diff options
| -rw-r--r-- | library/std/src/sys/windows/process.rs | 25 |
1 files changed, 22 insertions, 3 deletions
diff --git a/library/std/src/sys/windows/process.rs b/library/std/src/sys/windows/process.rs index 909017bcc0f..988801a78f0 100644 --- a/library/std/src/sys/windows/process.rs +++ b/library/std/src/sys/windows/process.rs @@ -46,9 +46,26 @@ pub struct EnvKey { utf16: Vec<u16>, } -// Windows environment variables preserve their case but comparisons use -// simplified case folding. So we call `CompareStringOrdinal` to get the OS to -// perform the comparison. +// Comparing Windows environment variable keys[1] are behaviourally the +// composition of two operations[2]: +// +// 1. Case-fold both strings. This is done using a language-independent +// uppercase mapping that's unique to Windows (albeit based on data from an +// older Unicode spec). It only operates on individual UTF-16 code units so +// surrogates are left unchanged. This uppercase mapping can potentially change +// between Windows versions. +// +// 2. Perform an ordinal comparison of the strings. A comparison using ordinal +// is just a comparison based on the numerical value of each UTF-16 code unit[3]. +// +// Because the case-folding mapping is unique to Windows and not guaranteed to +// be stable, we ask the OS to compare the strings for us. This is done by +// calling `CompareStringOrdinal`[4] with `bIgnoreCase` set to `TRUE`. +// +// [1] https://docs.microsoft.com/en-us/dotnet/standard/base-types/best-practices-strings#choosing-a-stringcomparison-member-for-your-method-call +// [2] https://docs.microsoft.com/en-us/dotnet/standard/base-types/best-practices-strings#stringtoupper-and-stringtolower +// [3] https://docs.microsoft.com/en-us/dotnet/api/system.stringcomparison?view=net-5.0#System_StringComparison_Ordinal +// [4] https://docs.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-comparestringordinal impl Ord for EnvKey { fn cmp(&self, other: &Self) -> cmp::Ordering { unsafe { @@ -84,6 +101,8 @@ impl PartialEq for EnvKey { } } +// Environment variable keys should preserve their original case even though +// they are compared using a caseless string mapping. impl From<OsString> for EnvKey { fn from(k: OsString) -> Self { EnvKey { utf16: k.encode_wide().collect(), os_string: k } |