 Compare November 16, 2025 14:16

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huitseeker added 2 commits November 19, 2025 09:20

fix: remove unused rustc_version dependency and simplify build.rs

9bdf23a

- Remove rustc_version build dependency that was no longer needed - Remove nightly detection since nightly cfg flag wasn't used anywhere - Simplify build.rs to only set track_caller cfg flag

refactor: use #[track_caller] directly instead of conditional cfg_attr

09e1090

		@@ -1,6 +1,8 @@
		#![no_std]
		#![deny(missing_docs, missing_debug_implementations, nonstandard_style)]

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cgettys-microsoft commented Nov 15, 2025

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