flang: update support and add flang-rt

[Acture]

Supersedes #391942

Summary

Package LLVM Flang as a first-class Fortran compiler in nixpkgs, with runtime library, cc-wrapper integration, and driver compatibility patches backported from upstream.

Based on the excellent initial implementation by @picostove.

Key Changes

Packages:

• Update the flang package with standalone build, passthru metadata (langFortran, isFlang, isClang, hardeningUnsupportedFlags)
• Add flang-rt runtime library (libFortranRuntime, libFortranDecimal)

cc-wrapper integration:

• Recognize -fc1 as frontend invocation (analogous to clang's -cc1)
• Strip -frandom-seed=* when wrapping flang (injected by reproducible-build hook; flang doesn't accept it)
• Use existing hardeningUnsupportedFlags to disable unsupported hardening flags

Driver patches (backported from upstream):

• warn-on-fbuiltin-and-fno-builtin — accept -fbuiltin/-fno-builtin with warning instead of hard error
• accept-and-ignore-some-gfortran-optimization-flags — handle -fexpensive-optimizations etc.
• use-xflang-in-diagnostics — suggest -Xflang instead of -Xclang in error messages

MLIR: fix MLIRConfig.cmake for standalone builds

MLIRConfig.cmake unconditionally overwrites MLIR_TABLEGEN_EXE, breaking standalone
builds that provide their own pre-built mlir-tblgen (e.g. in Nix sandboxed builds).
The patch adds guards to respect caller-set values and auto-creates an imported
mlir-tblgen target for downstream consumers. This replaces the previous dummy target
workaround in flang's CMakeLists.txt.

Upstream issue: llvm/llvm-project#150986

flang-rt: remove unused dependencies

flang-rt does not link against MLIR or libclang; removed unused CLANG_DIR/MLIR_DIR
cmake flags, build inputs, and the empty dev output.

Tests:

• compile-smoke — basic compilation and response file (@file) support
• driver-flags — wrapper flag filtering, driver diagnostics, and regression coverage for flang: update support and add flang-rt #452306 (comment) issues

Design

No user-supplied flags are silently dropped. Flag incompatibility is handled at three layers:

1. cc-wrapper — flags that should never reach flang are not injected in the first place (hardeningUnsupportedFlags, -frandom-seed filtering)
2. Upstream patches — flags commonly passed by Fortran build systems produce warnings, not hard errors
3. Passthru metadata — downstream tooling can detect flang and adapt accordingly

Test plan

• nix-build -A llvmPackages_21.flang
• nix-build -A llvmPackages_21.flang-rt
• nix-build -A llvmPackages_21.flang.passthru.tests.compile-smoke
• nix-build -A llvmPackages_21.flang.passthru.tests.driver-flags
• @VincentVanlaer's reproducer (basic Fortran compilation via mkDerivation)

Checklist

• I have read the contributing guidelines.
• The PR is formatted using nix fmt.

[Acture]

Continuing the work from @picostove's seemly inactive PR #391942. Requesting reviews from original reviewers in case I miss something. Any feedback would be greatly appreciated.

[Acture]

Build took an extremely long time. wasn't expecting this though

[RossComputerGuy]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 452306 --package llvmPackages_21.flang --package llvmPackages_21.flang-rt --package llvmPackages_21.clang --package llvmPackages_21.mlir
Commit: 17f5d754b59666c9558db102c824b481b0578047

---

aarch64-linux

❌ 6 packages failed to build:

• llvmPackages_21.clang
• llvmPackages_21.flang
• llvmPackages_21.flang-rt
• llvmPackages_21.flang-rt.dev (llvmPackages_21.flang-rt.dev.dev)
• llvmPackages_21.mlir
• llvmPackages_21.mlir.dev (llvmPackages_21.mlir.dev.dev)

[RossComputerGuy]

I think I just ran into a bug with nixpkgs-review, the build is still running.

[RossComputerGuy]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 452306 --package llvmPackages_21.flang-rt --package llvmPackages_21.flang --package llvmPackages_21.mlir --package llvmPackages_21.clang
Commit: 17f5d754b59666c9558db102c824b481b0578047

---

aarch64-linux

✅ 6 packages built:

• llvmPackages_21.clang
• llvmPackages_21.flang
• llvmPackages_21.flang-rt
• llvmPackages_21.flang-rt.dev (llvmPackages_21.flang-rt.dev.dev)
• llvmPackages_21.mlir
• llvmPackages_21.mlir.dev (llvmPackages_21.mlir.dev.dev)

[alyssais]

I think this should be split into multiple commits. There's a lot going on here.

[Acture]

The adapter had two issues for me:

• -nostdlibinc was missing in the arguments that needs to be removed
• Arguments read in from files (the @filename thing) have all commas escaped (these can occur for example with -Wl. Either these need to be unescaped or written out again to a file.

I've updated the wrapper to explicitly filter -nostdlibinc, and rewrite @file arguments to a tmp file.
Seems to work fine with my local test. Could you take another look?

[VincentVanlaer]

It works for me too now

[Acture]

It works for me too now

Great. Let's see if someone could help with merging.

[Copilot]

Pull request overview

Updates the LLVM Flang Fortran toolchain packaging to include the new flang-rt runtime and adjusts cc-wrapper/toolchain plumbing so Flang can be used as a wrapped compiler in nixpkgs (including handling Flang’s -fc1 mode).

Changes:

• Update flang packaging to include flang-rt sources and export language capability passthru flags for cc-wrapper.
• Add a new flang-rt derivation (runtime library) and wire it into the LLVM toolchain set.
• Extend cc-wrapper to recognize -fc1 as cc1-like and add initial Fortran wrapper support for flang.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pkgs/development/compilers/llvm/common/flang/default.nix	Adds flang-rt to the extracted monorepo source and exports passthru.lang* fields for wrapper behavior.
pkgs/development/compilers/llvm/common/flang-rt/default.nix	Introduces the flang-rt runtime build from LLVM runtimes.
pkgs/development/compilers/llvm/common/default.nix	Adds flang-unwrapped, flang-rt, and a Flang adapter wrapper; wires wrapped flang to include runtime libs.
pkgs/development/compilers/llvm/common/clang/default.nix	Explicitly marks clang as providing C and C++.
pkgs/build-support/cc-wrapper/default.nix	Updates Fortran wrapping logic to optionally wrap flang; adjusts a libc++ flags condition to use cc.langCC.
pkgs/build-support/cc-wrapper/cc-wrapper.sh	Treats -fc1 like -cc1 to avoid injecting wrapper flags in that mode.

---

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[Acture]

@alyssais I've reworked the approach based on your feedback. The standalone wrapper script is gone.

Instead of silently dropping flags, the new revision handles flag compatibility at three levels:

1. cc-wrapper — natively recognizes flang (isFlang), strips only -frandom-seed=* (which the reproducible-build hook injects and flang can't accept), and uses the existing hardeningUnsupportedFlags mechanism for the rest
2. Driver patches backported from upstream — flang now warns on common gfortran flags (-fbuiltin, -fexpensive-optimizations) instead of hard-erroring, and suggests -Xflang instead of -Xclang in diagnostics
3. Tests — driver-flags test verifies all of the above behavior, including that no flags are silently swallowed

No user-supplied flags are dropped. Flags that flang doesn't support either produce a proper diagnostic (via backported patches) or are never injected in the first place (via cc-wrapper / hardeningUnsupportedFlags).

Ready for another look when you have time.

[Acture]

@emilazy Take a look when you have time. It seems requested changes somehow stucked.

No time to re‐review

[emilazy]

I think I won’t be able to get around to re‐reviewing this PR in a timely manner – apologies! – so I’ve dismissed my blocking review to not hold it up. (Looks like @alyssais still has a blocking review, though.)