Add min_$TARGET_API_version cfg predicates

text/0000-min-target-api-version.md

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+- Feature Name: `min_target_api_version`
+- Start Date: 2020-11-02
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+
+[summary]: #summary
+
+A new `cfg` predicate `min_target_api_version` that allows users to declare the minimum target API version they would like to support.
+
+For instance, the standard library's Windows Mutex implementation could potentially take advantage of this mechanism instead of relying on runtime API detection:
+
+```rust
+pub unsafe fn unlock(&self) {
+    *self.held.get() = false;
+    if cfg!(min_target_api_version >= "6.1.7600") { // `cfg!(min_target_api_version = "Windows7")` is also possible 
+        c::ReleaseSRWLockExclusive(raw(self))
+    } else {
+        match kind() {
+            Kind::SRWLock => c::ReleaseSRWLockExclusive(raw(self)),
+            Kind::CriticalSection => (*self.remutex()).unlock(),
+        }
+    }
+}
+```
+
+# Motivation
+
+[motivation]: #motivation
+
+The target API version is the version number of the "API set" that a particular binary relies on in order to run properly. An API set is the set of APIs that a host operating systems makes available for use by binaries running on that platform. With newer versions of a platform comes newer APIs in the API set. 
+
+Crates including the standard library must account for the *minimum* API version that is required in order for the crate to be able to run. Rust currently has no mechanism for crates to compile different code (or to gracefully fail to compile) depending on the minimum targeted API version. This leads to the following issues:
+
+* Relying on dynamic detection of API support has a runtime cost. The standard library often performs [dynamic API detection](https://github.com/rust-lang/rust/blob/f283d3f02cf3ed261a519afe05cde9e23d1d9278/library/std/src/sys/windows/compat.rs) falling back to older (and less ideal) APIs or forgoing entire features when a certain API is not available. For example, the [current `Mutex` impl](https://github.com/rust-lang/rust/blob/234099d1d12bef9d6e81a296222fbc272dc51d89/library/std/src/sys/windows/mutex.rs#L1-L20) has a Windows XP fallback. Users who only ever intend to run their code on newer versions of Windows will still pay a runtime cost for this dynamic API detection. Providing a mechanism for specifying which minimum API version the user cares about, allows for statically specifying which APIs a binary can use. 
+* Certain features cannot be dynamically detected and thus limit possible implementations. The libc crate must use [a raw syscalls on Android for `accept4`](https://github.com/rust-lang/libc/pull/1968), because this was only exposed in libc in version 21 of the Android API. In the future there might be similar changes where there is no way to implement a solution for older versions.
+* Trying to compile code with an implicit dependency on a API version greater than what is supported by the target platform leads to linker errors. For example, the `x86_64-pc-windows-msvc` target's rustc implementation requires `SetThreadErrorMode` which was introduced in Windows 7. This means trying to build the compiler on older versions of Windows will fail with [a less than helpful linker error](https://github.com/rust-lang/rust/issues/35471).
+
+# Guide-level explanation
+
+[guide-level-explanation]: #guide-level-explanation
+
+Rust targets are often thought of as monoliths. The thought is that if you compile a binary for that target, that binary should be able to run on any system that fits that target's description. However, this is not actually true. For example, when compiling for `x86_64-pc-windows-msvc` and linking with the standard library, my binary has implicitly taken a dependency on a set of APIs that Windows exposes for certainly functionality. If I try to run my binary on older systems that do not have those APIs, then my binary will fail to run. Platforms usually expose a set of APIs in backward compatible with each release. When compiling for a certain target, you are therefore declaring a dependency on a minimum target API version that you rely on for your binary to run. 
+
+By default, the standard library uses a sensible minimum API version. For example, for `x86_64-pc-windows-msvc` the minimum API version is "6.1.7600" which corresponds to Windows 7. However, there's good reason why you might want control of how your code is compiled depending on what the minimum API version is set as. For instance, if you want to:
+
+* set your crate's minimum API version higher than that of the standard library. 
+* change certain implementation details of your crate depending on what a downstream user sets their minimum API version to be.  
+* have some sensible compiler error if users of your crate require a lower minimum API version that you require.
+
+This is where the `cfg` predicate `min_target_api_version` comes in. This allows you to conditionally compile code based on the set minimum API version. For example an implementation of mutex locking on Windows might look like this:
+
+```rust
+pub unsafe fn unlock(&self) {
+    *self.held.get() = false;
+    if cfg!(min_target_api_version >= "6.0.6000") { // API version greater than Vista
+        c::ReleaseSRWLockExclusive(raw(self)) // Use the optimized ReleaseSRWLockExclusive routine
+    } else {
+        (*self.remutex()).unlock()  // Fall back to an alternative that works on older Windows versions
+    }
+}
+```
+
+End users can set the `min_target_api_version` in the Cargo configuration file `.cargo/config` under the [`target key`](https://doc.rust-lang.org/cargo/reference/config.html#target)  like so:
+
+```toml
+[target.x86_64-pc-windows-msvc]
+min_target_api_version = "6.0.6000" # Vista 
+```
+
+If and end user sets their `min_target_api_version` to an incompatible version then the user receives an error. For instance, in the example above where the user is setting their `min_target_api_version` to Windows Vista, they will receive an error when linking with the standard library which imposes Windows 7 as its `min_target_api_version` by default for the `x86_64-pc-windows-msvc` target. 
+
+Many targets don't support a `min_target_api_versison` and any `min_target_api_version` check will return false. 
+
+`min_target_api_version` does not assume any semantic versioning information. The only semantics that are assumed is that code compiled with `min_target_api_version` works for all versions greater than or equal to that version. 
+
+# Reference-level explanation
+
+[reference-level-explanation]: #reference-level-explanation
+
+`min_target_api_version` is a `cfg` predicate option that allows users to conditionally compile code based on what the minimum platform API version they are willing to support is. In many ways, this predicate functions exactly like other `cfg` predicates with the exception that the predicate implicitly operates on an `>=` comparison and not a plain `=` equality check meaning that the list of target API versions needs to be ordered. 
+
+The implementation does an `Ord` comparison of the provided `min_target_api_version` defaulting to the platform's default if version cannot be parsed. The set `min_target_api_version` is set by the user through flags to rustc (or equivalently through a key in the `target` section of the Cargo configuration file). If none is provided, the version defaults to the target's default. Targets with no support for `min_target_api_version` should be defaulted to a special "any version" target API version which is less than all others.
+
+### Versioning schema
+
+Each platform (i.e., collection of targets that share an operating system component of their target triple) should have its own versioning schema. This means that how `min_target_api_version`s are represented for Windows targets need not be related at all to how they are represented on Linux versions. 
+
+For Windows, it most likely makes sense to use `<major>.<minor>.<build>` versioning that Microsoft uses to specify OS versions. These version numbers are monotonically increasing. In addition to this aliases for marketing names can be provided for convenience (e.g., "6.1.7600" == "Windows7"). 
+
+An example where another schema may be used entirely is Android which typically only ships major versions (e.g., API Level 21). It is also possible for a vendor to switch versioning schemas entirely (e.g., from date releases to version numbers).
+
+Implementors will likely want to implement `Ord`, `FromStr` and `Dispay` on some enum which represents a given platform's api version numbers. 
+
+# Drawbacks
+
+[drawbacks]: #drawbacks
+
+There are no known large drawbacks to this proposal. Some small drawbacks include:
+* No all targets will strictly follow an ever increasing versioning scheme where more recent versions (i.e., versions with larger version numbers) are supersets of less recent versions. APIs may be deprecated and completely removed between versions. 
+   * This proposal does not seek to address these situations. The mechanism does not provide any semantic versioning scheme on the versioning numbers. Maintainers must ensure that any feature they use can work for _all_ versions greater than or equal to the `min_target_api_version`.
+   * This does mean that code compiled with `min_target_api_version` is assumed to continue to work with all future versions of a target API. This is not considered a drawback per say since this is currently implicitly true for any target API use. 
+* Incremental complexity of the language
+
+# Rationale and alternatives
+
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The overall mechanism proposed here builds on other well established primitives in Rust such as `cfg`. No other alternatives have been seriously considered except for the status quo. 
+
+Small changes could be considered:
+* The use of `>=` in cfg annotations is new syntax. Using `=` might be simpler though possibly slightly more confusing. 
+
+# Prior art
+
+[prior-art]: #prior-art
+
+Adding new `cfg` predicates has been proposed and accepted in [previous](https://github.com/rust-lang/rfcs/blob/3071138d4ed510d6dfc1f8e1d7e9d4b099ea12e8/text/2495-min-rust-version.md) [RFCs](https://github.com/rust-lang/rfcs/blob/2b0c911d55cf4095036b984ae30d8b632718241e/text/2523-cfg-path-version.md). This proposal is similar to these previous proposals in mechanics. The actual semantics of this RFC have been discussed several times though this is the first time it is being formally proposed in an RFC. This document hopes to capture the entire state of the discussion up until this point. For a detailed history on the discussion, take a look at the following resources:
+
+* [Pre-RFC Zulip Discussion](https://rust-lang.zulipchat.com/#narrow/stream/242869-t-compiler.2Fwindows/topic/(Pre-RFC.3F).20target-api.2Fos-version)
+* [Proof of concept implementation](https://github.com/rust9x/rust/commit/cccfa575a4bb449defdcbf362757f1e161f6cdf5)
+* [Discussion as it relates to dropping XP Support](https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/Drop.20official.20support.20for.20Windows.20XP.20compiler-team.23378)
+* [More XP Support Discussion](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/On.20WinXP.20support)
+
+# Unresolved questions
+
+[unresolved-questions]: #unresolved-questions
+
+There are still some unresolved questions:
+
+### Should there be new targets that only differ from existing targets in their default `min_target_api_version`?
+
+In the [future possibilities](#future-possibilities) section we discuss the possibility of allowing users specialized versions of the std library depending on the specified `min_target_api_version`. In the meantime, should we also create new targets that different from the existing targets in that they have different default `min_target_api_version`s? This would allow users to more directly benefit from this feature. 
+
+### Should failing to parse the provided `min_target_api_version` produce a warning?
+
+The list of acceptable `min_target_api_version`s for a target should be well known, and thus if a user provides an unknown version, the compiler could be in position to warn the user about this mistake. This could lead to possible scenarios where the code emits a warning on an API version that exists in the wild but that the compiler does not yet know about. 
+
+### How do custom targets specify ordered valid list of `min_target_api_version`s?
+
+While it may be possible to have built in compiler support for major targets like `x86_64-pc-windows-msvc`, it is less clear how this should work for custom targets which usually specify such configuration in a target specification JSON file. We propose leaving this for a future RFC. 
+
+# Future possibilities
+
+[future-possibilities]: #future-possibilities
+
+### std aware Cargo
+
+This proposal should work nicely with std aware cargo, allowing for different builds of the std library based on what the `min_target_api_version` is set to. For now, the standard library shipped with each target through rustup will be set to some sensible default. 
+
+### `max_target_api_version`
+
+This provides a max bounds on the use of a target API. This would protect against use of later APIs where certain features had been modified or removed.